114 THE PHYSICAL GEDGRAPHY OF THE SEA. 



or at least to that degree which obstructs or prevents, we all 

 know. 



206. For example, open the window of a warm room in winter, 

 and immediately there are two currents of air ready at once to set 

 through it ; viz., a current of warm air flowing out at the top, and 

 one of cold coming in below. 



But the brown fields in summer afford evidence on a larger 

 scale, and in a still more striking manner, of the fact that, in na- 

 ture, columns, or. streamlets, or curdles of air do readily move 

 amonof 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 columns of air at different tempera- 

 tures, 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. 



207. If the air brought down by the northeast trade-w4nds differ 

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

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

 currents would not readily commingle. 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 curdles, does thread its way 

 through the air of the other without difficulty. Now, if the air of 

 these two currents differs as to magnetism, might not that be an 

 additional reason for their not mixing, and for their taking the di- 

 rection of opposite poles after ascending? 



208. 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, cross- 

 ing over, each i^i its proper direction, without mingling. 



209. Thus, having shown that there is nothing to prevent the 

 crossing of the air in these calm belts, I return to the process of 

 reasoning by induction, and offer additional circumstantial evi- 

 dence to prove that such a crossing does take place. Let us 

 therefore catechise, on this head, the waters which the Mississippi 

 pours into the sea, inquiring of them as to the channels among the 

 clouds through which they were brought from the ocean to the 

 fountains of that mighty river. 



