BASTING OF THE TEADB-WINDS, ETC. 171 



dred, or even tliree hundred miles. A very slow motion upward 

 there will cany 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 inter- 

 fering 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 difierent directions, crossing over, 

 each in its proper direction, without mingling. 



357. TJie rain ivinds in the Mississippi Valley. — The same pro- 

 cess of reasoning which conducted us (§ 355) into the trade-wind 

 region of the northern hemisphere for the sources of the Pata- 

 gonian rains, now invites us into the trade-wind regions of the 

 South Pacific Ocean to look for the vapour springs of the Missis- 

 sippi. 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 sup- 

 posed to feed the air with moisture for the fountains of that 

 river ; but if the rains come from the west, where, short of the 



