dred, or even three hundred miles. A very slow motion upward there will carry off the air in that direction as fast as the two systems of trade-winds, with their motion of twenty miles an hour, can pour it in; and that curds or flakes of air can readily cross each other and pass in different directions without interfering the one with the other, or at least without interfering to that degree which prevents, we all know. The brown fields in summer afford evidence in a striking manner of the fact that, in nature, flakes, or streamlets, or curdles of air do really move among each other without obstruction. That tremulous motion which we so often observe above stubble-fields, barren wastes, or above any heated surface, is caused by the ascent and descent, at one and the same time, of flakes of air at different temperatures, the cool coming down, the warm going up. They do not readily commingle, for the astronomer long after nightfall, when he turns his telescope upon the heavens, perceives and laments the unsteadiness they produce in the sky. If the air brought to the calm belt by the north-east trade-winds differ in temperature (and why not?) from that brought by the south-east trades, we have the authority of nature for saying that the two currents would not readily commingle (§ 98). Proof is daily afforded that they would not, and there is reason to believe that the air of each current, in streaks, or patches, or flakes, does thread its way through the air of the other without difficulty. Therefore we may assume it as a postulate which nature concedes, that there is no physical difficulty as to the two currents of air, which come into those calm belts from different directions, crossing over, each in its proper direction, without mingling.
357. The rain winds in the Mississippi Valley.—The same process of reasoning which conducted us (§ 355) into the trade-wind region of the northern hemisphere for the sources of the Patagonian rains, now invites us into the trade-wind regions of the South Pacific Ocean to look for the vapour springs of the Mississippi. If the rain winds of the Mississippi Valley come from the east, then we should have reason to suppose that their vapours were taken up from the Atlantic Ocean and Gulf Stream; if the rain winds come from the south, then the vapour springs might, perhaps, be in the Gulf of Mexico; if the rain winds come from the north, then the great lakes might be supposed to feed the air with moisture for the fountains of that river; but if the rains come from the west, where, short of the
