166 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 northeast trade-winds differ in 

 temperature (and why not ?) from that brought by the southeast 

 trades, we have the authority of nature for saying that the two 

 currents would not readily commingle (§ 98). Proof is daily af- 

 forded 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 difficulty as to the two currents of air, which come into those 

 calm belts from different directions, crossing over, each in its prop- 

 er direction, without mingling. 



857. The same process of reasoning which conducted us (§ 355) 

 The rain winds in the i^to the tradc-wiud TCgiou of the northern hemi- 

 Missisaippi Valley, gplierc for the sources of the Patagonian rains, now 

 invites us into the trade-wind regions of the South Pacific Ocean 

 to look for the vapor springs of the Mississippi. If the rain winds 

 of the Mississippi Yalley come from the east, then we should have 

 i^eason to suppose that their vapors were taken up from the Atlan- 

 tic Ocean and Gulf Stream ; if the rain winds come from the south, 

 then the vapor 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 

 great Pacific Ocean, should we look for the place of evaporation ? 

 Wondering where, I addressed a circular letter to farmers and 

 planters of the Mississippi Yalley, requesting to be informed as to 

 the direction of their rain winds. I received replies from Virginia, 



