Great Disasters and Horrors in the World's History

by Allen Howard Godbey

“The world do move.” The illustration so full of meaning two thousand years since has lost much of its force. The truth of yesterday is the error of to-day. The fact of to-day may be the phantasy of to-morrow. So it has come to pass that in our day the origin and laws of air currents are believed to be as well understood as those of any other forces in nature. Yet scientific theorists are, after all, divided on not a few points.

Two general classes of winds are recognized: the constant, and variable. Constant winds are those that blow all the year in the same direction. The beautiful concept of Kingsley, in the preceding chapter, contains the leading points of our knowledge concerning them.

All the various phenomena of air currents are dependent upon one unchanging law: that gaseous bodies—and all but two others—always greatly expand under the influence of heat. There are two noted partial exceptions: one of these prevents our globe from becoming a complete iceberg, and is as important as the law itself. Iron expands, till its melting point; but in its liquid state it occupies less space than when solid. Water contracts under the influence of cold, until the temperature of 39° is reached; after that it expands: and when frozen occupies about one-eighth more space than before. This wise provision of the Creator is second to none in importance, as regards its influence upon the climate of the earth at large. Had it been otherwise—did ice sink instead of float, our rivers and seas would in time become solid masses of ice; for water is so poor a conductor of heat, that its under-currents warm very slowly. Any one who plunges into a lake in mid-summer may often find the water warm at the surface, and of almost icy coldness a short distance beneath. The great Polar current comes down from Baffin’s Bay, and off the coast of Newfoundland it plunges beneath the warm, lighter current of the Gulf Stream; but it is not warmed by it. Registering thermometers detect its icy coldness almost unchanged in the realms of the tropics, far beneath the surface.

Note some simple illustrations of the expansive force of freezing water. Every housewife knows that a bottle left full of water will burst when the water freezes. The same power is shown in the gradual disintegration of rocks by alternate freezing and thawing. Water freezing in the crevices bursts off small particles, or even large fragments; so that rocks long exposed to the weather, crumble more or less. Every one is familiar with the appearance presented by steep clay-banks, in late winter and early spring, of ragged masses and fragments ready to fall at any time. Still another instance of this destructive power is shown in the killing of vegetation by freezing. Plants are built of myriads of tiny cells. The moisture within freezes and bursts the cell-walls, destroying the plant life. Certain plants have cells more elastic than others, which in consequence are not destroyed by freezing. But as an expanded cell does not readily shrink to its former size, subsequent freezings, when the cell contains more water than before, may finally destroy it. So wheat is “winter-killed,” by too frequent freezing. So globes of steel may be burst by this force.

To show the poor qualities of water as a conductor of heat, take a long glass tube and fill with water. Then put a piece of ice in one end. The water at the other end may now be brought to the boiling point by means of the flame of a lamp, ere the ice at the other end is melted.

Every one is familiar with the fact that heated air rises; but not all inquire why it does so. Take a foot-ball or bladder and partially inflate it; then hold it near a hot fire, and it may be swollen almost to bursting. Now, there is no more air in it than before; and if it be laid in a cold place, it will shrink to its first inflation. This shows how great is the expansive power of heat on the atmosphere. The same weight occupying a much larger bulk, we perceive that heated air is much lighter, and must rise. This, then, is the cause of what are known as constant winds.

As the earth revolves on its axis, the air is unequally heated, that nearest the equator becoming the warmest, in consequence of its receiving the most direct rays. Here, then, the air rises most rapidly; while the cooler air to the north or south must flow southward or northward to fill the vacuum. Now, the earth turning on its axis from west to east, whirls the northward and southward currents to the westward, so that they appear to blow from the northeast and southeast. The result of this loss of direction is gradual; so that when first perceptible, they are almost from a due northerly or southerly direction. As they near the equator, they are more rapid, and turn more decidedly to the west, never becoming violent, however; rarely exceeding fifteen to eighteen miles per hour.

It would appear that at the point where these meet each other, or come in contact with the ascending warm current, there must be a region of calms or light, variable winds, and occasional tempests. Such, in fact, is the case. This belt is from two hundred and eighty to four hundred miles in width, and lies along the thermal equator, or line of greatest average heat. This is not the same at the earth’s equator, properly so called; for, as the land has greater capacity for absorbing and retaining heat than the sea, and as most of the land lies in the northern hemisphere, it is evident the highest mean temperature must be north of the equator. So this belt of calms must lie in the same region; and, in fact, in the Atlantic ocean it lies between 3 and 9° north latitude, and in the Pacific, between 4 and 8°. As the sun travels northward during the first half of the year, this region of calms shifts slightly, also, so as to always nearly coincide with belt of the greatest mean heat.

At first sight, it appears curious that the motion of the earth should deflect these winds to the west. It would appear that the earth, atmosphere and all, must revolve as a unit about its axis; else, if the atmosphere lose time, its speed to the westward should be constantly accelerated, and long ago should have reached a velocity that would shake the mountains themselves; while, in fact, there is no variation perceptible.

It should be remembered that at the equator the earth is about twenty-four thousand miles in circumference; and as one complete revolution is made every twenty-four hours, a point on the equator is carried eastward at the rate of one thousand miles an hour. But if a circle be drawn around the earth parallel to the equator, at some distance from it, it is at once seen that any object in this circle, having a shorter distance to traverse, is carried eastward at a slower rate; so that a point only a few yards from either pole must necessarily advance but a few feet per hour. So then, a body of air moving from either pole toward the equator, must needs advance very slowly if the friction of the upper reverse currents and of the surface of the globe are to have opportunity to overcome its relative inertia and give it the same velocity as that of any point over which it may pass.