Rising up from the earth in the tropical belt, the air attains the height of several thousand feet; it then begins to curve off toward the north and south, and at the height of somewhere about three to five miles above the surface is again moving horizontally toward either pole; attaining a distance on that journey, it gradually settles down to the surface of the earth, and ceases to move toward higher latitudes. If the earth did not revolve upon its axis the course of these winds along the surface toward the equator, and in the upper air back toward the poles, would be made in what we may call a square manner--that is, the particles of air would move toward the point where they begin to rise upward in due north and south lines, according as they came from the southern or northern hemisphere, and the upper currents or counter trades would retrace their paths also parallel with the meridians or longitude lines. But because the earth revolves from west to east, the course of the trade winds is oblique to the equator, those in the northern hemisphere blowing from northeast to southwest, those in the southern from southeast to northwest. The way in which the motion of the earth affects the direction of these currents is not difficult to understand. It is as follows: Let us conceive a particle of air situated immediately over the earth's polar axis. Such an atom would by the rotation of the sphere accomplish no motion except, indeed, that it might turn round on its own centre. It would acquire no velocity whatever by virtue of the earth's movement. Then let us imagine the particle moving toward the equator with the speed of an ordinary wind. At every step of its journey toward lower latitudes it would come into regions having a greater movement than those which it had just left. Owing to its inertia, it would thus tend continually to lag behind the particles of matter about it. It would thus fall off to the westward, and, in place of moving due south, would in the northern hemisphere drift to the southwest, and in the southern hemisphere toward the northwest. A good illustration of this action may be obtained from an ordinary turn-table such as is used about railway stations to reverse the position of a locomotive. If the observer will stand in the centre of such a table while it is being turned round he will perceive that his body is not swayed to the right or left. If he will then try to walk toward the periphery of the rotating disk, he will readily note that it is very difficult, if not impossible, to walk along the radius of the circle; he naturally falls behind in the movement, so that his path is a curved line exactly such as is followed by the winds which move toward the equator in the trades. If now he rests a moment on the periphery of the table, so that his body acquires the velocity of the disk at that point, and then endeavours to walk toward the centre, he will find that again he can not go directly; his path deviates in the