AIR.—PHYSICAL PROPERTIES.
To make the weight for the winds.—Job xxviii:25.
One day an old Florentine pump-maker came to Galileo[1] to inquire why he could not make a pump work effectively when it was more than thirty-four feet long. The philosopher could not answer, nor did he solve the problem during his lifetime, but bequeathed it to his pupil, Torricelli.[2] This famous Italian succeeded in partially answering the question in 1643. He performed the following experiment: Taking a glass tube thirty-six inches long and one-fourth of an inch in diameter, closed at one end, he filled it with mercury, and holding his finger over the open end, inverted and placed it in a cup of mercury, then removing his finger, discovered that the quicksilver settled in the tube six inches, leaving a column of the shining metal thirty inches high. He thus demonstrated that air has weight, equal to that of a column of mercury thirty inches in height.
On this supposition it was argued that if the whole height of the air should be lessened the column would fall. Such was the opinion of Blaise Pascal,[3] who in 1646 requested M. Périer, his brother-in-law, to ascend the Puy de Dome, a summit near Clermont, and repeat the experiment of Torricelli. To his delight, upon reaching the top of the mountain, the column stood three inches lower. Pascal then used a tube fifty feet long, which he filled with water, and found that this liquid could be supported by the air to the height of thirty-four feet. Water is 13.6 lighter than mercury, and it will be observed from the foregoing statement, that it was supported 13.6 higher than quicksilver. Here was a full answer to the pump-maker’s query! A column of water, thirty-four feet long and one square inch at the base, weighs fifteen pounds. A column of mercury, thirty inches long and one square inch at the base, weighs fifteen pounds. A column of air the whole height of the atmosphere, one square inch at the base, weighs fifteen pounds.
Any influence, therefore, which varies the weight of the air, will vary the height of a column of quicksilver; and the reverse will of course be true, that any fluctuation in the column of mercury indicates a change in the condition of the atmosphere. Thus, a “falling barometer” predicts foul weather, for it shows that the air is becoming lighter, and will therefore rise, while other air will rush in, with varying speed, to take its place, producing breezes, gales, and possibly tornadoes. The warm air rising may come in contact with a cold stratum above and its moisture be condensed into rain or snow. We shall presently refer to this again.
SHOWING DENSITY OF ATMOSPHERE AT DIFFERENT HEIGHTS.
A quart of air, at ordinary temperature, weighs about eight hundred times less than a quart of water, yet the aggregate pressure of the atmosphere is equal to fifteen pounds on every square inch. A person of average size presents a surface of about two thousand square inches. This would receive a pressure of fifteen tons, a weight more crushing than that of all the shields cast upon the traitorous Tarpeia[4] at the Roman gate.
Herschel calculates that the total weight of the atmosphere is one twelve-hundred-thousandth of that of the earth.
Why does not such enormous pressure destroy life? Because it is counterbalanced by the pressure of air, gases and blood within the body. That this is true may readily be seen in the process of dry-cupping. Bare the arm, take a bit of writing paper an inch and a half long, dip it in alcohol, light, and instantly place in a small wine glass, and at once apply the glass to the soft part of the arm. The flesh under the glass will rise like a pin-cushion, and become red from the pressure of the blood within. Persons going down in diving-bells suffer from the condensation of the air in the bell, while on a high mountain they experience a pressure in the opposite direction, on account of the rarefaction of the atmosphere, the blood often gushing from the nose and ears.
We shall better understand the phenomena of the air by first considering some of its distinctive properties.
MAGDEBURG HEMISPHERES.
AIR PRESSES EQUALLY IN ALL DIRECTIONS.
This great principle, which applies to all gases as well as to fluids, has many illustrations in nature. The haliotis[5] is held to the rock with a tenacity which sometimes resists the strength of the collector, who would add its iridescent beauty to his cabinet of shells.
Alas for the fisherman’s line whose bait has been swallowed by a skate![6] Quickly descending to the bottom, this broad, flat fish expels the air from beneath it, and defies all effort at capture.
The most complete demonstration of this law is shown by the Magdeburg hemispheres,[7] invented by Otto von Güricke[8] and used by him before Charles V. and his brilliant court. They are still preserved in the ancient city which gave them their name, are twenty-four inches in diameter, and after the air in them had been removed, required twelve horses to separate them.
The pressure of the air varies greatly at different altitudes. At the height of three and one-half miles the column of mercury in a barometer falls to fifteen inches, showing that below that elevation we have as much air as in all the space above.
SHOWING TORRICELLI’S HISTORICAL EXPERIMENT AND THE PRINCIPLE OF THE BAROMETER.
The boiling point of liquids is materially influenced by the pressure of the atmosphere. On high mountains potatoes and even eggs can not be cooked by boiling, as the water will all evaporate before it is heated sufficiently to cook them.
Partially fill a glass flask with water, heat it until steam begins to escape, then remove the lamp and insert a stopper, the boiling will cease. Now pour cold water upon the flask, and the water within begins again to boil vigorously. The cold water condenses the steam, creating a partial vacuum, thus relieving the heated water from pressure, and it boils at a lower temperature than 212°. This illustrates the famous culinary paradox that “cold water will make hot water boil.”
The buoyancy of substances in air depends upon the same principle that determines their buoyancy in liquids. It will be proportioned to the amount of air which they displace.
It is correct to say that a balloon rises because the air is heavier, and therefore pushes under the balloon and forces it up; or, that it rises because it displaces more than its own weight of air. Thistle-down may be compressed so that it will fall like shot. The resistance offered by the air to the fall of bodies led men long to hold to the fallacy that the rapidity of the descent of falling bodies was proportioned to their weight. This error was at length exploded by Galileo in his interesting experiment on the leaning tower of Pisa.[9] In a vacuum, a cannon ball and a feather fall in the same time.
AIR PUMP.
COMPRESSIBILITY.
The statement that air can be expanded involves the counter-truth that it may be compressed.
Mariotte[10] announces the law as follows: Doubling the pressure upon a given amount of gas will halve the space it occupies, and double its expansive energy. The application of this principle in one form gives us the air-gun.[11] If the air in a gun-barrel forty inches long were compressed into the space of half an inch it would press with eighty times its previous force, or with a power equal to twelve hundred pounds to the square inch. Compressed air is often used as a power in mines and excavations, and its advantages are many; it was so employed in the Hoosac tunnel. Though the engine that compressed the air was three miles away, the loss from friction was very slight, and the air, having performed its work in driving the drill, was then liberated to purify the atmosphere of the tunnel and expel noxious gases which accumulated from continuous blasting. The apparatus for compressing air is called a condenser. It consists essentially of a cylinder and piston, with a valve in the bottom of each, opening downward. A precisely opposite arrangement of valves is found in the air-pump, a machine for exhausting air from a given space, usually a receiver. As the piston is raised in removing the air, the valve closes, and the air is thus forced out of the cylinder; the air in the receiver then expands, opens the valve at the bottom of the cylinder, and rises into it; as the piston descends its valve is opened; rising, it again removes the cylinder full of air; the air in the receiver again expands, opens the lower valve, and so continues, until the air in the vessel becomes too much rarefied to lift the delicate valve and make its escape. The vacuum thus produced is by no means so perfect as the “Torricellian vacuum,”[12] the name given to the unoccupied space above the column of mercury in a barometer.
Various substances may be placed in a receiver to show the expansive tendency of air. A piece of wood immersed in a jar of water will throw off thousands of little bubbles. A shriveled apple will become round and plump. The air in an empty rubber bag will often expand so as to fill the receiver. Air in a thin glass vessel, tightly corked, will expand so as to burst the vessel into fragments.
The following simple but useful piece of apparatus can easily be made: Take a pint bottle with a nicely fitted cork, through the cork insert a small glass tube so as to be perfectly air-tight (melted sealing wax is convenient for making tubes or glass tight;) a perforated rubber stopper is better. Let the end of the tube inserted be drawn out in the flame of an alcohol lamp (this is not essential, but will make the experiments more interesting), suck the air from the bottle, close the end of the tube at once with the finger and place it in a glass of water, and a miniature fountain, in vacuo, will be revealed. After performing this pretty experiment remove the tube and reinsert it with the larger end down, having filled the bottle two-thirds full of water. With the lips force a quantity of air into the bottle, upon removing the mouth the water will rise in the tube and fall in a fine spray from the small aperture at the top. This last experiment is particularly interesting, as it is a perfect illustration of a flowing oil well. Closely allied to the expansibility of air is its
ELASTICITY,
Or tendency to regain its former volume after being compressed. Many a school-boy has observed this property, while manipulating his fascinating popgun. When he places his finger over the open end of the piece of elder, utilized as a gun, and suddenly pushes down the piston upon the wad, he notices that it quickly flies back. An inflated bladder thrown upon the floor bounds like a rubber ball; force pumps in our houses act upon this principle. The air in the chamber of the pump is first compressed by the entrance of the water; it reacts like a spring, and forces the water through the pipes to the rooms above.
EXPANDING RUBBER BAG IN AN EXHAUSTED RECEIVER.
The hydraulic ram is another application of the same principle. Perhaps the reader may know some place where this apparatus can be used. Let us briefly describe the conditions of its operation. Near your house, at a lower elevation, may be a beautiful spring, so situated that, within the distance of about seventy feet, a fall of from five to ten feet can be obtained. Now run a large pipe from the spring to the spot where the ram is to be placed, below the level of the spring. The ram is a pear-shaped, cast iron cylinder, open at the small end, at which point a valve is placed, opening upward. The pipe coming from the spring is screwed into the bottom of the ram below this valve, in such a manner as to conduct the water past the valve, and out through an opening beyond. At this point, however, is placed a metallic valve, against which, as the water escapes, it continues to crowd. Presently the rushing stream obtains sufficient momentum to close this valve, and thus prevent for a moment its further escape. The accumulated force of the water then raises the valve in the bottom of the ram and it rises into the chamber, which is partially filled with air. This air is compressed, but on account of its elasticity at once reacts upon the water and forces it through another pipe to the required height. Only about one-eighth of the water is sent through the last pipe, as seven-eighths of it is required to force the remainder to the desired elevation. I have a great respect for this useful apparatus, the invention of the elder Montgolfier.[13] I know of one hydraulic ram which for fifteen years has raised, through a pipe twenty-two hundred feet long, to an elevation of seventy-five feet, an average of twenty-four barrels of water daily. Its total cost for repairs has not exceeded twenty-five dollars, and yet it has done every day the work of four men. If men had been hired to do this labor at $1.50 per day each, their wages would have amounted to the snug sum of $32,850.
A FOUNTAIN MADE BY COMPRESSING AIR IN A BOTTLE.
Atmospheric pressure is employed in many of our cities to convey packages from one part of a building to another, and to even greater distances. This “Pneumatic Dispatch”[14] system, as it is called, was first tried successfully in Paris, in 1865. A company was then established, which now claims to send eight hundred and thirty packages daily. In our own country this curious appliance may be seen in operation at the United States Express office in New York City, in the mammoth establishment of Mr. Wanamaker, in Philadelphia, and doubtless in many other places. For many years attempts have been made to propel cars by compressed air, but as yet the expense of such a plan greatly exceeds that of steam.
LIGHT.
Among the most gracious and beautiful offices performed by the atmosphere is the reflection and refraction of light. The blue dome of the sky, the magnificent coloring of the clouds, and all the delicate and ever varying tints of the morning and evening twilight are due to its influence. Without the air we should be in complete darkness until the sun rose, a fiery ball, above the horizon. All day long the only light we should receive would come directly from the sun, or be reflected from objects on the earth. At sunset, darkness would instantly be spread over us like a pall. No gentle gradations of light and deepening shade would usher in and close the day.
All must have observed during the past year the remarkable appearance of the western sky after the sun had set. Cities were more than once supposed to be burning, reflecting their lurid blaze upon the clouds. The cause of this is still a matter of dispute, but is generally attributed to the presence of star dust, or some minute mineral matter suspended in the higher atmosphere.
It will be remembered that color is not an inherent property of a substance, but depends upon what portion of the light rays it absorbs. Snow is white, as it absorbs none of the prismatic colors, but reflects them all to the eye. Whatever, then, varies the absorbing or reflecting power of an object varies its tints. Thus, objects seen on the horizon are red, because the dense atmosphere has turned aside the violet, indigo, blue, green, yellow and orange, and only the red color reaches the eye.
Observe that the initial letter of the prismatic colors taken in their order make the word “vibgyor.”
Again, were there no atmosphere, there could be no
CLOUDS NOR RAIN.
The moon is destitute of these, or at least that half of it which is always turned toward us. The most powerful telescopes can detect there the presence of neither atmosphere nor cloud.
A most remarkable proof of divine wisdom can be seen in the nice adjustment by which the pressure of the air prevents undue evaporation from the lakes and seas, and at the same time furnishes the medium by which moisture is conveyed to the remotest parts of the earth. The fact that water, in the form of mist or clouds, should float, and not fall in a substance many times lighter than itself, is one of the most wonderful of nature’s phenomena. When shot are dropped into water, we expect that they will sink; yet lead is but eleven times heavier than water, while water is eight hundred times heavier than air.
The following seems to be the most satisfactory explanation of the matter: It is a well known fact that the air has the power to absorb and hold, in an invisible form, a certain amount of moisture. The quantity which it can contain depends upon its temperature. If the air is cooled, it parts with a portion; thus if the grass radiates its heat, dew is deposited upon it; if it is very cold, the frost covers it with sparkling crystals. It is thought that when cooling from any cause takes place in higher altitudes, the atmospheric moisture changes from the invisible to the visible form, and assumes the physical condition of spheroids or vesicles, minute bubbles of water in point of fact, each bubble filled with air. These bubbles, heated by the sun’s rays, would become lighter than the medium in which they float, for the same reason that soap bubbles float while they are warm. In this condition they are drifted along by currents until they reach a colder stratum of air, when they are condensed and fall as rain. If cooled sufficiently, snow would be formed.
Cloud forms are four in number, cirrus, cumulus, nimbus, and stratus, all of which may sometimes be seen at once, in the sky of a summer’s day. At times they float above the loftiest mountains. Gay-Lussac,[15] rising in his balloon to an elevation of 21,600 feet, perceived clouds drifting far above him.
AIR CURRENTS—SHOWING THE PRINCIPLE ON WHICH MINES ARE OFTEN VENTILATED.
The work performed by the atmosphere in supplying water to the soil is worthy of profound attention. Close observation shows that it varies in amount, year by year, much less than one would suppose. For example, the average yearly rain-fall in western New York, for the last thirteen years, has been thirty-six inches. Contrary to the general impression, the record shows a slight yearly increase in the amount. While this is true, the quantity of water carried down by our rivers is constantly diminishing. All must have observed the lessening size of our streams. Many a mill has ceased to run from lack of its former supply of water. This has resulted from the destruction of forests, and clearing of land, which have greatly increased evaporation of moisture from the soil. So grave a matter has this become, that it is attracting the attention of governments, because of its relation to agriculture and the navigation of rivers.
One can have but little idea, unless he carefully calculates it, of the inestimable blessings conferred by the atmosphere upon man, in furnishing to the soil its supply of water.
MR. THOUGHTFUL THANKFUL,
Being disposed properly to acknowledge the care of a kind Providence, in carrying on the work of his farm, one day sat down to figure out the value of a recent shower, which had refreshed his crops. The leaves of his corn had begun to curl, the oats and wheat were growing prematurely yellow, a few more days of the scorching heat and drouth would have made his harvests a failure, but to his great relief a plenteous shower fell. The rain gauge showed half an inch of water. Mr. Thankful took out his pencil and, after careful mathematical calculations, arrived at this astounding conclusion: An unseen hand had conveyed from a remote distance, and deposited upon every acre of his little farm, more than fifty-six tons of water. He owns a hundred acres. There must therefore have been scattered upon the entire farm over 5,600 tons of rain, an amount so large that if he had been compelled to pay for its transportation it would have required more than all the income of his farm.
MAKING WATER BOIL BY APPLYING COLD WATER.
SOUND.
Our atmosphere is the medium of sound. Upon lofty mountains its vibrations become faint, while in a vacuum all sound ceases. The world of music, with which we are surrounded, were the air removed, would become forever silent. No song of birds, no murmur of the brook, no sighing of the trees, no thunder of the cataract, no grand diapason of the sea, no sweet voice of friendship, no thrilling words of love could ever again fall upon human ear. Gather together in one heap of useless rubbish (for they will never more be needed), harp, lyre, flute, flageolet, violin and guitar, piano and organ. Even that harp of three thousand strings, which the divine hand has placed in the human ear, shall not again vibrate to the delicate touch of nature’s hand.
ELECTRICITY AND METEORS.
We will close our present article by mentioning two other interesting atmospheric phenomena.
Dr. Franklin proved that lightning and electricity are identical. This wonderful agent manifests itself in a variety of ways. The zigzag track of light across the darkened sky, with its accompanying crash, is one of nature’s exhibitions of tremendous power. The irregularity of its path is due to the resistance of the air, compressed by the electric motion. The beautiful illumination called heat or sheet lightning, is caused by the reflection of the electric flash, at a great distance from the observer.
A very curious form of electricity is that known as St. Elmo’s fire, which appears as a glowing ball, often poising itself on the spars of ships, to the great consternation of superstitious sailors.
Judge Dana, in his admirable book, “Two Years Before the Mast,” more than once alludes to the sensation caused by these weird visitors, as they rounded stormy Cape Horn.
The northern lights, or aurora borealis, with their throbbing, shifting, crimson and purple tints, sometimes called “the merry dancers,” are supposed to be produced by the discharge of electricity in high altitudes and in rarefied air. All around us there is slumbering this power, which science may some day awaken to do the common work of the world.
Meteors, or “shooting stars,” as they are often incorrectly called, are small bodies, often not larger than grains of sand, which rush into our atmosphere at a speed equal to the earth’s motion, eleven hundred miles a minute, and by friction are set on fire, and blaze for a moment in the sky. Lockyer[16] says that seven millions of these, visible to the naked eye, traverse our atmosphere in a single day, and that a powerful telescope would reveal in the same time not less than four hundred millions.
Once in thirty-three years an astonishing display of these celestial fireworks takes place. The last was in 1866. At that time these bodies chanced to cross the track of the earth’s orbit, and were thus brought into collision with it. The largest of them, called meteorites, sometimes pass through the atmosphere unconsumed and reach the earth. They have been known to kill both men and cattle.
In 1866 one thousand of these stones, the largest weighing six hundred pounds, fell in Hungary.
It is very incorrect to call these flashing bodies in the air shooting stars, for they are extremely minute in size, while stars are vast suns; again, in point of distance, they are different, being near at hand, while the latter are millions of miles away. It would be difficult to find an instance in which language can convey a greater error than this phrase, which constantly implies that vast worlds, by thousands, are flying hither and thither, like sky-rockets. Often a single glance at the sky on a clear night, would show how unsafe this world would be as the object of such tremendous cannonading.
End of Required Reading for January.