opposite direction as they move from the equator towards the poles. This turning is caused by the rotation of the earth on its axis.
207. Effect of diurnal rotation on the course of the trade-winds.—The earth, we know, moves from west to east. Now if we imagine a particle of atmosphere at the north pole, where it is at rest, to be put in motion in a straight line towards the equator, we can easily see how this particle of air, coming from the very axis of diurnal rotation, where it did not partake of the diurnal motion of the earth would, in consequence of its vis inertæ, find, as it travels south, the earth slipping from under it, as it were, and thus it would appear to be coming from the north-east and going towards the south-west; in other words, it would be a north-east wind. The better to explain, let us take a common terrestrial globe for the illustration. Bring the island of Madeira, or any other place about the same parallel, under the brazen meridian; put a finger of the left hand on the place; then moving the finger down along the meridian to the south, to represent the particle of air, turn the globe on its axis from west to east, to represent the diurnal rotation of the earth, and when the finger reaches the equator, stop. It will now be seen that the place on the globe under the finger is to the southward and westward of the place from which the finger started; in other words, the track of the finger over the surface of the globe, like the track of the particle of air upon the earth, has been from the northward and eastward. On the other hand, we can perceive how a like particle of atmosphere that starts from the equator, to take the place of the other at the pole, would, as it travels north, and in consequence of its vis inertiæ, be going towards the east faster than the earth. It would therefore appear to be blowing from the south-west, and going towards the north-east and exactly in the opposite direction to the other. Writing south for north, the same takes place between the south pole and the equator.
208. Two grand systems of currents.—Such is the process which is actually going on in nature; and if we take the motions of these two particles as the type of the motion of all, we shall have an illustration of the great currents in the air (§ 204), the equator being near one of the nodes, and there being at least two systems of currents, an upper and an under, between it and each pole. Halley, in his theory of the trade winds, pointed out the key to the explanation, so far, of the atmospherical circulation;