Nature's Miracles

Familiar Talks on Science

BY

ELISHA GRAY, Ph. D., LL. D.

Vol. I

World-Building and Life

Earth, Air, and Water

NEW YORK

FORDS, HOWARD, & HULBERT

Copyright, 1899,
BY
FORDS, HOWARD & HULBERT.
THE MERSHON COMPANY PRESS,
RAHWAY, N. J.

CONTENTS.

CHAPTER PAGE
Introduction, [v]
Earth.
I. World-Building and Life, [1]
II. Limestone, [12]
III. Coal, [22]
IV. Slate and Shale, [31]
V. Salt, [36]
Air.
VI. The Atmosphere, [42]
VII. Air Temperature, [51]
VIII. Cloud Formation, [60]
IX. Cloud Formation (Continued), [69]
X. Wind—Why It Blows, [79]
XI. Wind (Continued), [88]
XII. Local Winds, [100]
XIII. Weather Predictions, [110]
XIV. How Dew Is Formed, [115]
XV. Hailstones and Snow, [124]
XVI. Meteors, [129]
XVII. The Sky and Its Color, [134]
XVIII. Liquid Air, [146]
Water.
XIX. Rivers and Floods, [152]
XX. Tides, [161]
XXI. What Is a Sponge? [167]
XXII. Water and Ice, [177]
XXIII. Stored Energy in Water, [182]
XXIV. Why Does Ice Float? [192]
XXV. Glaciers, [198]
XXVI. Evidences and Theories of an Ice Age, [207]
XXVII. Glacial and Preglacial Lakes and Rivers, [217]
XXVIII. Some Effects of the Glacial Period, [230]
XXIX. Drainage before the Ice Age, [239]

INTRODUCTION.

Dear Reader: Please look through this "Introduction" before beginning with the regular chapters. It is always well to know the object, aim, and mode of treatment of a book before reading it, so as to be able to look at it from the author's view-point.

First: A word about the title—"Nature's Miracles." Some may claim that it is unscientific to speak of the operations of nature as "miracles." But the point of the title lies in the paradox of finding so many wonderful things—as wonderful as any miracle that was ever recorded—subservient to the rule of law.

"But," you say, "a miracle does not come under any rule of law."

Ah! are you sure of that? It is true that we may not understand the law that the so-called miracle comes under, but the Author of all natural law does. We do not pretend to dispute but that the Power that made nature's laws can change them if He sees fit; but we cannot believe that He will ever see fit. It would destroy all order and harmony, all advancement in science and knowledge of God's works, not to be able to rely implicitly upon the laws of nature as consistent and continuous.

In putting out these little volumes, it is not to be understood that the subjects treated will be more than touched upon, at the most salient points. To do much more would require volumes of immense size, and life would be too short for me to write or for you to read them.

Again: these volumes are "familiar talks." The Author wishes to sit down with you—so to speak—and not hold you at arm's length.

It will be his aim to use the language of common life and to avoid all technical names so far as possible, or, when they are necessary, to explain their meaning. The object is to reach the thousands of readers who have not and cannot have the advantages of a scientific education, but who can by this means get at least a rudimentary idea of some of the natural laws with which they are coming in contact every hour, and through which the inner man has constant communication with the outer world. It may be, too, that many young students will be helped by these plain general views of topics which their text-books will give them in detail.

A knowledge of the real things in the objective world about us and the laws that govern them in their inter-relations is of practical value to every man, whatever his calling may be. Not only will it be of value practically, but it will also be a constant source of interest and pleasure. Man is so constituted that he must have something to be interested in, and if he has no resources within himself he looks elsewhere, and often to his hurt, mentally, morally, or otherwise. If he could have an interest awakened in him for the study and contemplation of the natural world he would then have a book to read that is always open, always fresh, always new. He is dealing with facts and not theory, except as he uses theory for getting at facts.

A man who is all theory is like "a rudderless ship on a shoreless sea." All he really knows is that he is afloat, and if he lands at all it is likely to be in an insane asylum. The mind, in order to keep its balance, must have the solid foundation of real things. Theories and speculations may be indulged in with safety only so long as they are based on facts that we can go back to at all times and know that we are on solid ground.

It is the desire and aim of all good men to make their nation a truly great people, with a civilization the highest possible. The character of all kinds of growth is largely determined by the character of the material upon which it feeds. The study of natural law can never be harmful, but is always beneficial, for the student is then working in harmony with law. It is the violation of law that makes all the trouble in the world—whether physical, moral, or social. When we speak of natural law we do not confine ourselves to what is commonly known as chemistry and physics, and the laws that govern the material world, but include as well the laws of our own being, as intellectual and spiritual units. For all law, physical, intellectual, and spiritual, is in a sense natural.

All departments of science are simply branches of one great science, and all phases of human activity are touched by it. The preacher is a better preacher, the doctor a better doctor, the lawyer a better lawyer, the editor a better editor, the business man a better merchant, and the mechanic a better workman, if they follow scientific methods. Indeed, any man will be a better husband, father, and citizen, if he has some trustworthy knowledge of the laws under which this great universe, down to his own little part of it, lives, moves, and has its being.

NATURE'S MIRACLES.

EARTH.

CHAPTER I.

WORLD-BUILDING AND LIFE.

"In the beginning God created the heaven and the earth. And the earth was without form, and void."

Whatever our speculations may be in regard to a "beginning," and when it was, it is written in the rocks, that, like the animals and plants upon its surface, the earth itself grew; that for countless ages, measured by years that no man can number, the earth has been gradually assuming its present form and composition, and that the processes of growth and decay are active every hour.

The science that deals with the formations and stratifications that are found on the earth and under the earth, and all the forces that have been and are now active in their formation, is called Geology (earth science). It is a science about which little is known by the average individual, and yet it is one of transcendent interest, from the study of which the lover of nature can obtain a vast amount of profit and pleasure. When the uncultured man sees a stone in the road it tells him no story other than the fact that he sees a stone and that it would better be removed; and all the satisfaction he gets out of it is in the thought that he has saved some unlucky wagon wheel from being wrenched or broken. The scientist looking at the same stone perhaps will stop, and with a hammer break it open, when the newly exposed faces of the rock will have written upon them a history that is as real to him as the printed page. He is carried back to a far-off time, where he sees the processes and forces at work that have formed this stone and made it what it is, not only in its outward form, but in its constitution, down to its molecules and atoms. (The word "atom" is used in chemistry to mean the smallest particle of an elementary substance that will combine with the atoms of another substance to form new compounds of matter. And molecules are made up of atoms.) The scientist looking at this stone sees in it not only that mechanical and chemical agencies have cooperated in the work of its formation, but that animal life itself may have been the chief agency in bringing the materials together and giving form to the peculiar architecture employed in its formation. If it is a piece of limestone this latter statement will be eminently true.

Here is a powerful motive for the study of physical science. It is not to be expected, nor is it possible, that every individual can be a scientist in the strict sense of the word, but it is possible for everyone of ordinary intelligence to become familiar with the salient facts of science, if only a small portion of the time that is now devoted to the reading of literature that is rather harmful than helpful be spent in studying the phenomena and works of nature.

The acquirement of such knowledge would furnish every individual with a constant source of instructive amusement that would never lose its interest. He would not be dependent every hour upon people and things outside of himself; because he would carry about with him inexhaustible sources of instruction and pleasure that would furnish him continual and helpful diversion and save him from a thousand morbid tendencies that are always ready to seize upon an unemployed mind. There are many men and women in the insane asylum to-day for the simple reason that they have not made intelligent use of the mental powers that nature has endowed them with.

Sermons are not always preached from pulpits. They are written in the rocks and on the flowers of the field and the trees of the forest.

Let us then look a little at the underground foundation of all this beautiful earth. And before attempting that, the question may arise in some minds how we know what is so deep down under the surface. Fortunately this is a question very easily answered. At some period after the rocks were formed the crust of the earth was broken by volcanic eruptions at various places and times, and turned up, as in the formation of mountains, so that the edges of the various stratifications of the rocks, from those near the surface down to the lowest rocks, are exposed to view. Another means of knowing what the various formations are has been by borings of deep wells. These borings, however, are only confirmatory of what was well known before through the upheavals that are plentiful in all parts of the world. There is abundant evidence that all of the rocks and all of the strata of every name and nature (except perhaps igneous rocks) were originally laid down in water. This is evidenced not only by the stratifications themselves, but by the evidences of sea-life everywhere present in the earth's crust. Before the upheavals in the earth's crust began, the whole surface of the globe was a great ocean of hot water. The substances of which the rocks were formed were undoubtedly held in suspension in the air and in the water, and by a gradual process were deposited in the bottom of the ocean in layers, forming rocks of various kinds, according to the nature of the substance deposited. Gradually the crust of the earth was built up until it acquired a certain thickness; when, either from shrinkage under the crust a great void was formed until it could not sustain its own weight, or the pressure caused by confined gases and molten matter produced an upheaval which broke the crust of the earth outward, causing great wrinkles that we call mountain ranges. Undoubtedly both forces were active in producing these results. When the gases and molten matter had escaped through the rifts in the rocks caused by the upheaval there must have been great voids formed that were filled up by the shrinkage of the earth, causing much irregularity in its surface.

In some places there were enormous elevations, and in others correspondingly deep depressions. The water that before was evenly distributed over the surface of the globe, after the upheavals ran off into the lower levels, filling up the great valleys, forming the seas, and leaving about one-third of the land surface uncovered. It must not be supposed, however, that the appearance of the land was caused by one grand movement or upheaval, but that it has been going on in successive stages through long ages of time. This is clearly evidenced by the rock formations. The deposition of rock strata is still active in the bottoms of the oceans, although not to the same degree as in former times. When the upheaval took place the old stratifications were thrown out of level, but the new ones that were then formed remained in a level position until they were in their turn disturbed by some subsequent upheaval.

The laws of gravitation would tend to precipitate the matter held in suspension by the water straight down to the bottom, toward the center of the earth, so that the plane of these stratifications would tend to be parallel to the surface of the water, that is horizontal, until disturbed. Then they would be tilted in many directions. Hence it will be easily seen why the seams in the rocks, especially in and near mountainous regions, do not lie in a horizontal position after an upheaval, but are found standing at all angles, up to a perpendicular.

Viewed from this standpoint, the solid portion of the old world has gone all to pieces. Wherever there is a chain of mountains it marks a breakage in the earth's crust, and these mountains are not all on the land, but extend under the seas so deeply that they are unable to lift their heads above the surface of the water. The earth is no longer round, except in general outline, but broken up into all sorts of shapes that give the varied conditions of landscape that we find whichever way we turn.

There are but few volcanoes that are active in this age, while in former times they extended for thousands of miles. We still have occasional earthquakes, but undoubtedly they are very slight as compared with those that shook the earth millions of years ago.

If, now, we study the constitution of the earth's crust so far as it has yet been penetrated, we find it divided up into periods called Primary, Secondary, and Tertiary. The primary period reaches down to the line where the lowest forms of animal fossils begin to be found. This is called the "Paleozoic" period, which means the period of "ancient life." From here let us first go downward. Immediately under this lies a stratum of "Metamorphic" rocks. To metamorphose is to change; and metamorphic rocks are those which have been changed by heat or pressure from their original formation. This class of rocks lie on top of what are called "Igneous" rocks, which means that they have been formed by or subjected to heat. All lava-formed rocks are igneous. They are unstratified,—not in layers or strata, but in a formless mass,—and in this they differ from water-formed rocks.

If there is a molten center to the earth these igneous rocks are undoubtedly the offspring of this great internal furnace. The metamorphic rocks were primarily igneous and are changed somewhat in their structure by the lapse of time. For instance, marble is a metamorphic limestone. The difference between common limestone and marble is in its molecular structure—the way in which its smallest particles are put together. They are both carbonates of lime. But the marble is made up of little crystals and will take a polish, while ordinary uncrystallized limestone will not. The igneous rocks are chiefly granite; and granite is formed of orthoclase-feldspar, mica, and quartz. (The word "orthoclase" means straight fracture, and the orthoclase-feldspar has two lines of cleavage at right angles to each other.) This is the ordinary composition of granite, but there are a great many variations, chiefly as to color and proportions of the ingredients named.

The igneous rocks, then, are the lowest of all; then come the metamorphic rocks; and as before stated, on top of metamorphic rock begins the first evidence of life in its lowest form. The Paleozoic (ancient life) or Primary period is made up of a number of subdivisions. The first and oldest division is called the "Silurian" age, which is underlaid by the metamorphic rocks and overlaid by the rocks of the Devonian period. It is called Silurian, from the name of a kind of fish, fossils of which are found in the rocks of this age, which are distinguished for the absence of land-plant fossils and vertebrate animals.

In the Silurian strata are found limestones, slate, flagstones, shales, etc. On top of the Silurian begins the "Devonian" age, in which is found the old red sandstone, as well as limestone and slate; and here begin to be found the fossils of land-plants. On top of the Devonian lies the "Carboniferous" series, which complete the series of the primary period. In the lower part of this stratum is found carboniferous limestone, which is overlaid by a kind of stone called millstone grit, and on top of this lie the true carboniferous strata or coal-bearing measures. In the coal strata are found the first reptile fossils.

On top of the coal measures begins the Secondary period, or "Mesozoic" (middle life). This period is distinguished for the great development of reptiles, and is called the "age of reptiles." In this age occur the first traces of mammals, and birds, and fishes with bony skeletons. Among plants we find here the first evidence of palms. The formation is chiefly chalk, sandstones, clays, limestone, etc. We now come to the last or "Tertiary" period, which brings us to the top earth. This is chiefly formed of sedimentary rocks—those which have been formed by the settling of sediment, in water.

While we are forced to these general conclusions in regard to the building of the world, and to its subsequent distortion by the series of upheavals that have occurred from time to time, and to the successive "ages" of the layers of rock foundation of its crust, there are many mysteries that remain unsolved and many questions will present themselves to the mind of the reader. One of these questions is, Where was the water and where was the earthy matter before its precipitation? Matter, including water, can exist in the gaseous form, and we only need to assume that there was a core of intense heat, to understand how all the material that we find on the earth and in the earth could have been held in suspension in the gaseous state until the cooling process had reached a stage where the various combinations and recombinations could take place in the great laboratory of nature. If we study the constitution of the sun (and with the modern appliances we are able to do so), we find that it is made up of some and perhaps all of the same materials that are found here on earth. If there is no water existing, in the sun, as water, there are the gases present which would produce it if the conditions were right. And, for all we know, that flaming mass of burning gases may some time go through the same kind of cooling and building up in solids that our earth has experienced.

We thus have what may be called an outline sketch of the process of World-building.

CHAPTER II.

LIMESTONE.

A large part of the structure of the earth's crust is formed of a substance called limestone. Ordinary limestone is a compound of common lime and carbon dioxide, a gas that is found mixed with the air to a very small degree. Carbon dioxide will be better known by the older people as carbonic acid. It is a gas that is given off whenever wood and coal are burned, or any substance containing carbon. It is composed of one atom of carbon to two of oxygen. Every ton of coal that is burned sends off three and two-thirds tons of this gas. The increase in weight comes from the fact that every atom of carbon unites with two of oxygen, which it takes from the air, and the oxygen is heavier than the carbon.

In comparing the relative weights of atoms (the smallest combinable particle of a solid, liquid, or gas) we use the hydrogen atom as the unit of comparison and call it "one," because it is the lightest of all atoms. The carbon atom is twelve times heavier than the hydrogen atom, and the oxygen atom is sixteen times heavier. Hence it will be seen readily how a ton of coal will form two and two-thirds times its weight of carbonic dioxide. Lime, having a strong affinity or attraction for this gas, has absorbed it from the air and water, forming what is known as carbonate of lime—which is the ordinary limestone. Chalk and the various marbles are also carbonates of lime. Limestone strata in the crust of the earth are found in all the periods of the earth's formation. All forms of sea shells that were once the homes of animal life are constructed of this compound; and in the later formations of limestone, in the Secondary and Tertiary periods, we find this rock to be made up almost entirely of marine shells, some of them microscopic in size. The earlier or older formations of limestone that are found deeper down in the earth's crust are less mingled with these marine shells. This comes from the fact that the first deposition of limestone strata occurred before the later forms of sea life had developed. Whatever signs of life are found in these lower stratifications are of the very lowest order. It is not to be understood that animal life is a necessary factor in the formation of limestone, but it has been an incidental feature which no doubt has been the chief means of gathering up from the water this compound and precipitating it into the great limestone strata that are everywhere found.

Carbonate of lime is found in solution in nearly, if not quite, all of the mineral waters, and is also found in the water of the ocean. In earlier times it must have been held in solution in much greater quantities than at present. The myriads of sea animals that existed, and that still exist, gathered from the water this substance, which formed their shells, and served as a house in which they lived. New germs were continually forming new shells, while the older ones ceased to live as animals, and their houses in which they lived were precipitated to the bottom of the ocean, where they were bound together as limestone rock. These sea animals no doubt caused a much more rapid formation of limestone than would or could have been the case without their existence.

One can thus readily see what an important factor animal life has been in the process of world-building. This process is still going on, but probably not to the same extent as in former ages, because it is not likely that there is so much carbonate of lime held in solution as there was before these great limestone beds were formed. Limestone, however, is easily disintegrated by the action of water. We find the spring water impregnated with it as well as that of the small streams and rivers. Pure water is a powerful solvent. When the rains fall upon the earth the water percolates through it and through the limestone strata, which gradually wears away the limestone and carries it back to the ocean, so that the process of tearing down and building up is continually going on. The great caves that are found everywhere in the limestone regions were formed by the action of water. The great Mammoth Cave of Kentucky, which is said to have 200 miles of underground passages, has been entirely worn out by the action of running water.

Some years ago the writer visited this cave and had an opportunity to study the wonderful eroding or gnawing-out effect of water on limestone. At some period earlier in the history of the earth there was evidently an underground river or large stream of water that found its way through the crevices of the rocks, and gradually wore out a great bed for itself, which was fed by lateral streams pouring into the main branch, each one of which lateral branches cut its own channel. A plan view of the Mammoth Cave presents a picture not unlike that of a great river with numerous branches emptying into it, all of them showing the windings such as we see in a river and its feeders upon the surface of the earth. There are three sets of these channels, one above the other, and we do not find the water till we get to the bottom of the third underground story, so to speak. There is one place in this system of underground channels where the dripping from the roof of the upper channels has cut a great well hole many feet in diameter perpendicularly down through the whole system to a great depth. The sides of this great well hole are fluted into grooves caused by the constant downflow of the water. Although the amount of water flowing down through this well hole is very small, it is continually at work. Like interest on money, it never rests, each minute that passes has eaten away some of the great rock.

In other portions of the cave the dripping of the water is so gradual that the carbonate of lime hardens and forms what are called stalactites, that hang like icicles from the roof of the cave. Sometimes the water runs down so slowly upon these stalactites that it evaporates as fast as it appears, leaving behind its little load of carbonate of lime. If, however, there is a drip, there are formations built also from the lime in the dropping water on the floor of the cave, and these are called stalagmites. In time the stalactites and the stalagmites will meet, forming a great column reaching from floor to ceiling. Some of these formations, when they are free from foreign substances, are very beautiful. They are also very hard, giving off a metallic musical tone when struck by any hard substance.

We have already stated that limestone is a compound of ordinary lime and carbon dioxide, forming a carbonate of lime. This statement does not give a complete analysis of all the elements entering into limestone. In the first place lime itself is a compound formed of two elementary substances, calcium and oxygen. The lime molecule is composed of one atom of calcium and one of oxygen. Neither calcium nor lime is found pure in nature. Inasmuch as carbon dioxide is composed of one atom of carbon and two of oxygen, and lime is composed of one atom of calcium and one of oxygen, when we have the two combined the molecule of carbonate of lime, or, as it is technically called, calcic carbonate, is composed of one atom of calcium, one of carbon and three of oxygen, (lime plus carbon dioxide).

As before stated, lime is not found un-combined with other substances in nature. And as it is of great economic importance, it will be profitable to know how it is formed. Lime is produced from ordinary limestone by burning it in kilns where it is subjected to a heat of a certain temperature for a number of hours. The heat drives off the carbon dioxide, which, as we have seen, has taken away from each molecule of the compound all of the carbon and two atoms of the oxygen, while all of the calcium is retained with one atom of oxygen, leaving ordinary lime. Lime, then, is simply oxide of calcium.

As all know, it is used almost exclusively for making mortar for building purposes. In order to do this we have to put it through the process of "slacking," by pouring water upon it, and here another chemical change takes place. The water unites with the lime, when immediately the heat that was expended in throwing off the carbon dioxide and was stored in the lime as energy is now given up again in the form of heat. When a considerable bulk of lime is slacked very rapidly the heat that is given off is so great that it will produce combustion. Here is a beautiful illustration of what has been erroneously called "latent heat." It is "heat stored as potential energy," that is released by the combination of lime with water. Slackened lime, then, is called calcic hydrate.

Very little of the limestone that we find is absolutely pure. It is considered good when it does not contain over five or six per cent. of foreign substance. When more than this is present the lime is considered poor, and when it reaches fifteen per cent. or more of impurities it assumes the property of hardening under water and is called cement.

Carbonate of lime is found in several other forms; for instance, the various kinds of marble and chalk are carbonates of lime. The composition of marble and chalk is exactly the same as that of limestone. The difference is chiefly one of molecular rather than chemical structure. Marble is what chemists would call an allotropic or changed form of limestone; and, as before stated, the difference seems to consist in the fact that the marble assumes a crystalline arrangement of its atoms and will therefore take a high polish, which is not true of ordinary limestone. Marble varies greatly in coloring and texture, all of which differences are explainable under the one head of molecular arrangement. Nearly pure carbon exists in three distinct forms—the diamond, graphite, and charcoal. As is the case with marble, these differences in the different forms of carbon are not chemical, but molecular differences. The substances are the same, but their infinitesimal particles are differently arranged.

Carbonate of lime—as it exists in its various forms, as limestone, from which lime and cement are made, and marble, which is such an important element in the arts—is a substance of great importance to man. We have already noted some of the processes that nature uses in gathering up these substances from the ocean by the employment of various forms of animal life. Here is another. Whoever has visited the Bermudas has seen an island wholly formed of what is called coral rock. Coral is a structure produced by a peculiar form of sea animal that gathers up the calcareous or lime-like matter floating in the sea water, and builds a house of it in which to live during the little lifetime that is allotted to him. When he dies his children do not occupy the old home, but build a new one, which is a superstructure planted upon the old one as a foundation. This process of growth sometimes takes the form of a tree or plant, and coral trees grow upon trees and plants upon plants, until a structure is erected having its foundation upon the bottom of the ocean, that finally reaches up until it rises above the surface of the water; and here—after through years the water has brought sea-weed and drift to decay and form soil, and the birds have brought seeds and fertilization, and vegetable life is prospering—another animal called man builds his home upon it. The material that the coral is formed of is substantially the same as that we find in the minute shells of the limestone rocks.

The great chalk cliffs that are found on the coasts of the English channel are the work of a sea animal microscopic in size. At one time it was a question among scientists how these chalk cliffs were formed, but when the microscope was invented this mystery, as well as many others, was solved. The chemical components of chalk are precisely the same as those of limestone. The microscope shows that chalk is almost wholly a product of very small organized shells. The animals who are the architects of the chalk cliffs are called "foraminifera"—bearing shells perforated with little holes. The chief difference between chalk and limestone seems to be in the size of the shells of which they are respectively made up and in the manner of the bonding of these shells together. The shells in a lump of chalk are held much more loosely than those in a lump of limestone. These intrepid workers are still actively changing the structure of the bottoms of seas and oceans, and forming new islands, which in turn become the substructure that supports new life, animal and vegetable. And when we consider the great part performed by these microscopic architects and builders it is not a misnomer to speak of the building of a world.

CHAPTER III.

COAL.

Some time, long ago, some man made the discovery that what we now call coal would burn and produce light and warmth. Who he was or how long ago he lived we do not know, but as all earthly things have a beginning, we know that such a man did live and that the discovery that coal would burn was made. Coal, in the sense that we use the word here, is not mentioned in the Scriptures. According to some authorities, coal was used in England as early as the ninth century. It is recorded that in 1259 King Henry III. granted a privilege to certain parties to mine coal at Newcastle. It is further stated that seven years after this time coal became an article of export. In 1306 coal was so generally used in London that a petition was sent to parliament to have the use of it suppressed on the ground that it was a nuisance. Coal was used in Belgium, however, about 1200. There is a tradition that a blacksmith first used it in Liège as fuel. It was first used for manufacturing purposes about 1713.

Coal is found laid down in great veins, varying in thickness, in various parts of the world in the upper strata of the Paleozoic period. The age in which it was formed is called by geologists the Carboniferous (coal-bearing) age.

Before going on to account for the deposits of coal, let us stop a moment and consider what it is. Chemists tell us that coal is chiefly constructed of carbon, compounded with oxygen, hydrogen, and nitrogen. There are many varieties, but all may be classified under two general headings—bituminous and anthracite. Bituminous coal contains a large amount of a tarry substance, a kind of mineral pitch or bitumen, which burns with a brilliant flame and a black sooty smoke, exceedingly rich in carbon. Anthracite coal is hard and stone-like in its texture, burning with scarcely any flame and no smoke. It produces a fire of intense heat when it is once ignited. There is another form of coal called cannel coal, which is a corruption of "candle coal," so called because a piece of this kind of coal when ignited will burn like a match or pine knot and give light like a candle. This is the richest of all the coal deposits in gases that are set free by heat, and for this reason is extensively used in the manufacture of what is commonly called coal gas. England produces a large amount of cannel coal, as well as another variety of bituminous coal, which latter, however, does not burn with such a black smoke as the coal found in the Ohio valley and the Western States of America. East of the Alleghany Mountains there is a region of anthracite coal that is very extensively worked and finds great favor in all parts of the country as fuel for domestic heating, especially on account of its great cleanliness.

All of the coal beds have a common origin, and the difference in the quality of coal found in different parts of the country is due to many circumstances, some of which have never been explained. There is indisputable proof, however, that all coal beds are of vegetable origin. Geologists tell us that these coal beds were formed during an age before the earth had cooled down to the temperature that it has at the present time—an age when vegetation was forced by the internal heat of the earth instead of having to receive all its warmth from the sun's rays as we do now. Some of our readers are familiar with what is commonly termed a hotbed. A hotbed is made by putting soil on top of substances that will ferment and create heat underneath the soil. This heat from beneath will force vegetation and cause a much larger growth than there will be if left to the sun's rays alone. During the carboniferous age the earth was a great hotbed.

The fossils of trees and plants, as well as reptiles, that we find in the great coal measures of the world, show that they were of large tropical growth, and this is shown not only in the temperate zone, but in the zone farther north. For ages and ages this rank growth of vegetation grew up and fell down until a great layer of vegetable matter was formed, which at a later time was covered over by other stratifications of earth material, so that these great layers of vegetable formation were hermetically sealed and pressed down by an enormous weight that increased as time went on. The formation of coal may be studied even at this day (for it is now going on) by visiting and examining the great peat beds that are found in various parts of the world. It is well known that peat is used as a fuel by many people, especially the peasantry of the old countries. If peat is pressed to a sufficient degree of hardness it burns in a manner not unlike some forms of coal. Peat is a vegetable formation and has been formed by the rank growth of various kinds of vegetation in swampy places. Of course, it lacks the purity of the coal that was formed during the carboniferous age, because of the much slower growth of vegetation now than during that time, and the opportunity that peat bogs offer for an intermixture of earthy with the vegetable matter. The fact that we find the imprint of trees and ferns and other vegetable growth of tropical varieties, as well as the fossils of reptiles, imbedded in the coal measures, proves that at one time this stratum was at the land surface of the earth. We also find that all of the formations of the Secondary and Tertiary periods are on top of the coal—and this shows that after the age of rank vegetable growth there was a sinking of the earth in many places far down into the ocean—so that vast layers of rock formed on top of these beds of vegetable matter. In England great chalk beds crop out in cliffs on the southern coast, and, as we have seen, these chalk rocks are largely made up of the shells of marine animals. London stands on a chalk bed, from six hundred to eight hundred feet thick. Indeed, England has been poetically called Albion, White-land, from this appearance of her coast.

All of the great chalk beds were formed ages after the coal beds, as the latter are found in the upper strata of the Paleozoic period.

A study of these strata will show that there are many layers of coal strata varying in thickness and separated by layers of shale and sandstone. How the shale and sandstone layers are formed will be the subject of a future chapter.

From the position that the coal measures occupy, being entirely under the Secondary and Tertiary formations, it will be observed that they are very old. If we should examine a piece of ordinary bituminous coal we should find that there are lines of cleavage in it parallel to each other, and that it is an easy matter to separate the lump on these lines. If we examine the outcrop of a coal bed we will find that these lines of cleavage are horizontal. This indicates that the great bulk of vegetable matter of which the coal formation is made up has been subjected to tremendous pressure during a long period of time. If we further examine the structure of a body of coal we find the impressions of limbs and branches as well as the leaves of trees and various kinds of plants. We shall further find that these impressions lie in a plant in the same direction as the line of cleavage. This is a point to be remembered, as it helps to explain the nature and structure of other formations than those of coal. Not only are leaves and branches of vegetable matter found, but fossils of reptiles, such as live on the land. Sometimes there is found the fossil of a great tree trunk standing in an erect position, with its roots running down into the rock below the coal bed, while the trunk extends upward entirely through the coal and high up into the other strata. All of these facts lead us to the firm conclusion that when the trees were grown that formed these beds they were above the surface of the ocean. This, taken in connection with the fact that the vegetable fossils that are found indicate a tropical growth of great size, drives us to the conclusion that the climate at the time these coal measures were formed was much warmer than it is now.

As already remarked, this extra warmth came from the earth itself before it had cooled down to its present temperature, rather than from the heat of the sun. There is nothing inconsistent in the thought that the sun may have been warmer in a former age than now. We may conceive that the earliest coal formations took place when the land stood above the surface of the water, and that the conditions were favorable for a rapid and luxuriant growth of vegetation; after this had gone on for a very long period of time, by some convulsion of nature the land surface was submerged under the ocean, when other mineral substances were deposited on top of this layer of vegetable growth, which hardened into a rock formation. At a later period the earth was again elevated above the surface of the water and the same process of growth and decay was repeated. These oscillations of the earth up and down occurred at enormously long intervals, until all of the various coal strata with their intermediate formations were completed. After this we must suppose that the whole was submerged to a great depth and for a very long period of time, because of the great number and various kinds of rock formations laid down by water that lie on top of the coal measures. This tremendous weight, as it was gradually builded up, subjected these vegetable strata to an inconceivable pressure. In some places this pressure was much greater than in others, which undoubtedly is one of the reasons why we find such differences in the structure and quality of coal. There were no doubt many other reasons for differences, one of them being the character of the vegetable growth out of which they were formed. Again, in some parts of the world these coal strata may have been subjected to a considerable degree of heat, which would change the structure of the formation, and in some cases drive off the volatile gases. One can easily imagine that heat was thus a factor in the formation of what is known as anthracite coal, so much less gaseous than the bituminous kinds. The anthracite beds seem to be denser and of a more homogeneous character. The lines of cleavage are not as prominent, but there are the same evidences of vegetable origin that we find in the bituminous formations.

It will be seen from what has gone before that coal was first wood. But wood is a product of sunshine. Thus the sun was the architect and builder of the trees and plants that were finally hermetically sealed under the great earth strata. The sun gathered up the material and set the forces in play which made the chemical combinations of the various elements in nature that enter into vegetable growth.

After the lapse of untold ages of time these great beds of stored-up sun-energy were discovered by man and their contents are dragged out to the earth's surface, to warm our houses, to drive the machinery of our factories, to send the locomotives flying across the continents and the steamships over the oceans. So important has this article become that if any one nation could control the output it would be able to paralyze all the navies and the manufacturing of the world.

If the coal of the world should become exhausted we should be confronted with a great problem. Fortunately for us, this is a problem that will have to be solved by the people of some future age, as the growth of wood will scarcely keep pace with the consumption of fuel. By that time the genius of man will have devised an economical means of storing the energy of the sunbeams directly for purposes of heat, light, and power.

CHAPTER IV.

SLATE AND SHALE.

Slate is one of the great commercial products of the world. As far back as the year 1877 the output of slate was not less than 1,000,000 tons per annum. The chief use to which slate is put is for covering buildings, and for this purpose it is better than any other known material. It is also used in the construction of billiard tables and for writing-slates; these latter uses are very insignificant as compared to its use in architecture. Slate, like building-stone and limestone, is quarried from the earth's crust and is found in the strata close above the Metamorphic rocks, near the beginning of what is called the Primary, or Paleozoic period. As compared with the coal formations it is very, very old.

There are different substances called slate that are not slate in the scientific use of that word. In general all stone formations are called slates that split up into thin layers. But the true slate is a special material which is formed by special processes of nature. The difference between slate and shale, for instance, is not one of ingredients, but of the process by which the ingredients are put together. All of the sedimentary rocks are formed by a deposit of sediment from the water on the bottom of the ocean. At one period the floods have brought down a certain kind of material in greater profusion than at others, and this is deposited in thin layers, and as it hardens there will be seams in it and the stratifications will be differently colored, the color depending upon the deposit at any particular time.

A bed of shale, like a bed of coal, has lines of cleavage in it, and if it is examined under a microscope it will be found that the sedimentary particles, like the twigs and leaves in the coal veins, lie with their longest dimensions in line with the plane of cleavage. Shale in color looks like slate, and an analysis of the material of which it is formed shows that shale and slate are both made from the same. There is, however, a structural difference between the two which is very peculiar and very interesting. The slate is ordinarily a denser material and the lines of cleavage are often at right angles with those that we find in ordinary shale.

A slab of shale will be of a uniform color on any one line of cleavage. The color may change at the next line, and generally does, to a slight extent. It is easy to see, then, if we could change the lines of cleavage in the shale, so as to run at right angles with their present lines, the face of a slab would show bands of different colors or shadings, such as we often see in slate. If you take a piece of clay that has been thoroughly mixed, and subject it to a very great pressure, and then examine the piece that has been submitted to pressure under a microscope and compare it with a piece of the clay after it has been thoroughly mixed, but has not been submitted to pressure, you will find that the two are very different in structure. The pressed clay will show that the particles of which it is made up have all turned, so that their longest dimensions are in a line at right angles with the direction of pressure. Here is an interesting fact that we must remember. And it is in this that we find the reason for the structural difference between shale and slate. The lines of cleavage in shale are not formed necessarily by pressure, but because in the disposition of the material of which it was formed the particles naturally laid themselves down so that their longest dimensions were on a horizontal line.

Ages after, when other rock and other formations had been laid down on top of the bed of deposited mud, the upheavals of the earth have so changed the lines of pressure upon this material and the pressure is so great that a rearrangement of the particles of which the slate is made up has taken place, so that their longest dimensions now are in a direction that crosses the stratifications as originally laid down.

The effect of this is twofold. First, the material is compressed into a denser, closer form, and then, the lines of cleavage are changed, or to express it in more common language, the grain has been changed. So that when it splits up it runs crosswise of the original layers as the water deposited them, and this produces the different shadings so often seen in different slate. Shale splits in line with its layers; slate splits across that line.

Let us go back a moment to our experiment with the lump of clay. If we examined the mixture before submitted to pressure we should find that the oblong particles of which it was made up would stand in all directions, hit or miss, and if we should dry this lump of clay it would have no special lines of cleavage. But the moment we have submitted it to a certain amount of pressure we find that lines of cleavage have been established, and that the particles have been rearranged so that their longest dimensions are all in one direction, which coincides with the cleavage lines. If we should now take this same piece of clay and subject it to a pressure at right angles to that of the first experiment we should find that the lines of cleavage had also changed and that the particles had all been rearranged. Apply the principle to the formation of slate, and we can understand how it happens that what we call the grain runs crosswise of the deposits that were made at different times. It is not a chemical, but purely a mechanical difference. Or, to express it differently—the difference is a structural one produced by mechanical causes.

The origin of cleavage in slate has been the subject of much speculation and investigation, but like many other problems it was solved through the invention and application of the microscope. Thin layers of slate have been made, the same as with limestone and chalk, so thin that the light would readily pass through it and that an examination of the particles could be readily made, showing their arrangement under varied conditions. Science is indebted to the microscope for the solution of very many problems that for ages before had puzzled philosophers.

CHAPTER V.

SALT.

It may seem curious to the reader that we should care to discuss a subject seemingly so simple as common salt. But it is a very usual thing for us to live and move in the presence of things that are very common to our everyday experience, and yet know scarcely anything about them, beyond the fact that they in some way serve our purpose.

Salt is one of the commonest articles used in the preparation of our food. It has been questioned by some people whether salt was a real necessity as an animal food, or whether the taste for it is merely an acquired one. All peoples in all ages seem to have used salt, and reference to it is made in the earliest histories. Travelers tell us that savage tribes, wherever they exist, are as much addicted to the use of salt as civilized people. One of the early African travelers, Mungo Park, tells us that the children of central Africa will suck a piece of rock salt with the same avidity and seeming satisfaction as the ordinary civilized child will a lump of sugar.

All animals seem to require salt, and it is claimed by those who have tried the experiment that after one has refrained from the use of salt for a certain length of time the craving for it becomes exceedingly painful. It is most likely that the taste for salt is a natural craving. In any event, whether it is a natural or an artificial taste, it has become an article of the greatest importance in the preparation of food, as well as on account of its use in the arts. Salt is a compound of chlorine and sodium. In chemical language it is called sodium chloride. The symbol is NaCl, which means that a molecule of salt is composed of one atom of sodium and one of chlorine. Chlorine is an exceedingly poisonous gas.

Formerly the chemist when he wished to obtain sodium extracted it from common salt and discharged the chlorine gas into the air. It was found that in establishments where the manufacture of sodium was conducted on a large scale the destructive properties of the chlorine discharged into the air was such that all vegetation was killed for some distance around the manufactory. This came to be such a nuisance that the manufacturers were either compelled to stop business or in some way take care of the chlorine. This is done at the present day by uniting the chlorine gas with common lime, forming a chloride of lime, which is used for bleaching and purifying purposes.

Salt is found in great quantities as a natural product under the name of rock salt. It is found in some parts of the world in great veins over 100 feet in thickness. In some cases the rock salt is mined, when it has to be purified for commercial purposes. The common mode of obtaining salt, however, is by pumping the solution from these great beds where it is mingled with water—salt water; the water is then evaporated, and when it reaches a certain stage of evaporation the salt crystallizes and falls to the bottom.

Different substances crystallize in different forms. The crystallization of water when it freezes, as we shall see hereafter, arranges its molecules in such a form as to make a lump of ice of given dimensions lighter than the same dimensions of water would be. Salt in crystallizing does not follow the same law; the salt crystal is in the shape of a cube and is denser in its crystalline form than in solution, hence it is heavier and falls to the bottom.

It is said that there is a deposit of rock salt in Galicia, Austria, covering an area of 10,000 square miles. There are also very large deposits in England, the mining of which has become a great industry. There are also great beds of salt in various parts of the United States, notably near Syracuse, N. Y., where large salt deposits were exposed in an old river bed formed in preglacial times. The common mode of preparing salt for domestic purposes is by the process of evaporation from brine that has been pumped from salt wells. The quality of the salt is determined largely by the temperature at the time of evaporating the water from it. Ordinary coarse salt, such as is used for preserving meat or fish, is made at a temperature of about 110 degrees; what is known as common salt is made at a temperature of about 175 degrees; while common fine or table salt is made at a temperature of 220 degrees. Thus it will be seen that the process of granulation with reference to its fineness is determined by the rapidity of evaporation. Salt is one of the principal agents in preserving all kinds of meats against putrefaction. It will also preserve wood against dry rot. Vessel builders make use of this fact to preserve the timbers used in the construction of the vessels.

Salt at the present day is very cheap, but at the beginning of the present century it was worth from $60 to $70 per ton. The methods of decomposing salt to obtain its constituents, which are used in various other compounds, are very simple to-day as compared with the processes that prevailed in the days before the advent of electricity in large volume, such as is produced by the power of Niagara Falls. It is curious to note that a substance so useful and so harmless as common salt should be made out of two such refractory and dangerous elements as chlorine and sodium. Both of these elements, standing by themselves, seem to be out of harmony with nature, but when combined there are few substances that serve a better purpose.

These great salt beds that are found to exist in England and America and other parts of the world were undoubtedly deposited from the water of the ocean at some stage in the formation of the earth's crust. It is well known that sea water is exceedingly saline; 300 gallons of sea water will produce a bushel of salt. Undoubtedly beds of salt are also formed by inland lakes, such as the Great Salt Lake in Utah. Only about 2.7 per cent. of ocean water is salt, while the water of the Great Salt Lake of Utah contains about 17 per cent. When there is so much salt in water that it is called a saturated solution, salt crystals will form and drop to the bottom, which process will in time build up under a large body of salt water a great bed of rock salt.

The water in all rivers and springs contains salt to a certain degree, and where it runs into a basin like that of a lake with no outlet, through the process of evaporation pure water is being constantly carried off, leaving the salt behind. It is easy to see that if this process is kept up long enough the water will become in time a saturated solution, when crystallization sets in and precipitation follows, accounting for the deposits of rock salt.

AIR.

CHAPTER VI.

THE ATMOSPHERE.

Meteorology is a science that at one time included astronomy, but now it is restricted to the weather, seasons, and all phenomena that are manifested in the atmosphere in its relation to heat, electricity, and moisture, as well as the laws that govern the ever-varying conditions of the circumambient air of our globe. The air is made up chiefly of oxygen and nitrogen, in the proportions of about twenty-one parts of oxygen and seventy-nine parts nitrogen by volume, and by weight about twenty-three parts oxygen and seventy-seven of nitrogen. These gases exist in the air as free gases and not chemically combined. The air is simply a mixture of these two gases.

There is a difference between a mixture and a compound. In a mixture there is no chemical change in the molecules of the substances mixed. In a compound there has been a rearrangement of the atoms, new molecules are formed, and a new substance is the result.

About 99-1/2 per cent. of air is oxygen and nitrogen and one-half per cent. is chiefly carbon dioxide. Carbon dioxide is a product of combustion, decay, and animal exhalation. It is poison to the animal, but food for the vegetable. However, the proportion in the air is so small that its baneful influence upon animal life is reduced to a minimum. The nitrogen is an inert, odorless gas, and its use in the air seems to be to dilute it, so that man and animals can breathe it. If all the nitrogen were extracted from the air and only the oxygen left to breathe, all animal life would be stimulated to death in a short time. The presence of the nitrogen prevents too much oxygen from being taken into the system at once. I suppose men and animals might have been so organized that they could breathe pure oxygen without being hurt, but they were not, for some reason, made that way.

Air contains more or less moisture in the form of vapor; this subject, however, will be discussed more fully under the head of evaporation. The air at sea-level weighs fifteen pounds to the square inch, and if the whole envelope of air were homogeneous—the same in character—it would reach only about five miles high. But as it becomes gradually rarefied as we ascend, it probably extends in a very thin state to a height of eighty or ninety miles; at least, at that height we should find a more perfect vacuum than can be produced by artificial means. The weight of all the air on the globe would be 11-2/3 trillion pounds if no deduction had to be made for space filled by mountains and land above sea-level. As it is, the whole bulk weighs something less than the above figures.

As we have said, the air envelopes the globe to a height at sea-level of eighty or ninety miles, gradually thinning out into the ether that fills all interstellar space. We live and move on the bottom of a great ocean of air. The birds fly in it just as the fish swim in the ocean of water. Both are transparent and both have weight. Water in the condensed state is heavier than the air and will seek the lowest places, but when vaporized, as in the process of evaporation, it is lighter than air and floats upward. In the vapor state it is transparent like steam. If you study a steam jet you will notice that for a short distance after it issues from the boiler it is transparent, but soon it condenses into cloud.

If we could see inside of a boiler in which steam had been generated, all the space not occupied with water would seem to be vacant, since steam before it is condensed is as transparent as the air. We will, however, speak of this subject more fully under the head of evaporation and cloud formation. It is not enough that we have the air in which we live and move, with all of its properties, as we have described: something more is needed which is absolutely essential both to animal and vegetable life—and this essential is motion. If the air remained perfectly still with no lateral movement or upward and downward currents of any kind, we should have a perfectly constant condition of things subjected only to such gradual changes as the advancing and receding seasons would produce owing to the change in the angle of the sun's rays. No cloud would ever form, no rain would ever fall, and no wind would ever blow. It is of the highest importance not only that the wind shall blow, but that comparatively sudden changes of temperature take place in the atmosphere, in order that vegetation as well as animal life may exist upon the surface of the globe. The only place where animal life could exist would be in the great bodies of water, and it is even doubtful if water could remain habitable unless there were means provided for constant circulation—motion.

The mobility of the atmosphere is such that the least influence that changes its balance will put it in motion. While we can account in a general way for atmospheric movements, there are many problems relating to the details that are unsolved. We find that even the "weather man" makes mistakes in his prognostications; so true is this that it is never safe to plan a picnic for to-morrow based upon the predictions of to-day. The chief difficulty in the way of solving the great problems relating to the sudden changes in the weather and temperature lies in the fact that two-thirds or more of the earth's surface is covered with water; thus making it impossible to establish stations for observation that would be evenly distributed all over the earth's surface. Enough is known, however, to make the study of meteorology a most wonderfully interesting subject.

We have already stated that air is composed of a mixture of oxygen and nitrogen chiefly, with a small amount of carbon dioxide. So far as the life and health of the animal is concerned we could get along without this latter substance, but it seems to be a necessity in the growth of vegetation. There are other things in the air which, while they are unnecessary for breathing purposes, it will be well for us to understand, as some of them are things to be avoided rather than inhaled.

As before mentioned, air contains moisture, which is a very variable quantity. In a cold day in winter it is not more than one-thousandth part, while in a warm day in summer it may equal one-fortieth of the quantity of air in a given space. There is also a small amount of ammonia, perhaps not over one-sixty-millionth. Oxygen also exists in the air in very small quantities in another form called ozone. One way to produce ozone is by passing an electric spark through air. Anyone who has operated a Holtz machine has noticed a peculiar smell attending the disruptive discharges, which is the odor of ozone. It is what chemists call an allotropic form of oxygen, just as the diamond, graphite, and charcoal are all different forms of carbon, and yet the chemical differences are scarcely traceable. It is more stimulating to breathe than oxygen and is probably produced by lightning discharges.

As has been before stated, the oxygen of the air is consumed by all processes of combustion, and in this we include the breathing of men and animals and the decay of vegetable matter, as well as the more active combustion arising from fires. A grown person consumes something over 400 gallons of oxygen per day, and it is estimated that all the fires on the earth consume in a century as much oxygen as is contained in the air over an area of seventy miles square. All of these processes are throwing into the air carbon dioxide (carbonic acid), which, however, is offset by the power of vegetation to absorb it, where the carbon is retained and forms a part of the woody fiber and pure oxygen is given back into the air. By this process the normal conditions of the air are maintained.

One decimeter (nearly 4 inches) square of green leaves will decompose in one hour seven cubic centimeters of carbon dioxide, if the sun is shining on them; in the shade the same area will absorb about three in the same time.

There is another substance in the form of vegetable germs in the air called bacteria. At one time these were supposed to be low forms of animal life, but it is now determined that they are the lowest forms of vegetable germs. Bacteria is the general or generic name for a large class of germs, many of them disease germs. By analysis of the air in different locations and in different parts of the country it has been determined that on the ocean and on the mountain tops these germs average only one to each cubic yard of air. In the streets of the average city there are 3000 of them to the cubic yard, while in other places where there is sickness, as in a hospital ward, there may be as many as 80,000 to the cubic yard. These facts go to prove what has long been well known, that the air of a city furnishes many more fruitful sources for disease than that of the country. Some forms of bacterial germs are not considered harmful, and they probably perform even a useful service in the economy of nature. Within certain limits, other things being equal, the higher one's dwelling is located above the common level the purer will be the air. This rule, however, has its limits, as the oxygen of the air is heavier than the nitrogen, so that the air at very great altitudes has not the same proportion of oxygen to nitrogen that it has at a lower level. An analysis that was made some years ago of the air on the west shore of Lake Michigan, especially that section where the bluffs are high, shows that it compares favorably with that of any other portion of the United States.

In view of the foregoing, it is of the highest importance to the sanitary condition of any city, town, or village that it be not too compactly built. If more than a certain number of people occupy a given area, it is absolutely impossible to preserve perfect sanitary conditions. And there ought to be a State law, especially for all suburban towns, which are the homes and sleeping places for large numbers of business men who spend their days in the foul air of the city, stipulating that the houses shall be not less than a certain distance apart. Oxygen is the great purifier of the blood, and if one does not get enough of it he suffers even though he breathes no impurities. The power to resist the effects of bad air is much greater when one is awake and active than when asleep, and this is why it is more important to sleep in pure air than to be in it during our waking hours. It is best, however, to be in good air all of the time. By pure air I do not mean pure oxygen, but the right mixture of the two gases that make air. Too much of a good thing is often worse than not enough. Pure food to eat, pure water to drink, and pure air to breathe would soon be the financial ruin of a large class of doctors.

CHAPTER VII.

AIR TEMPERATURE.

The most recent definition of heat is that it is a mode of motion; not movement of a mass of substance, but movement of its ultimate particles. It has been determined by experiment that the ability of any substance to absorb heat depends upon the number of atoms it contains, rather than its bulk or its weight.

It has also been stated that the atmosphere at sea-level weighs about fifteen pounds to the square inch, which means that a column of air one inch square extending from sea-level upward to the extreme limit of the atmosphere weighs fifteen pounds. The density of the air decreases as we ascend. Each successive layer, as we ascend, is more and more expanded, and consequently has a less and less number of air molecules in a given space. Therefore the capacity of the air for holding heat decreases as we go higher.

We deduce from these facts that the higher we go the colder it becomes; and this we find to be the case. Whoever has ascended a high mountain has had no difficulty in determining two things. One is that the air is very much colder than at sea-level, and the other that it is very much lighter in weight. We find it difficult, when we first reach the summit, to take enough of oxygen into our lungs to carry on the natural operations of the bodily functions. To overcome this difficulty, if we remain at this altitude for a considerable time, we shall find that our lungs have expanded, so as to make up in quantity what is lacking in quality.

If a man lives for a long time at an altitude of 10,000 feet he will find that his lungs are so expanded that he experiences some difficulty when he comes down to sea-level. And the reverse is true with one whose lungs are adapted to the conditions we find at sea-level, when he ascends to a higher altitude. There is a constant endeavor on the part of nature to adapt both animal and vegetable life to the surroundings. While no exact formula has been established as to the rate of decrement of temperature as we ascend, we may say that it decreases about one degree in every 300 or 400 feet of ascent. There is no exact way of arriving at this, as in ascending a mountain the temperature will be more or less affected by local conditions. If we go up in a balloon we have to depend upon the barometer as a means of measuring altitude, which, owing to the varying atmospheric conditions, is not a reliable mode of measurement. It is easily understood that a cubic foot of air at sea-level will contain a great many more atoms than a cubic foot of air will at the top of a high mountain; or, to state it in another way, a cubic foot of air at sea-level will occupy much more than a cubic foot of space 10,000 feet higher up. Suppose, then, that the amount of heat held in a cubic foot of air at sea-level remained the same, as related to the number of atoms. In its ascent we shall find that at a high altitude the same number of atoms that were held at sea-level in a cubic foot have been distributed over a so much larger space that the sensible heat is greatly diminished or diluted, so to speak. It was an old notion that heat would hide itself away in fluids under a name called by scientists latent heat. This theory has been exploded, however, by modern investigation.

If we place some substance that will inflame at a low temperature in the bottom of what is called a fire syringe (which is nothing but a cylinder bored out smoothly, with a piston head nicely fitted to it, so that it will be air-tight) and then suddenly condense the air in the syringe by shoving the plunger to the bottom, we can inflame the substance which has been placed in the bottom of the cylinder. In this operation the heat that was distributed through the whole body of air, that was contained in the cylinder before it was compressed, is now condensed into a small space. If we withdraw the plunger immediately, before the heat has been taken up by the walls of the syringe, we shall find the air of the same temperature as before the plunger was thrust down. This, however, does not take into account any heat that was generated by friction.

Let us further illustrate the phenomenon by another experiment. If we suddenly compress a cubic foot of air at ordinary pressure into a cubic inch of space, that cubic inch will be very hot because it contains all the heat that was distributed through the entire cubic foot before the compression took place. Now let it remain compressed until the heat has radiated from it, as it soon will, and the air becomes of the same temperature as the surrounding air. What ought to happen if then we should suddenly allow this cubic inch of air to expand to its normal pressure, when it will occupy a cubic foot of space?

Inasmuch as we allowed the heat to escape from it when in the condensed form, when it expands it will be very cold, because the heat of the cubic inch, now reduced to the normal temperature of the surrounding air, is distributed over a cubic foot of space.

This is precisely what takes place when heated air at the surface of the earth (which is condensed to a certain extent) rises to the higher regions of the atmosphere. There is a gradual expansion as it ascends, and consequently a gradual cooling, because a given amount of heat is being constantly distributed over a greater amount of space. At an altitude of forty-five miles it will have expanded about 25,000 times, which will bring the temperature down to between 200 and 300 degrees below zero.

When we get beyond the limits of the atmosphere we get into the region of absolute cold, because heat is atomic motion, and there can be no atomic motion where there are no atoms.

We have now traced the atmosphere up to the point where it shades off into the ether that is supposed to fill all interplanetary space. As Dryden says:

There fields of light and liquid ether flow,
Purg'd from the pond'rous dregs of earth below.

By interplanetary space we mean all space between the planets not occupied by sensible material. It is the same as interatomic space, or the space between atoms, except in degree, as the same substance that fills interplanetary space also fills interatomic space, so that all the atoms of matter float in it and are held together from flying off into space by the attraction of cohesion. What this ether is, has been the subject of much speculation among philosophers, without, however, arriving at any definite conclusion, further than that it is a substance possessing almost infinite elasticity, and whose ultimate particles, if particles there be, are so small that no sensible substance can be made sufficiently dense to resist it or confine it. It is easy to see that a substance possessing such qualities cannot be weighed or in any way made appreciable to our senses. But from the fact that radiant energy can be transmitted through it, with vibrations amounting to billions per second, we know that it must be a substance with elastic qualities that approach the infinite. Assuming that the ether is a substance, the question arises how is it related to other forms of substance? This is a question more easily asked than answered. The longer one dwells upon the subject, however, the more one is impressed with the thought that after all the ether may be the one element out of which all other elements come.

Chemistry tells us that there are between sixty and seventy ultimate elements. This is true at least as a basis for chemical science. Chemical analysis has never been able to make gold anything but gold, or oxygen anything but oxygen, and so on through the whole catalogue of elements. It may be, however, that the play of forces under and beyond those that seem to be active in all chemical processes and relations, are able to produce certain affections of the ether, the result of which in the one case is an atom of gold and in the other an atom of oxygen, etc., to the end of the list. In this case all of the so-called elements may have their origin in one fundamental element that we call the ether. I am aware that we are wading in deep water here, but sometimes we love to get into deep water just to try our swimming powers. The above is a suggestion of a theory called "the vortex theory," that is taking root in the minds of many philosophers to-day, and yet there is almost nothing of known facts to base such a theory upon, and nearly all we can say about it is that it seems plausible, when viewed through the eye of imagination.

We do know that substances, such as fluids or gases, assume very different qualities when put into different rates of motion. A straw has been known to penetrate the body of a tree endwise by the extreme velocity imparted to it when carried in the vortex of a tornado. Instances of the terrific solid power of substances that are mobile when at rest are often exhibited during the progress of a tornado, especially when confined in very narrow limits. Sometimes a tornado cloud will form a hanging cone, running down to a sharp point at the lower end, which lower end may drag on the ground, or it may float a little distance above the ground, but more frequently it moves forward with a bounding motion, now touching the earth and now rising in the air. This cone is revolving at a terrific speed. The substance revolving is chiefly air, carrying other light substances that it has gathered up from the ground. If it comes in contact with a tree or building it cuts its way through as though it were a buzzsaw revolving at a high rate of speed. This is not simply the force of wind, but a kind of solidity given to the fluent air by its whirling motion.

I remember a case in Iowa, where one of these revolving cones passed through a barnyard, striking the corner of the barn, cutting it off as smoothly as though done with some sharp-edged tool, but it in no other way affected the rest of the building. One would suppose that the centrifugal force developed in this whirling motion would cause the cone to fly apart, and why it does not no one certainly knows. But we are obliged to accept the fact.

These cases are cited to show that motion gives rigidity to substances that in the quiescent state are mobile or easily moved, like the straw or the air. If we should assume that there are infinitesimal vortices or whirling rings in the ether, of such rapidity as to give it different degrees of rigidity, we can get a glimmering idea of how an atom of matter may be formed from ether.

Referring to the rigidity which motion gives to ordinary matter, it is well known that when two vessels at sea collide the one having the higher speed is not so liable to injury as the one with the lower. The reader will perhaps remember a circumstance said to have occurred a few years ago on the Lake Shore Railroad, between Buffalo and Cleveland. The limited express was going west, and while rounding a curve the engineer suddenly came in sight of a wrecked freight train, a part of which was lying on the track where the express train had to pass. The engineer saw that he was too near the wreck to stop his train and that the only way to save his own train and the lives of his passengers would be to cut through the wreck. He pulled out the throttle and put on a full head of steam, and when the train struck the wreck it was going at such a high rate of speed that it cut through without seriously damaging the train and without harm to the passengers.

There are other heroes beside those who lead armies in battle.

CHAPTER VIII.

CLOUD-FORMATION—EVAPORATION.

Water exists in different forms without, however, undergoing any chemical change. It is when condensed into the fluid state that we call it "water," and then it is heavier than the atmospheric air and therefore seeks the low places upon the earth's surface, the lowest of which is the bed of the ocean. Wherever there is water or moisture on the face of the globe there is a process going on at the surface called evaporation. This process is much more rapid under the action of heat than when it is colder. In other words, as the heat increases evaporation increases within certain limits and bears some sort of a ratio to it. Evaporation is not confined to water, but as our subject has to deal with atmospheric phenomena we will speak of it only in its relation to aqueous moisture.

The heat that is imparted to the earth's surface by the rays of the sun is able to separate water into minute particles, which, when so separated, form what is called vapor, which is transparent, as well as much lighter than the air at the surface of the earth. Being lighter than the air, it rises when disengaged and floats to the upper regions of the atmosphere. The atmosphere will contain a certain amount of these transparent globules of moisture in the spaces between its own molecules. If the air is warm the molecules will be farther apart and it will contain more moisture than when it is cold.

The process of evaporation is one of the most important in the catalogue of nature's dynamics. Without it there would be no verdure on the hills, no trees on the plains, no fields of waving grain, and no animal life upon the land surface of the globe. Evaporation is nature's method of irrigation, and the system is inaugurated on a grand scale, so that there are but few neglected spots upon the face of the earth which moisture, carried up from the great reservoirs of water, does not reach. The rate of evaporation, other things being equal, depends upon the extent of surface; therefore a smooth surface like that of the lake or ocean will not send up as much vapor from a given area in square miles as an equal area of land will do, when it is saturated with moisture, for the reason that there is a much larger evaporating surface on a square mile of land, owing to its inequalities, than upon an equal area of smooth water. Of course, if the earth is dry there can be but little evaporation. One of the effects of evaporation is to withdraw heat, and so to produce cold in the substance from which the evaporation takes place.

If we put water into a vial and drop regularly upon it some fluid that evaporates readily it will extract the heat from the vial and the water in it to such an extent that in a short time the water will be frozen. In hot countries ice is manufactured on a large scale upon the principle that we have just described. Water is put into shallow basins, excavated in the earth, over which is placed some substance like straw that readily radiates heat, and on the straw are placed porous bricks, that are kept wet, thus furnishing a very large evaporating surface. In this way the process of evaporation is carried on very rapidly and the heat is extracted from the water to such an extent that it freezes, often forming ice in one night over an inch in thickness, and this in the hottest climates on the globe. Evaporation cannot go on in places where the air is already saturated with moisture. When the air is dry evaporation is very rapid, but as it becomes more and more filled with moisture the evaporation is checked to the same degree. This fact accounts for the difference of bodily comfort that we experience at different times in the year when the temperature is the same. Sometimes we are very uncomfortable although the temperature is not above 75 degrees Fahrenheit, more so even than we are at other times when the temperature is ten or fifteen degrees higher. If the air is saturated with moisture, even though the temperature is not above 70 or 75 degrees, the perspiration is not readily evaporated from the surface of the body. If the air is dry the temperature may be much higher and we be much more comfortable, because evaporation goes on rapidly, which keeps the body not only dry, but cool. I remember passing through a desert in Arizona where there was scarcely a green thing in sight in any direction, and the temperature was said to be 140 degrees. I did not suffer as much as I often have done in the East with the thermometer at 80 or 90 degrees, and there was very little show of sensible perspiration; it was going on rapidly, however, but was being absorbed by the dry air. This goes to show that temperature is not the only factor to be considered when we are making an estimate of the good or bad qualities of a climate.

Evaporation is carried on much more rapidly when the wind blows than at other times, for the reason that the moisture is carried off laterally as fast as it is formed, all resistance to its escape into the upper air being removed. If the air is charged to saturation with moisture at a certain temperature, it will remain so, and evaporation stops so long as the temperature remains unchanged. If its temperature rises the process of evaporation can start up, because the capacity of the air for holding moisture has been increased. But if a temperature is perceptibly lowered another phenomenon will manifest itself.

In the uncondensed state vaporized moisture is quite transparent, so that we are able to see through it as we do through a pane of glass. If, however, the body of air that is saturated with this invisible moisture becomes suddenly chilled, the moisture condenses into cloud or mist.

If we watch a passing railroad train we shall notice a mass of fleecy white mist floating away from the smokestack, assuming the billowy forms of some of the clouds in summer. This cloud is produced by the sudden condensation of steam, which was transparent before it came in contact with the cold, outside air, the effect being much more pronounced in cold than in warm weather. We may liken these floating globules of mist to the dust of the earth which floats in the air, and it has not been inaptly called water-dust. Anyone who has seen an atomizer used or has stood at the foot of a great waterfall, like Niagara, has seen the fluid so finely divided that it will float in the air, instead of falling to the ground. What takes place is that a number of these transparent atoms of moisture that are released in the process of evaporation coalesce into one small drop or particle of water, and they will continue to float in the air as mist or cloud until a sufficient number have combined into one solid mass to render that mass heavier than the air, when it falls in the form of rain.

If we live in a region—and there are such on the face of the earth—where there is very little evaporation and consequently very little moisture in the air, there is rarely ever a cloud seen nor is there any rainfall, for the reason that there is no material existing out of which to form clouds, and the clouds precede the rain. Hence, all the artificial attempts to produce rain in these arid regions have been futile. If a body of warm air, when saturated with invisible moisture, is suddenly chilled by coming in contact with a cold wave, it is squeezed like a sponge, so to speak, and the invisible particles become visible because a number of them have coalesced as one particle; the particles gather in a large mass, and we have the phenomenon of cloud formation.

Clouds more generally form in the upper regions of the atmosphere because it is normally colder in the higher regions. In some cases clouds float very high in the air and in others very low. This is due to two causes:

If we should send up a balloon containing air rarefied to a certain extent it would continue to ascend only until it reached a point where the outside air and that contained in the balloon are of the same density. If we should send up this same balloon on different days with the same rarefaction of internal air we should find that on some days it would float higher than others, because the density of the air is constantly fluctuating, as is indicated by the rise and fall of the barometer. Now let us consider the balloon as a globule of moisture of a definite weight, and this globule only one of an aggregation of globules sufficient to form a cloud. We can readily see from what has gone before that a cloud thus formed, having a definite density and weight, would float higher some days than others.

Assuming again that the density of the air remains the same from day to day, the clouds will still float high or low in the atmosphere from another cause. Let us go back to our illustration of the balloon. If we have a fixed condition of atmosphere, external to the balloon, and vary the conditions internally, which means varying its weight, the balloon will float higher or lower as the internal conditions are varied. Now apply this principle to the moisture globules of which a cloud is formed and we can understand why a cloud will float high or low from the two causes that we have described. Clouds are of different color and density, and this is due to the differences of the make-up of the moisture globules of which the clouds are formed. If these globules are in an advanced stage of condensation the cloud is darker and more opaque. In earlier conditions of condensation the cloud will have a bright look, which shows that it reflects most of the light, whereas in the case of the dark cloud the light is largely absorbed.

There is a sort of notion prevailing that clouds come up from the horizon, and in many cases they do, but they may form directly over our heads. There always has to be a beginning, and that occurs wherever the conditions are most favorable for condensation of vapor. If the earth is wet and the sun is hot the evaporation may be very rapid as well as the ascent of the invisible moisture, which carries with it the air, which in turn expands the higher it rises, thus producing cold. This, taken with the normal cold that exists in the higher regions, may be sufficient to produce a sudden condensation of this ascending vapor, which is all that is necessary to form a cloud.

The inquiry may arise, Why is the moisture condensed, almost always, in the upper regions of the air, where it is rare? Because the more rare and therefore expanded it is, the more moisture it will hold. This, taken with the fact that cold currents are encountered high up, sufficiently answers the question.

It is interesting to know that the processes of nature are interdependent. It is not enough that we have the evaporation of moisture that will ascend into the higher regions of the air and there be condensed into cloud and possibly rain, but we must have the means for distributing these conditions over a large area, and for this purpose we have the phenomenon of wind. Why the winds blow can be accounted for to a certain extent,—we might say to a large extent,—but there yet remain many unsolved problems relating to wind and weather. Of the phenomena of wind we will speak more fully in a future chapter.

CHAPTER IX.

CLOUD FORMATION—CONTINUED.

As water in its condensed state is 815 times heavier than air, the question naturally comes to one why it does not immediately fall to the earth when it condenses. There are at least two and probably more stages of condensation. Investigators into the phenomenon of cloud formation claim to have ascertained that the first effect of condensation is to form little globes of moisture that are hollow, like a bubble, with very thin walls. Everyone has recognized the ease with which a soap bubble will float in the air, and yet it is simply a film of moisture. These little balloons, so to speak, are called spherules. It is undoubtedly the case that mingled with these little bubbles of moisture there are fine particles of solid water hanging on and carried along with them. Undoubtedly this is true; at least just before the final act of condensation takes place; and when the little hollow spherules collapse they are gathered together in drops of water larger or smaller according to the rapidity of condensation. There is probably another power at work to prevent the too ready precipitation of moisture when condensed, and that is the wind. A cloud never stands still, although in some cases it may appear to do so. If we take a stone in our hand and allow it to drop without applying any force to it, it will fall directly to the ground. But if we give it an impetus in a horizontal direction it will travel some distance before striking the ground. If we could give the same impetus to a body as light as a globule of water-dust it would probably travel indefinitely without falling. Dust that would settle directly to the ground from an elevation in still air would travel thousands of miles without falling, before a wind having any considerable velocity.

Suppose the sun to be shining with intense heat upon a certain area of the earth's surface and the conditions to be right for very rapid evaporation of moisture. The air which is heated close to the ground, being expanded, will rise, together with the invisible particles of moisture, and there will be a column of moisture-laden air continually ascending until it reaches a point in the upper atmosphere where it is condensed into a cloud that takes on the billowy form which in summer time we call a thunder cloud, but which in the science of meteorology is called cumulus, or heap-cloud. If there were no air currents this billowy cloud would stand as the capping of an invisible pillar of ascending vapor, but as it is never the case that air is not moving at some velocity in the upper regions, it floats away as rapidly as it is formed. This peculiar kind of cloud is formed in the mid-regions of the atmosphere, and it is a summer cloud as well as a land cloud. Of course, it may float off over the ocean and maintain its peculiar shape for a certain distance, but it is rare that such a cloud would ever be seen in mid-ocean or in midwinter. As the warm season advances in summer, and evaporation from the earth is less than the rainfall, there is less and less moisture in the air, when, of course, the conditions for cloud formation, especially inland, are not so favorable as in the early spring or summer. Frequently there comes a time when we have a long season of dry, settled weather. Probably during most of the days clouds will form and we think it is going to rain, but before night they have vanished, and the same thing is repeated the next day and the next, perhaps for weeks at a time.

The explanation is this: We have already said that so long as the air remains in a uniform condition as to temperature it will absorb moisture in a transparent state until it is filled to the measure of its capacity at a given temperature. If there were no change of temperature, it would not condense into cloud. Clouds may be absorbed into the atmosphere—or evaporated—and become invisible; and this process is going on to a greater or less degree continually. If we watch the steam as it escapes from a steam boiler, the first effect is condensation into cloud, but as it floats away it gradually melts and is absorbed into the atmosphere as invisible vapor. This is especially true on a warm day; the same process takes place in the air that is going on at the level of a body of water or at the surface of moist earth.

As before stated, condensation always takes place when a body of moisture-laden air comes in contact with cold. When the steam escapes from a boiler, even on the hottest day, it is hotter than the surrounding air; the first effect is condensation, and then evaporation takes place the same as it would at the surface of the earth when the condensed particles of moisture are separated into the invisible atoms that accompany evaporation.

In settled, dry weather as the sun approaches the zenith, the earth becomes intensely heated, and there is an ascending column of air partly laden with moisture; but not to the same extent as earlier in the season. Condensation takes place and clouds are formed, but as there is not sufficient moisture to carry them to the point of a further condensation,—which would result in precipitation,—as the sun lowers in the west and the heated air becomes more evenly distributed this condensed vapor is reabsorbed into the air as invisible moisture by a process allied to that of evaporation. This condition of things would extend to a much longer period than it does in our latitude if it were not for the gradual changing of the seasons, which finally destroys the balance in the dynamics of cloud-land and allows the cold—that has been held back for the time—in the great northern zone to get the upper hand. Then we have what is termed in common parlance a change in the weather, or, more properly in this case, a change in the season.

We have already spoken of the cloud called cumulus (which means heap) and of its performance during the dry season of summer. There is another form of cloud that is seen at this season of the year called cirrus (a curl). It takes the form of a curl at its ends. This cloud usually has a threaded shape and sometimes takes the form of a feather, and frequently forms are seen that remind you of frost pictures on a window pane. These clouds float very high in the atmosphere, away above the tops of the highest mountains, from six to eight miles above the level of the sea. They are formed only at a season of the year when the atmospheric conditions are most uniform. At certain times of the day and night the moisture will rise to this height before it condenses and when it does condense it immediately freezes, which makes it take on these peculiar forms that would no doubt conform very closely to the frost pictures on the window pane if it were not for the disturbing influences of air currents at this altitude. The fact that they are ice or frost clouds instead of water clouds gives them that peculiar whiteness and brightness of appearance. If ordinary clouds are water-dust these high clouds may be called ice-dust. Sometimes we see them lying in bands or threads running across the sky in the direction that the wind blows. Their form is undoubtedly a resultant of the struggle between the air currents and the tendency of crystallized water to arrange itself in certain definite lines or forms. This cloud may be said to be one extreme, having its home in the highest regions of cloud-land, while the cumulus, or thunder cloud, is the other extreme and occupies the lower or mid regions of the air.

There is a still lower cloud of course, as ordinary fog is nothing more than cloud, which under certain conditions lies on the surface of the ocean or dry land. Fogs prevail when the barometer is low. As soon as it rises from the source of evaporation the moisture condenses almost to the point of precipitation. There is not enough buoyancy in its globules when the air is light, as it is when we have a low barometer, to cause the fog to float into the higher regions of the atmosphere.

The high clouds, which are called cirrus, under certain conditions drop down to where they begin to melt into ordinary moisture globules, and while this process is going on we have a combined cloud effect which is called cirro-stratus. This form of cloud may be recognized, when looking off toward the horizon, by its being formed into long straight bands. It is sometimes called thread-cloud. As it further descends it takes on a different form called the cirro-cumulus, or curl-heap. This is just the opposite in its appearance to the cirro-stratus, as it is broken up into flocks of little clouds separated from each other and in the act of changing to the form of the cumulus, or billowy form of cloud; and this latter takes place when it drops to a still lower stratum of warmer air and is there called the cumulo-stratus, which is the form of cloud we most often see in the season of thunderstorms. The lower edge of the cloud is straight, parallel with the horizon, while the upper part is made up of great billowy masses, having high lights upon their well defined projections and blending into darker shades caused by the shadows in the valleys between the mountains of cloud.

The rain cloud is called the nimbus, and may be said to be the extension of a cumulo-stratus. When it reaches this condition it is condensed to a point where the vesicular globules collapse and a number of them run together, forming a solid drop of water, and here it begins to fall. It may be very small at first, but in its fall other condensed globules will adhere to it and if the conditions are right, sometimes the rain drops will have the diameter of a quarter of an inch by the time they reach the earth.

Under other conditions, such as we have sometimes during dry weather, the rain drops will start to fall, but instead of growing larger, they grow smaller by absorption into the thirsty air, and will not be allowed to reach the earth. Often there are showers of rain in the air that fall to a certain distance and are taken up, as in the process of evaporation, to again be formed into cloud, without ever having touched the earth.

Thus it will be seen that clouds assume various forms under various conditions of atmosphere, as it is related to moisture, temperature, and density. Clouds sometimes appear to be stationary when they are only continually forming on one side and continually being absorbed into invisible moisture on the other. I remember seeing some wonderfully beautiful cloud effects in the regions of the Alps. Almost every day in summer there appears above the peak of Mount Blanc a beautifully formed cloud cap standing some distance above it and hollowed out underneath like an inverted cup. Although this cloud appears to be stationary, it is undergoing a rapid change; the moisture rises from the snow-capped peak as invisible vapor to a certain distance, where it is condensed into a cloud of wonderful brilliancy. As the cloud globules float upward they are absorbed into the atmosphere again, as invisible moisture at the upper limit of the cloud. If the wind happens to be blowing, another phenomenon takes place, giving the appearance somewhat of a volcano. It is blown off from the peak in the direction of the wind, but within a short distance it strikes a warmer stratum of air, where it is absorbed and assumes the transparent condition.

If we ascend a high mountain, we get some idea of the altitude of the various forms of cloud. A thunderstorm may be in progress far below us, while the sun may be shining from a clear sky above, with perhaps the exception of the frost clouds that we have referred to floating high above the mountain tops.

We have now described in a general way how clouds are formed, how they are condensed into rain, and how moisture is distributed over large areas by these rain clouds being borne on the wings of the wind; and now you ask, Whence the wind? In our next and following chapters we will try to answer this question.

CHAPTER X.

WIND—WHY IT BLOWS.

We have said that globules of moisture, released by the action of the sun's rays in the process of evaporation, tend to rise because they are lighter than the air. Right here let it be said that all material substances have weight; even hydrogen, the lightest known gas, has weight, and is attracted by gravitation. If there were no air or other gaseous substances on the face of the earth except hydrogen, it would be attracted to and envelop the earth the same as the air now does. Carbon dioxide is a gas that is heavier than the air. If we take a vessel filled with this gas and pour it into another vessel it will sink to the bottom and displace the air contained in it until the air is all driven out. If we fill a jar with water up to a certain height and then pour a pint of shot into it the water will be caused to rise in the vessel because it has been displaced at the bottom by the heavier material. Now if we remove the shot the water will recede to the level maintained before the shot was put in. On the contrary, if we should pour an equal bulk of cork or pith balls into the jar the water would not be displaced, because the balls are lighter than the water and would lie on top of it; if, however, the water is removed from the jar, the cork will immediately go to the bottom of the jar, because the cork is heavier than the air which has taken the place of the water. We wish to impress upon the mind of the reader the fact, that all substances of a fluidic nature, whether in the fluid or gaseous state, have weight, and obey the laws of gravitation, and the heavier portions will always seek the lower levels, and in doing this will displace the lighter portions, causing them to rise. There is no tendency in any substance to rise of itself, but the lighter substance rises because it is forced to do so by the heavier, which displaces it. This law lies at the bottom of all of the phenomena of air currents.

If we are at certain points on the seashore in the summer time we may notice that about 9 o'clock in the morning a breeze will spring up from the ocean and blow toward the land; this will increase in intensity until about 2 o'clock in the afternoon, when it has reached its maximum velocity, and from this time it gradually diminishes, until in the evening there will be a season of calm, the same as there was in the early morning. The explanation of this peculiar action of the air is found in the fact that during the day the land is heated much more rapidly on its surface than the water is.

The radiant energy from the sun is suddenly arrested at the surface of the earth, which is heated to only a very shallow depth, while in the water it is different; being transparent it is penetrated by the radiant energy to a much greater depth and does not suddenly arrest it, as is the case on land. As the sun rises and the rays strike in a more and more vertical direction the earth becomes rapidly and intensely heated at its surface, and this in turn heats the stratum of air next above it, which is pressing on it with a force of fifteen pounds to the square inch at sea-level. When air is heated it expands, and as it expands it grows lighter. The stratum lying upon the earth as soon as it becomes heated moves upward and its place is occupied by the heavier, cooler air that flows in from the sides. We can now see that if there is a strong ascending current of air on the land near the ocean the cooler air from the surface of the ocean will flow in to take the place of the warmer and lighter air that is driven upward, really by the force of gravity which causes the heavier fluid to keep the lowest level. As the earth grows hotter this movement is more and more rapid, which causes the flow of colder air to be quickened, and hence the increasing force of the wind as the sun mounts higher in the heavens. But when it has passed the point of maximum heating intensity and the earth begins to cool by radiation, the movements of air currents begin to slow up, until along in the evening a point is reached where the surface of the earth and that of the ocean are of equal temperature, and there is no longer any cause for change of position in the air.

The earth heats up quickly, and it also cools quickly, especially if there is green grass and vegetation. While they are poor conductors of heat, they are excellent radiators, so that when the sun's rays are no longer active the earth cools down rapidly and soon passes the point where there is an equilibrium between the land and water. The water possesses the opposite quality. It is slow to become heated, because of a much larger mass that is affected, and is equally slow to give up the heat. And the consequence is that after the sun has set, the land cools so much faster than the water that we soon have the opposite condition, and the sea is warmer than the land, which makes the air at that point lighter, and which in turn causes the denser or colder air from the land to flow toward the ocean, and displace the lighter air and force it upward; hence we have a land instead of a sea breeze. So that the normal condition in summer is that of a breeze from the ocean toward the land during part of the day and a corresponding breeze from the land to the ocean during part of the night, with a period of no wind during the morning and evening of each day.

The forces that work to produce all the varying phenomena of air currents on different portions of the earth are difficult to explain, as there are so many local conditions of heat and cold, and these are modified by the advancing and receding seasons. The unequal distribution of land and water upon the earth's surface; the readiness with which some portions absorb and radiate heat as compared with others; the tall ranges of mountains, many of them snow-capped; the lowlands adjacent to them that become intensely heated under the sun's rays; the diversity of coastline and the fact that there is a zone of continually heated earth and water in the tropical regions—all these conditions, coupled with the fact that the earth rotates on its axis once in twenty-four hours, are certainly sufficient to account for all the complicated phenomena of aërial changes on the various portions of the earth's surface.

The trade winds are so called because they blow in a certain definite direction during certain seasons of the year, and can be reckoned upon for the use of commerce. If you trace the line of the equator you will notice that for more than three-quarters of the distance it passes through the water. The water, as we have explained in the last chapter, becomes gradually heated to a considerable depth, and when once saturated with heat is slow to give it up. It can easily be seen that there will be a zone extending each way from the equator for a certain distance that will become more intensely heated than any other parts of the earth, with the exception of certain circumscribed portions of the land. The result is that this heated equatorial zone is constantly sending up warm air caused by the inrush of colder air, which is heavier than the air at the equator, expanded by the heat. The warm air at the equator is forced up into the higher regions of the atmosphere, and here it overflows each way, north and south, causing a current of air in the upper regions counter to that of the lower. As it travels north and south it gradually drops as it becomes cooler, and finally at some point north and south its course is changed and it flows in again toward the equator. As a matter of fact, the trade winds do not flow apparently from the north and south directly toward the equator, but in an oblique direction. On the north side of the equator we have a northeasterly wind, and a southeasterly wind on the south side. This is caused by the rotation of the earth from west to east. The direction of the trade wind, however, is more apparent than real.

The earth in its diurnal revolutions travels at the rate of a little more than 1000 miles an hour at the equator. But if we should travel northward to within four miles, say, of the north pole, the surface point would be moving at the rate of only about a mile an hour. At some point equidistant between the north pole and the equator the surface of the earth will be moving at a rate, say, of 500 miles an hour. If we could fire a projectile from this point that would have a carrying power to take it to the equator some time after the projectile was fired, although it would fly in a perfectly direct line, it would appear to anyone at the equator who observed its approach to be moving from a northeasterly direction. The reason is that the earth is traveling twice as fast at the equator as it is at the point whence the projectile is fired. Therefore it will overshoot, so to speak, at the equator, and not be dragged around by the increased motion we find there.

To make this still plainer, suppose the earth to be standing still and a projectile be fired directly across from the north pole in the direction of the lines of longitude and required one hour to reach the equator, the projectile would appear to anyone standing at the equator to come directly from the north. If, however, the earth is revolving at the rate of 1000 miles an hour at the equator to the eastward, and the projectile was fired from the pole, where there is practically no motion, in the same direction along the longitudinal lines as before, the observer would have to be in a position on the equator 1000 miles west of this longitudinal line in order to see the projectile when it arrived; therefore the apparent movement of the projectile would not be along the line at the instant that it was fired, but along a line that would cross the equator at a point 1000 miles west. When a southward impulse is given to the air it follows, to some extent, the same law, so that to one standing on the equator the northern trade wind will blow from the northeast and the southern trade wind from the southeast.

Owing to the fact that the air rises in the heated zone there is always a region of calms at this point where there is no wind and no rain. There are two other regions of calms in the ocean, one at the north at the tropic of Cancer and another at the south near the tropic of Capricorn. As has been stated, there are currents flowing back in the upper regions at the equator north and south, and these are called the upper trades—the lower currents being called the lower trades. These upper trades gradually fall till they reach the tropic of Cancer on the north, where the lower part of the current stops and bends back toward the equator, now becoming a part of the lower trade wind. This causes a calm at that point where it turns. The upper parts of this current continue on, in a northerly and southerly direction, on the surface until they meet with the cold air of the north and south polar regions, where there is a conflict of the elements—as there always is when cold and warm currents meet.

The only point where the trade wind has free play is in the South Indian Ocean, and this is called the "heart of the trades."

If the whole globe were covered with water there would be a more constant condition of temperature; but owing to the great difference between the land and water, both as to altitude and the ability to absorb and radiate heat, we have all of these varied and complicated conditions of wind and weather. The trade winds shift from north to south and vice versa with the advancing and receding seasons, due to the fact that the earth has a compound motion. It not only revolves on its axis once in twenty-four hours, but it rocks back and forth once a year, which is gradually changing the direction of its axis; and in addition to these motions it is traveling around the sun as well.

CHAPTER XI.

WIND—CONTINUED.

In our last chapter we discussed the winds that prevail in the regions of the tropics called trade winds, because they follow a direct course through the year, with the exceptions noted in regard to their shifting to the north or south with the changing seasons; we also described the phenomena of land and sea breezes, which during certain seasons of the year reverse their direction twice daily. We will now describe another kind of wind, called monsoons, that prevail in India.

India lies directly north of the great Indian Ocean, and the lower part of it comes within the tropical belt lying south of the Tropic of Cancer. During the summer season here the earth stores more heat during the day than it radiates or loses during the night. This causes the wind to blow in a northerly direction from the sea both day and night for six months each year, from April to October. During these months the land is continually heated day and night to a higher temperature than the water in the ocean south of it. The winds are probably not so severe during the night as through the day, as the difference between the temperature of the land and the water will not be so great during the night; and difference of temperature between two points usually means a proportional difference in the velocity of the wind. There is a time in the fall and spring, while there is a struggle between the temperature of the land and water for supremacy, when the winds are variable, attended with local storms somewhat as we have them in the temperate zone. But after the sun has moved south to a sufficient extent the land of India loses more heat at night than is stored up in the day; hence the conditions during the winter months are reversed, the water is constantly warmer than the land, and there is a constant wind blowing from the land to the ocean, which continues until April, when after a season of local storms the conditions are established in the opposite direction. These winds are called "monsoons."

The word monsoon is probably derived from an Arabic word meaning "seasons." It is a peculiarity of this monsoon that in summer it blows in a northeasterly direction from the sea and in the winter in a southwesterly direction from the land. This divergence from a direct north and south is caused by the rotation of the earth and the explanation is the same as that we have given for the trade winds.

In the southern latitudes there is a comparatively constant condition of wind and weather, because the surface of the globe in these regions is mostly water; but in the north, where most of the land surface is located, we have a very different and a very complicated set of conditions, as compared with the southern zones.

The freaks of wind and weather that we find prevailing upon the North American continent are not so easily accounted for as the phenomena heretofore discussed. In the northern part the land reaches far up toward the north pole, while on the west lies the Pacific Ocean, which merges into the Arctic Ocean at Bering Strait. The climate of the western coast is affected by a warm ocean current that sets up as far north as Alaska, while high ranges of mountains prevent the effects of this warm current from being felt inland to any great extent; all of which helps to complicate any theory that may be advanced regarding changes of weather. Aside from the changes of temperature that are due to the seasons, which are caused by the oscillating motion of the earth between the limits of the Tropic of Cancer on the north and the Tropic of Capricorn on the south, there are other changes constantly taking place in all seasons of the year. While it is not difficult to account for the change of seasons and the gradual change of temperature that would naturally follow—owing to the difference of angle at which the sun's rays strike the earth—it is more difficult to account for the violent changes that occur several times during the progress of a season, as well as the less violent ones that come every few days. In fact, it rarely happens that the temperature is exactly the same on any two successive days during the year. The diurnal changes are easily accounted for by the rotation of the earth on its axis each day. But there is another class of phenomena with which the "weather man" has to struggle when he is making up a forecast of the weather from day to day.

In order that we may proceed intelligently, let us say a word about the barometer. We speak of high and low barometer, and we make the instrument with graduations marked for all kinds of weather, which really mean but very little. The reading of a single barometer alone will give us but a faint idea of what is really going to happen from day to day. But if we have a series of barometers located at different stations scattered all over the continent and connected at headquarters by telegraph, so that we can have the readings from a whole series of barometers at once, then it becomes a very useful instrument. A barometer may read low at one station by the scale, but may be high with reference to some other barometer that reads very low.

What is a barometer? If we should take a glass tube closed at one end, the area of the cross section of which is one inch square, and fill it with mercury, and while thus filled plunge the open end into a vessel of mercury, it will be found that the amount of mercury remaining in the tube above the level of the mercury in the vessel will weigh about fifteen pounds, if the experiment has been performed at sea-level. This will vary, however, according to the temperature of the air. Of course barometers are tested when the air is at a certain temperature. If the weight of mercury in the tube is fifteen pounds, since it is sustained by the air pressing down on the mercury in the open vessel, it shows that the air-pressure on that open vessel is equal to fifteen pounds to the square inch. In practice, of course, the tubes are made very much smaller. If the air changes so that it is lighter than normal the mercury will fall in the tube, because the pressure on the mercury in the open vessel is less than fifteen pounds to the square inch. And, again, conditions may arise that will condense the air and make it for the time being weigh more than fifteen pounds to the square inch, in which case the mercury will rise in the tube. Thus it will be seen that the barometer will register the slightest change in air pressure.

Let us dwell for a moment on the causes of what are commonly called "changes of weather," when we will again revert to the use of the barometer.

The use of the telegraph in connection with the establishment of a weather bureau having stations for observation at convenient points throughout the country has contributed much to the science of meteorology. It is found that there are areas of high and low pressure existing at the same time in different parts of the country. These usually have their origin in the far northwest, and follow each other, sweeping down the eastern side of the Rocky Mountains and gradually bending easterly and from that to northeasterly by the time they reach the Atlantic coast. The areas of low pressure are called cyclones, while the areas of high pressure are called anti-cyclones. (By cyclone we do not mean those cloud funnels commonly called by that name that form at certain times of the year in certain sections of the country and produce such destruction of life and property. These storms are usually confined to a narrow strip and are short-lived. They arise undoubtedly from local conditions. A description of these tornadoes—for such is their true name—will be given in some future chapter.)

These centers of high and low pressure may be several hundred miles apart. In the area of high pressure, if it is in the winter season, the weather is unusually clear and cold, and generally clear and fairly cool at any season, and while there may be some wind it is not so strong as in the cyclone or low-pressure center. At this point it will be warmer and winds will prevail, with rain or snow, the winds varying in direction and intensity at a given point as the cyclone moves forward. In the center of these cyclones and anti-cyclones there will be a region of comparative calm, and the air is ascending at the center of the area of low pressure while it is pouring in on all sides from the area of high pressure where the air is compressed by a downward current from the upper regions.

The high-pressure or anti-cyclone system usually covers a larger area than the low-pressure system, where the air is ascending. While the air moves laterally from high to low, it does not move in a direct line. The air movement outside of the high-pressure center is usually not at a very high speed, but in northern latitudes in the direction of the hands of a clock. As it circles around it widens out spirally until it reaches the edge of a low-pressure system, when it bends in its course and moves in the other direction around this center, but constantly moving inward toward it in a spiral form and in a direction that is reverse to that of the hands of a clock. When the air current comes within the influence of a low-pressure or cyclonic system the velocity of its movement is very much accelerated until it has moved into the zone of quiet air in the center, where it is ascending.

In the upper regions of the atmosphere there are counter currents flowing in the opposite direction. The downward flow at the area of high pressure compresses the air near the surface of the earth and rarefies it in the higher regions of the atmosphere, while the opposite effect is going on over the center of low pressure, the air being rarefied nearer the surface of the earth, but condensed above normal in the higher regions by the upward current, which causes an overflow back toward the rarefied upper regions over the area of high pressure.

It will be observed that the ordinary storm has a compound motion. The whole system moves in an easterly direction, while the winds are blowing spirally about the storm center. If we should be in the track of a moving storm so that its center passed over us the winds at the beginning would blow in one direction and then there would come a subsidence until it had moved forward through the quiet zone, when we should feel the wind in the opposite direction until the area of low pressure had moved forward into the region of high pressure. The velocity of the wind will be determined by the difference of pressure between the areas and by the distance that the areas of high and low pressure are apart. The steeper the grade the more rapidly the fluid will flow.

Let us now have recourse, for a moment, to Figs. 1, 2, and 3 in order that the subject may be more fully understood. In looking at these diagrams we should imagine ourselves looking South, with the left hand to the East.

Fig. 1.

Fig. 1 shows the general direction of the air movement between two areas—one of high and the other of low pressure. The arrows show the general direction of the wind. You will notice that in the upper regions it blows in an opposite direction from the air movement on the surface of the earth.

Fig. 2 shows in a general way how the wind moves spirally around both centers. Over the area of high pressure the air descends spirally from the upper regions, circling around a large area—it may be one hundred miles or more in diameter—in the direction of the movement of the hands of a clock.

Fig. 2.

But then the wind at the high-pressure area is lighter than it is at the low, and circles outwardly until it finally moves off in the direction of a low-pressure area, gradually bending in the other direction until finally it moves the reverse of the hands of a clock—although now it is in a smaller circle, and with a more rapid motion. It moves spirally and upwardly about the low-pressure area until it reaches a point in the upper air, where it goes through the same gyrations in an opposite direction. Now imagine the whole combination moving from west to east at an average rate of thirty miles per hour, and imagine further that this system is linked to other systems that are following along, and you have some idea of the weather changes as they occur in the middle United States.

By referring to Fig. 3 you will see why the wind changes its direction when a storm center passes over any point. It has not only a spiral but also a forward movement.

Fig. 3.

Now let us go back to the barometer and see what part it plays in predicting changes in the weather. At the area of low pressure the air is ascending, as we have seen, and, owing to the peculiar way it ascends—by circling spirally upward around a region of comparative calm—it creates a partial vacuum, which is more pronounced in the center of the area. At the area of high pressure the air will be condensed by the descending current being arrested by the earth. The descending current—coming, as it does, from the upper and colder regions—accounts for the cool weather that most always prevails at a high-pressure area. In order to know how great the change of weather is likely to be, we must know what the readings of at least two barometers are—one at the high- and another at the low-pressure area. If the difference between the readings of the two barometers is very great, and the areas are comparatively close together, we may expect the change to be sudden and violent.

"High" and "low" as applied to a barometer are only relative terms. There is no fixed point on the index of the instrument that can be said to be arbitrarily high or low. For this reason a single barometer is not of much use. If it begins to fall from any point, and falls rapidly, it indicates that an area of a much lower pressure is approaching. The same is true of a high-pressure area, if the barometer rises rapidly from any point.

If we study the air motions in these systems sufficiently to get at least an inkling of the law of their movements, it becomes a very interesting subject.

Wind from whatever cause serves a wonderfully useful purpose in the economy of nature. Without wind, heat and moisture could not be distributed over the face of the earth and our globe would not be a fit habitation for man. How wonderful is the machinery of Nature, that can first forge a world into shape and afterward decorate it with green grass and flowers that are watered by the "early and latter rain"!

CHAPTER XII.

LOCAL WINDS.

There are so many causes that will produce air motion that it is often difficult to determine just what one is the chief factor in causing the direction of the wind at any particular time. There are very many instances, however, where the cause can be traced without difficulty; many of these have already been mentioned and there are many more that might be. Of course, as has been often stated, there is only one remote cause for all winds, and that is the sun, coupled with the movements of the earth. But there are certain local conditions that are continually modifying the phenomena of air movement. The velocity of winds as they occur from day to day varies very greatly with the height above the surface of the earth; ordinarily the velocity at 1000 feet above the earth will be more than three times greater than it is at 50 or 60 feet above, and even at 60 feet the velocity is much greater than at the surface of the earth. This is due partly to the retarding effect of friction caused by contact of the air with the earth's surface, but more particularly by trees, inequality of surface, and other obstructions on the earth.

There is a variety of wind called mountain winds that arise from different causes. As has been stated in a former chapter, under ordinary conditions the air is more dense at sea-level than at any point above, and the density is constantly changing from denser to rarer the higher we ascend. Suppose at a certain point, say halfway up a mountain side, the air has a certain density, and if it is at rest the lines of equal density or pressure will seek a level, just as water would under the same conditions. Suppose we start at a given point on the side of a mountain and run out on a level till we are 100 feet in a perpendicular line above the side of the mountain, the air contained within those lines will be in the shape of a triangle. If now the sun shines upon the side of the mountain the air is warmed and expands according to a well-known law, and the amount of expansion will depend upon the depth of the volume of air; hence the point of greatest expansion in our figure will be where the air is 100 feet deep, and will gradually decrease as we go toward the mountain till we come to the point where our horizontal line makes contact with the mountain side. At that point, of course, there is no expansion, because there is no depth of air; and the effect will be that the expanded air will overflow toward the mountain, and be deflected up its sloping side. If we apply this same principle to the whole mountain side we can see that there will be, during the day, a constant current of air flowing up the mountain. As night comes on this upward movement will cease and there will be a season of quiet until the earth has become colder than the air, and we have a phenomenon of exactly the opposite kind, when the air contracts instead of expands, which produces a downward current from the mountain top.

These currents are as regular at certain seasons of the year as the land and sea breeze. Of course, they may be obliterated for the time being, by the presence of a stronger wind due to some other cause, such as during the prevalence of a storm. In some of the regions of California hottest during the day time, the nights are made endurable, and even delightful, by the cool breezes that sweep down from the tops of the mountains. It often happens that on the shady side of a high and steep mountain where the sun's rays strike it so obliquely, if at all, that the earth will be but little heated, there will be a vast mass of cold air stored up. After the valley has become intensely heated by the sun there is an ascending current of air which in turn causes a down rush of the cold body of air from the mountain side. These local winds are frequently very severe, only lasting, however, for a short time, until an equilibrium of temperature and density has been established. A wonderful exhibition of this sort of wind is said to occur at certain times of the year on the coast at Tierra del Fuego, where a blast which they call the "Williwaus," comes down from the mountain side, without warning, with such tremendous force that no ship could stand the strain if it should continue for any length of time. Fortunately the shock does not last more than eight or ten seconds, when it is followed by a perfect calm. It is as though a great volume of air had been fired from some enormous cannon from the top of the mountain to the sea. The water is pulverized into a spray that is driven in every direction.

Sometimes these violent blasts occur in the Alps, but from a very different cause. Avalanches of great extent often take place on the sides of the mountains, when a vast amount of material, equal to three or four hundred million cubic feet of earth, will fall several thousand feet. Often an avalanche of this kind will produce a wind, which is confined, of course, to a restricted area, that is said to be so violent as to tear one's clothes into shreds. This is not caused by any difference of temperature, but by a violent compression.

There is a peculiar wind that occurs in Switzerland, often, between the months of November and March. These winds last from two to three days and are of great violence—especially near the mountains. They are warm and dry and are caused by an area of low barometer and an ascending current of air occurring at some point north of the Alps, which causes the air from Italy to flow over the Alpine range, causing a tremendous precipitation of snow and rain, which not only takes the moisture from the air, but sets free in the form of heat the energy that was stored in the process of evaporation, and this, together with the compression of the air as it flows down the slope of the mountains, makes it hot and dry. This wind is called the "Fohn."

There is a similar condition of things existing on the eastern slope of the Rocky Mountains which has a modifying effect upon the climate of parts of Colorado, Wyoming, Montana, also extending up into British America. This wind, which is here called "chinook," arises from causes similar to those that are active in Switzerland that give rise to the "fohn" wind.

There is a wind called the "blizzard" that is felt most keenly in Montana and the Dakotas during the winter, which is exceedingly cold and lasts sometimes for a period of 100 hours. The temperature falls at times 30 or 40 degrees below zero and the wind maintains a velocity of from forty to fifty miles an hour. These winds spread eastward as far as Illinois, but not with the same severity, and they move southward to the Gulf of Mexico, spreading over the States of Texas and Louisiana, and are there called "northers." It is exceedingly dangerous to be caught in a blizzard in the Dakotas, where the wind reaches its greatest velocity and the cold its lowest temperature—especially when the wind is accompanied, as it frequently is, by severe snowing. By the time it reaches the Gulf States it is very much modified as to temperature, but it is a very disagreeable wind in that portion of the country, because of the exceeding dampness of the air. One would be much more comfortable in dry, still air, even if it were many degrees below zero, than in an air freighted with moisture, although the temperature has not fallen to the freezing point.

There are hot winds called by different names according to the localities in which they occur. In southern California at certain seasons of the year the inhabitants are afflicted with what they call a desert wind that blows from the heated regions of Arizona toward the Pacific Ocean. The temperature sometimes reaches 120 degrees Fahrenheit, and persons have been known to perish from the effects of these hot winds in open boats out on the water before they could reach land.

Hot winds prevail on the plains of Kansas during the months of July and August that are phenomenal in their intensity, so much so that if they were widespread and of long continuance, like the northern blizzard, they would be attended with great loss of life and destruction to vegetation. Fortunately, they come in narrow streaks and in most cases do not blow more than from ten to thirty minutes at a time. These hot belts are sometimes not over 100 feet wide, and again they are as much as 500. They are so hot and dry that green leaves and grass are rendered as dry as powder in a few minutes. These winds are probably caused by the fact that at this season of the year, when the prevailing wind is southwesterly, the air becomes heated to a great height, and are the resulting effect of certain combinations of air currents in the higher regions of the atmosphere that force the already heated air toward the earth. As the air descends it is more and more compressed, which causes it to become more and more heated. We have already described the heating effect of compression upon air as shown by the experiment with the fire syringe. It was shown that air at normal temperature could be suddenly compressed into so small a space that the condensed heat, which was before diffused through the whole bulk of air at normal pressure, was sufficient to cause ignition. A cubic yard of air on the surface of the earth would occupy a much larger space if carried a mile above it. From this it is easy to see that if a volume of air at that height had a temperature of 70 or 80 degrees it would be very hot when condensed into a very much smaller volume, as it would be if it were forced down to the surface of the earth. These winds are the result of some superior force that is active in the upper regions of the atmosphere, because it is natural for heated air to rise, and this is what happens when the power that forced it down to the earth is no longer active to hold it there.

Reference has been made in a former chapter to tornado winds; they are rather exceptional phenomena and not thoroughly understood. The winds seem to blow in from all directions toward an area of very low pressure at a single point. The spiral motion that is common to all cyclones, in a tornado seems to be gathered up into a condensed form, like a funnel. The direction of movement is the same as that of the cyclone—that is, in the reverse direction to that of the hands of a watch. The upward motion of the air inside of the funnel is at a rate of over 170 miles an hour. The onward movement of the whole system is about thirty miles per hour.

Tornadoes occur with greater frequency in the United States than in any other section of the globe. Tornadoes seldom occur in winter, except perhaps in the Southern States. They are more frequent in the month of May than at any other time during the year, although they occur sometimes in April, June, and July.

Between 1870 and 1890 about sixty-five destructive tornadoes occurred in the United States, involving great loss of life and property. When a tornado moves off the land on to the ocean it may become what is termed a waterspout. These probably never originate on the water, but after they have once formed may be carried over the water to a considerable distance. A tornado was never known to originate on the shores of Lake Michigan, but there are a few instances (the most notable one being the Racine tornado) when they have reached the lake after having traveled from some distant point inland.

The Racine tornado—so called because it destroyed a large portion of that city—happened fifteen or more years ago. The tornado originated about 100 miles southwest of Racine, Wis., in northern Illinois. The funnel-shaped cloud passed over the lake, but the tornado character of the storm was broken up before it reached the other shore.

When a tornado passes from land to water it becomes a waterspout only when the cloud-funnel hangs low enough and the gyratory energy is sufficiently great. There is a great pressure on the water outside of the funnel and almost a perfect vacuum inside. This latter fact contributes largely to the destructive power of the tornado. When a funnel is central over a building a sudden vacuum is created outside of it and it bursts outwardly from the internal air pressure.

CHAPTER XIII.

WEATHER PREDICTIONS.

To predict with any great accuracy what the weather will be from day to day is a somewhat complicated problem, and, as all of us have reason to know, weather predictions made by those who have the matter in charge and are supposed to know all about it often fail to come to pass. The real trouble is that they do not know all about it. There are so many conditions existing that are outside of the range of barometers, thermometers, anemometers, and telegraphs that no one can tell just when some of these unknown factors will step in to spoil our predictions.

In very many cases, perhaps in a large majority of them, the predictions made by the weather bureau substantially come to pass. It has been stated in former chapters that the changes of weather accompany the movements of what are called cyclones and anti-cyclones, the cyclone being accompanied by low barometric pressure and the anti-cyclone by a higher one. The winds of the cyclone move spirally around the center of lowest depression with an upward trend, the motions being in a direction reversed to that of the hands of a clock. In the centers of high pressure the current is downward instead of upward and the direction of the wind around it is opposite to that around the low-pressure area. The fundamental factor in predicting the weather is the direction of movement of these areas of low pressure. In almost all cases the direction of movement is from the west to the east, but not always in a straight line. These movements, however, are classified so that after the direction has become established one can predict with considerable accuracy as to whether it will move in a curved or a straight line. By movement we do not refer to the direction of the wind at any particular point, but the onward movement of the whole cyclonic system, which is usually from twenty-five to thirty miles an hour, but in some cases the speed is much greater.

Not only does the upward movement of the whole system vary, but the velocity of the wind around any given cyclonic center varies. There are about eleven classes of cyclones that appear in the United States, each class having its own path of movement and origin. A large number of these appear to originate north of the Dakotas, and move directly east to the Gulf of St. Lawrence. Three other classes originate on about the same line, a little west,—say, north of Montana,—moving first in a southeasterly direction, passing over the center of Lake Michigan and bending northerly through Lake Ontario and finally landing in the Gulf of St. Lawrence. Two other classes start at the same point, one of them going as far south as Cincinnati, and the other as far south as Montgomery, Ala., and both turning at these points northeasterly to the Gulf of St. Lawrence. Two other classes originate in Colorado, one moving in a northeasterly direction slightly curved, and the other directly east. Still others have their origin farther south in the Gulf of Mexico, and move in a northeasterly direction. Very rarely they originate in the Atlantic east of Savannah, moving first in a northwesterly direction, but finally bending to the northeast.

Every day there is a weather map made up showing the locations of the high and low barometers, direction of wind, lines of equal pressure, as well as those of temperature. By study from year to year all of these phenomena have become systematized, so that by tracing an area of low barometer from its origin in its progress easterly it is soon seen to fall under one of these classes and we are able to predict about what its course will be. Knowing the speed of its movement as well as the velocity of wind and all the conditions attending it, taken in connection with the weather conditions in the region for which the prediction is made, an expert can ordinarily forecast with some degree of accuracy. After all that can be said, however, weather predictions based upon maps are and have been far from satisfactory. One who has been a close student of local conditions for a number of years will often predict with as great accuracy as the weather bureau. Areas of low pressure are followed sooner or later by a fall of temperature; this is especially true in the winter months. Sometimes this fall is very marked, and then it is called a cold wave. These sudden changes of temperature are not thoroughly understood, but are supposed to be due partly at least to rapid radiation of heat into the upper regions, as the clear atmosphere which usually attends areas of high pressure is favorable to such a condition. Undoubtedly, too, there are dynamic causes, forcing the colder air from the upper regions to the earth, when it immediately flows off toward an area of low barometer.

Long-time predictions are purely guesses. They sometimes guess on the right side, and this gives them courage to make another. It is an old saying that "all signs fail in dry weather." In time of a drought it is true that the indications which at ordinary times would be surely followed by a rain are of no value. When a season is once established, either as a rainy season or a dry season, it is likely to persist in this character until a change comes that is produced by the movement of the sun in its course northerly and southerly, and the change produced from this cause requires several weeks of time.

If accurate weather predictions could be made for a long time in advance, or for even a week, they would be of incalculable value. But it is doubtful if ever this will be brought about, as there are too many necessarily hidden factors which enter into the calculations. If stations could be established all over the oceans with sufficient frequency, and an equal number at a sufficient altitude in the air, I have no doubt that much that is now mysterious might be made plain.

CHAPTER XIV.

HOW DEW IS FORMED.

Reader, did you ever live in the country? Were you ever awakened early on a summer's morning to "go for the cows"? Did you ever wade through a wheat field in June—or the long grass of a meadow—when the pearly dewdrops hung in clusters on the bearded grain, shining like brilliants in the morning sun? Have you not seen the blades of grass studded with diamonds more beautiful than any that ever flashed in the dazzling light of a ballroom? If not, you have missed a picture that otherwise would have been hung on the walls of your memory, that no one could rob you of.

Everyone has noticed that at certain times in the year the grass becomes wet in the evening and grows more so till the sun rises the next day and dispels the moisture, and this when no cloud is seen. Dew is as old as the fields in which grass grows. It was as familiar to the ancients as it is to us, and yet it is only about three-quarters of a century since the cause of it has been understood. We even yet speak of the dew "falling" like rain. In former times some scientists supposed that it was a fine rain that fell from the higher regions of the atmosphere. Others supposed it to be an emanation from the earth, while still others supposed it was an exudation from the stars.

"By his knowledge the depths are broken up and the clouds drop down dew" (Prov. iii. 20).

The first experiments carried on in a scientific way were by Dr. Wells, a physician of London, between the years 1811 and 1814.

Everyone has noticed in warm weather the familiar phenomenon of water condensed into drops on the outside of a pitcher or tumbler containing cold water. This condensation is dew. It always forms when the conditions are right, summer and winter. In cold weather we call it frost. It has been stated in a former chapter on evaporation that the capacity of the air for holding moisture in a transparent form depends upon its temperature. If the temperature is at the freezing point it will contain the 160th part of the atmosphere's own weight as aqueous vapor. If it is 60 degrees Fahrenheit the air will retain six grains of transparent moisture to the square foot of air, while at 80 degrees it will contain nearly eleven grains. When the air is charged with this vapor to the point of saturation (which point varies with the temperature) a slight depression of the temperature is sufficient to condense this vapor into cloud or drops of water. Between 1812 and 1814 Dr. Wells made a series of experiments with flocks of cotton wool. He weighed out pieces of equal weight and attached a number of them to the upper side of a board and as many more to the lower side, and exposed it to the night air under varying conditions. One experiment was made with a board four feet from the earth, so that half of the bunches of cotton faced the ground and the other half the sky. He found upon weighing these after a night's exposure under a clear sky that the cotton wool on top of the board had gained fourteen grains in weight from the moisture, or dew, that had formed upon it, while the same amount of cotton on the under side of the board had only increased four grains. He tried further experiments by making little paper houses, or boxes, to cover a certain portion of grass or vegetation. He found that while there would be a heavy dew on the grass outside there was little or none within the inclosure. These experiments were conducted in various ways and closely watched to see that none of the phenomena were in any way connected with falling rain. It has been determined that substances like grass and green leaves of all kinds, hay and straw, while they are poor conductors of heat, are excellent radiators. In another chapter we have referred to this quality of straw, that is taken advantage of by the inhabitants of hot countries in the manufacture of ice and in our own land for storing it.

Perhaps everyone who has lived in the country has noticed that on a summer's morning when the grass is laden with dewdrops a gravel walk or a dusty road will be perfectly dry. This is due to the fact that the gravel will retain heat and not radiate it, for a much longer time than grass or green leaves. Dew begins to form upon the grass very soon after the sun is set because the moment the sun's rays are withdrawn the heat is rapidly radiated by the blades of grass, which cools the earth under it and the air above and surrounding it, so that if the air is anywhere near the moisture saturation point on cooling at the surface of the ground it will readily give up a part of its moisture, which condenses in drops upon the blades of grass.

If the night is still and clear and there is much moisture in the air, the dew will be heavy, but if the night is cloudy there will be little or no dew formed. The clouds form a screen between the earth and the upper regions of the atmosphere, which prevents the heat from radiating to a sufficient extent to form dew. For the same reason no dew will form under a light covering spread over the ground even at some distance above it. The covering acts as a screen, which prevents the heat from radiating to the dew point. From what has gone before it will be seen that if the atmosphere is not charged with moisture up to the point of saturation it will require a greater amount of depression of temperature to cause condensation, and this is why we usually have heavier dews in June when the air is more highly charged with moisture than we do in August when it is dry. This also accounts for the ice clouds, called cirrus, being formed so high up in the atmosphere during dry weather. There is so little moisture in the air that it requires a very great difference of temperature to cause condensation to take place, and the necessary depression is not reached in these cases except at an altitude of several miles.

Dr. Wells has shown that if we take the reading of two thermometers on a clear summer night, one of them lying on the grass and the other suspended two feet above it, we shall find that the one lying on the grass will read 8 or 10 degrees lower than the one suspended in the air. If the night is still there will be a cold stratum of air next to the earth, which will not tend to diffuse itself to a very great degree and dew will form. If, however, it is cloudy or the wind is blowing there is rarely any formation of dew. The reason in the former case, as we have explained, is that the radiated heat is held down to the earth in a measure, and in the latter case there is a constant change of air; so that in either case no part of it is allowed to cool down sufficiently to precipitate moisture.

It is a curious fact that often there will be a heavier dew under the blaze of a full moon on a clear night than at any other time. The moon has no screens about it of any kind to obstruct the free radiation of heat. It is supposed to be a dead cinder floating in space and not surrounded by an atmosphere, so that the sun's rays have full effect upon it during the time it is exposed to them, and at that time it becomes heated to a temperature of something like 750 degrees Fahrenheit. For half the month, say, the sun is shining continuously upon all or a part of it. In other words, the days and nights of the moon are about two weeks long. The moon does not revolve upon its own axis like the earth, therefore the same side or a portion of it is exposed to the sun for 14 days. During the time that the moon is in the earth's shadow it is supposed to fall to 187 degrees below zero, which is 219 degrees below the freezing point. When the moon is full and is heated up to over 700 degrees there is sufficient heat radiating from it to be felt sensibly upon the face of the earth, and it would be felt if it were not for the great envelope of atmosphere and its attendant cloud formations that surround the earth. There are but few days in summer when there is not a haze in the atmosphere, although we call the sky clear, which intensifies the light and gives everything a warmer tone. The heat coming from a full moon on a clear night is absorbed in causing the aqueous vapors that are partly condensed in the higher regions of the atmosphere, to be reabsorbed into transparent vapor. This clears away the heat screen in the atmosphere and allows radiation to go on more rapidly at the earth's surface, and thus cools it to a greater extent when the moon is shining brightly than when it is dark and in the shadow of the earth.

As we have already mentioned, the cold that is produced by radiation through the blades of grass and other radiating substances may be indicated by placing one thermometer on the ground and fixing another at some point in the air. Sometimes the difference is very marked, amounting to as much as 20 or 30 degrees. If under these conditions a cloud floats overhead, forming a heat screen, its presence will be readily noticed by a rise in the thermometer. Radiation into the upper regions of the atmosphere is checked, which causes a sudden rise in the temperature near the surface of the earth. By taking advantage of this principle of heat radiation from the earth's surface it is a very easy matter to protect tender vegetation from even quite a severe frost, if it occurs in the early fall, by a slight covering, such as thin paper. The paper will act as a heat screen and in a measure prevent the heat from radiating from the earth immediately under it. Frost—which of course is but frozen dew—at this season of the year will form on a still autumn night, although the atmosphere at some distance above the ground is some degrees above the freezing point. The reason for this will be obvious when we consider the facts that have been set forth concerning the power of radiation to produce cold.

It has been estimated by meteorologists that the amount of water condensed upon the surface of the earth in the form of dew amounts to as much as five inches, or about one-seventh of the whole amount of moisture that is evaporated into the air. It will thus be seen that dew performs an important part in supporting vegetation.

The same operation in nature's great workshop that forms the dews of summer creates the frosts of winter. The moisture in cold weather is condensed the same as in warm. When it is condensed at the surface of the earth we have the phenomenon of frost, but when condensed in the upper regions of the atmosphere we have that of snow.

Heat radiation from the earth goes on in winter, which is evidenced by the fact that a thick covering of snow is a great benefit to vegetation as a protection against the injurious effects of frost. The writer has seen flowers blooming abundantly at an altitude of 12,000 feet above the sea-level, protected only by the friendly shelter of a snowbank. In some cases the blooming flowers were in actual contact with the snow. By experiment it has been determined that the earth under a thick coating of snow is usually warmer by nine or ten degrees than the air immediately above the snow covering.

CHAPTER XV.

HAILSTONES AND SNOW.

A hailstone is a curious formation of snow and ice, and most of the large hailstones are conglomerate in their composition. They are usually composed of a center of frozen snow, packed tightly and incased in a rim of ice, and upon this rim are irregular crystalline formations jutting out in points at irregular distances. Frequently, however, we find them very symmetrically formed as to outline, and the snow centers are almost without exception round. Hailstones and hailstorms differ in different climates, but they are more pronounced in the torrid than in the temperate zone. Historians give accounts of hailstones of enormous size; the very large hailstones being undoubtedly aggregations of single stones that have been thrown together and congealed in the clouds during their fall to the earth.

It is recorded that on July 4, 1819, hailstones fell at Baconniere measuring fifteen inches in circumference, and very symmetrically formed, with beautiful outline. Hailstones in India are said to be very large—from five to twenty times larger than those in England or America—seldom less than walnuts and often as large as oranges and pumpkins. It is recorded that in 1826, during a hailstorm at Candeish, the stones perforated the roofs of houses like cannon shot, and that a single mass fell that required several days to melt, weighing over 100 pounds. It is further recorded that on May 8, 1832, a conglomerate mass of hailstones fell in Hungary a yard in length and nearly two feet in thickness. Still another instance is recorded of a hailstone having fallen in 1849 of nearly twenty feet in circumference. This hailstone is said to have fallen upon the estate of Mr. Moffat of Ord. We will only ask our readers to listen to one more hailstone story, in which it is related that during the reign of Tippoo, sultan, a hailstone fell as large as an elephant. Undoubtedly one of two things was true regarding this latter story; it was either a very large hailstone or a very small elephant. The historian fails to give the size of the elephant. There is no doubt, however, but that hailstones may adhere and form large masses owing to the violent agitation of the elements that always attends a hailstorm.

Hailstorms are almost universally attended by constant and heavy thunder and lightning, together with violent winds. They usually occur on a very hot day, and when the air is filled to saturation with moisture. When this is the case a column of air is very highly heated at some point, when it ascends with great force into the upper regions of the atmosphere to a greater altitude than is common in the case of ordinary thunderstorms. Here it meets with an intensely cold body of air, when it is suddenly condensed and readily frozen as soon as condensed, which not only forms hailstones, but sets free the energy that has been carried up in the moisture globules. This results in frequent electrical discharges, causing great waves of condensed and rarefied air, which, in the rarefied portions, produces still more intense cold; so that we have the conditions for a mighty struggle between the elements, which is intensified by a constant and terrific electric cannonade. Undoubtedly there are also whirlwinds in the cloud, similar to those that sometimes visit the earth, which would tend to gather up the hailstones and aggregate them into large masses. It is a mighty battle between the moisture-laden, superheated air, ascending from the surface of the earth, and the powers residing in the upper regions of cold. Nature is constantly struggling to find an equilibrium of her forces, and a hailstorm is only one of the little domestic flurries that take place when she is setting her house to rights. Hailstorms are usually confined to very narrow limits, and they can prevail on a grand scale only in hot climates, where we have the conditions for wide differences of temperature between the upper and lower regions of the atmosphere; and, also, where the conditions are favorable, for an enormous amount of absorption of moisture into the atmosphere.

When snow is formed in the atmosphere, the conditions are quite different from those of a hailstorm; it is usually in a lower plane of the atmosphere, and there is no violent commotion, as is the case with the latter. A volume of air laden with moisture comes in contact with a colder volume of air, when condensation takes place, as in the case of rain, except that the moisture is immediately frozen. In this case both volumes of air may be below the freezing point, but one is very much colder than the other. If the snow reaches the earth it will be because the air is below the freezing point all the way down. Snow is formed at all seasons of the year. We may have a snowstorm on a high mountain when we have extreme heat at sea-level.

In summer time of course the snow melts as soon as it falls into a stratum of air with a temperature above the freezing point, and continues its journey from that point as raindrops instead of snowflakes. In the formation of a snowflake Nature does some of her most beautiful work. A snowflake first forms with six ice spangles, radiating from a common center. Shorter ones form on these six spokes, standing at an angle of about sixty degrees, on each side of each spoke, of such length and arrangement as to form a symmetrical figure or flower. They do not always take the same form, but follow the same laws that govern the formation of ice crystals. The structure of a snowflake may be often found upon a window pane of a frosty morning. Here, however, the free arrangement of the parts of a snow crystal are interfered with by its contact with the window pane, but while floating gently in the air there is the utmost freedom for the play of nature's forces as they apply to the work of crystallization.

The difference in structure of snowflakes is chiefly due to the conditions under which they are formed. If the moisture is frozen too rapidly the molecular forces that are active in crystallization do not have time to carry out the work, in its completeness of detail, as it will where the freezing process, as well as the condensing process, goes on more slowly.

CHAPTER XVI.

METEORS.

Meteors are the tramps of interplanetary space. They sometimes try to steal a ride on the surface of the earth, but meet with certain destruction the moment they come within the aërial picket line of our world's defense against these wandering vagrants of the air. They have made many attempts to take this earth by storm, as it were, and many more will be made. They fire their missiles at us by the millions every year with a speed that is incredible, but thanks to the protecting influence of the great ocean of air that envelops our globe they become the victims of their own velocity.

Meteors or shooting stars are as old as the earth itself, and they are the material of which comets are made. Before it was determined what these meteors or shooting stars were, many theories were promulgated as to their origin. One was that they were masses of matter, large and small, projected by volcanic action from the face of the moon with such violence as to be brought within the attraction of the earth. Others supposed them to be the effect of certain phosphoric fluids that emanated from the earth and took fire in the upper regions of the atmosphere. This, however, was mere speculation and without any scientific basis of fact. Anyone who has been an observer of shooting stars will have learned that there are certain periods of the year when they are more numerous than at other times; notably in August and November. Then again there are longer periods of many years apart. By persistent observation it has been established that there are great numbers of schools or collections of cosmic matter that fly through interplanetary space, having definite orbits like the planets. Any one of these collections may be scattered through millions of miles in length. A comet is simply one of these wandering collections of meteoric stones having a nucleus or center where the particles are so condensed as to give it a reflecting surface something like the planets or the moon. This enables us to see the outline of the comet to the point where the fragments of matter become so scattered that they are no longer able to reflect sufficient light to reach our eyes. The fringe of a comet, however, may extend thousands or even millions of miles beyond the borders of luminosity.

There is scarcely a day or night in the year when more or less of these meteoric stones do not come within the region of our atmosphere, and when this happens the great velocity at which they travel is the means of their own destruction. They become intensely heated by friction against the atmosphere just as a bullet will when fired from a gun—only to a greater extent owing to the greater velocity. They disintegrate into dust which floats in the air for a time, when more or less of it is precipitated upon the surface of the earth. Disintegrated meteors, or star dust, as they are sometimes called, are often brought down by the rain or snow. Most of the shooting stars that we observe are very small, resembling fire-flies in the sky, but once in a while a very large one is seen moving across the face of the heavens, giving off brilliant scintillations that trail behind the meteor, making a luminous path that is visible for some seconds. These brilliant manifestations are due to one of two causes. Either there is a very large mass of incandescent matter or else they are so much nearer to us than in ordinary cases that they appear larger. It is more likely, however, that it is due to the former cause rather than the latter, from the fact of its apparently slow movement as compared with the smaller shooting stars. It has been determined by observation that the average meteor becomes visible at a point less than 100 miles above the earth's surface. It was found as far back as 1823 that out of 100 shooting stars twenty-two of them had an elevation of over twenty-four and less than forty miles; thirty-five, between forty and fifty miles; and thirteen between seventy and eighty miles. It was determined by Professor Herschel that out of sixty observations of shooting stars the average height of their first appearance was seventy-eight miles and their disappearance was at a point fifty-three miles above the earth.

It is a matter of history, however, that sometimes these meteoric stones descend to the surface of the earth before they are entirely disintegrated. A fine specimen of this kind is to be seen in the Smithsonian Institution. There are over forty specimens of these aërolites (air-stones) in the British Museum, labeled with the times and places of their fall. Instances of falling to the earth are so rare that there is little to fear from these wandering missiles of the air. We do not remember a case where life or property has suffered from the fall of a meteor.

This brings us to the consideration of the part which the great air envelope surrounding the earth plays as a protection against many outside influences. For instance, if it were not for the air, millions of these meteoric stones would be showered upon our earth every year and at certain times every day, which would render the earth untenable for human existence. We should be at the mercy of those wandering comets whose fringes strike our atmosphere more or less deeply at frequent intervals. It is not impossible that the earth may at some time pass directly through one, and yet there is little danger that in such a case there would be more than an unusual display of celestial fireworks.