Page:Physical Geography of the Sea and its Meteorology.djvu/35

Rh enough; let us see what results arise from them: As the constant exposure of the equatorial regions of the earth to the sun must necessarily there engender a vast amount of heat, and as his absence from the polar regions must in like manner promote an infinite accumulation of cold, to fit the entire earth for a habitation to similar races of beings, a constant interchange and communion betwixt the heat of the one, and the cold of the other, must be carried on. The ease and simplicity with which this is effected surpass all description. The air, heated near the equator by the overpowering influences of the sun, is expanded and lightened; it ascends into upper space, leaving a partial vacuum at the surface to be supplied from the regions adjoining. Two currents from the poles toward the equator are thus established at the surface, while the sublimated air, diffusing itself by its mobility, flows in the upper regions of space from the equator toward the poles. Two vast whirlpools are thus established, constantly carrying away the heat from the torrid toward the icy regions, and, there becoming cold by contact with the ice, they carry back their gelid freight to refresh the torrid zone.

29. Of diurnal rotation.—"Did the earth, as was long believed, stand still while the sun circled around it, we should have had directly from north and south two sets of meridional currents blowing at the surface of the earth toward the equator; in the upper regions we should have had them flowing back again to the place whence they came. On the other hand, were the heating and cooling influences just referred to to cease, and the earth to fail in impressing its own motion on the atmosphere, we should have a furious hurricane rushing round the globe at the rate of 1000 miles an hour—tornadoes of ten times the speed of the most violent now known to us, sweeping everything before them. A combination of the two influences, modified by the friction of the earth, which tends to draw the air after it, gives us the trade-winds, which, at the speed of from ten to twenty miles an hour, sweep round the equatorial region of the globe unceasingly.

30. Currents.—"Impressed with the motion of the air, constantly sweeping its surface in one direction, and obeying the same laws of motion, the great sea itself would be excited into currents similar to those of the air, were it not walled in by continents and subjected to other control. As it is, there are constant currents flowing from the torrid toward the frigid zone to supply