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.