Page:Popular Science Monthly Volume 9.djvu/198

178 and they tend more than anything else to that miserable "breaking-down afterward" of which I have already spoken.—Chambers's Journal.



HE element of all others most sensitive to the changes and impulses of ever) 7 kind of force is the earth's atmosphere. It is in a state of constant disturbance, and seems to be obedient to no laws or regularity. Yet, unstable as the winds appear, they are really, in their general movements, among the most orderly and effective agents in Nature. This is shown in a remarkable manner by their agency in impelling the great ocean-streams, and therefore their important influence on glacial phenomena. In order to make this evident, it will be necessary to explain in brief the general laws of their circulation.

The earth turns on its axis from west to east, and with it rotates daily the enormous envelope of the atmosphere. The velocity of rotation at the equator is something over 1,000 miles an hour; at thirty degrees distance it is about 150 miles an hour less. In higher latitudes it is still less; and at the poles nothing. Therefore, whenever the air moves north or south on the surface of the earth, it will carry with it a less or greater velocity of rotation than the places it passes over, and will turn into an easterly or westerly wind, according as it approaches or recedes from the equator. In the region of the sun's greatest heat, the air, rarefied and lightened, is continually rising, and cooler currents come in on both sides to take the place of the ascending volume. As these side-currents come from a distance of about thirty degrees from the equator, they have, at starting, an eastward velocity many miles an hour less than the localities they will eventually reach. Consequently they will appear to lag behind in all the course of their progress to the equator—that is, they will have a westerly motion united with their north and south movements. These are the great trade-winds, blowing constantly from the northeast on this side, and the southeast on the other side of the equator.

But the heated air, which has risen in immense volumes in the tropics, spreads out to the north and the south in the upper regions, passes entirely over the trade-winds, and comes down to the earth in the temperate zones. It, however, continues to have the velocity toward the east which it acquired at the equator, and, when it strikes the slower-moving latitudes, it will be traveling much faster than the regions it comes down upon. Hence the westerly winds that prevail almost constantly in the middle latitudes. 