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. Now, in the case of these constant winds, the inertia is very nearly overcome, as they start from a circle in which the velocity to the eastward is about 750 miles per hour. If the inertia were fully overcome, there would be no perceptible wind; as the velocity is actually but fifteen to eighteen miles per hour, it appears that the friction encountered actually destroys from thirteen-fourteenths to fifteen-sixteenths of the inertia. Hence, we find these constant air currents toward the west are, in reality, the result of the earth carrying any object on its surface a little more rapidly than the atmosphere moves; so that these winds are precisely the same in principle as the well-known fact that when you run rapidly in still air (so-called), it seems that the wind is blowing directly in your face. In like manner, it appears that a wind from west to east is merely an air-current moving a little more rapidly than the earth revolves at that point. The relative difference between the velocity of air-currents must vary greatly; for a violent easterly or westerly wind very near the poles may equal or even exceed the speed of the rotation of that point; while the most violent tropical storms average between one-twentieth and one-eighth of the local rotation. The latter is not often exceeded. But whatever the relation of the respective velocities, it is clear that the velocity of the wind in general must depend largely on the amount of air abnormally heated, and upon the rapidity with which it is heated. So men have come to recognize that a period of unusually oppressive heat forebodes a storm of some sort. But few regard the unusual warmth as a reason of the storm. They are linked, in the popular mind, as antecedent and consequent, rather than as cause and effect.