MAGNETISM AND CIRCULATION OF THE ATMOSPHERE. I37 



pour into it is only about three miles, for that is supposed to be 

 about the height to which the trade-winds extend, 



350. Thus we have the air passing into these calms by an open- 

 ing on the north side for the northeast trades, and another on the 

 south for the southeast trades, having a cross section of three 

 miles vertically to each opening. It then escapes by an opening 

 upward, the cross section of which is sixty or one hundred, or even 

 three hundred miles. A very slow motion upward there will car- 

 ry 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 columns of air can readily cross each other and 

 pass in different directions without interfering the one with the 

 other, or at least to that degTce which obstructs or prevents, we 

 all know. 



351. 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. 



352. 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 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 columns of air at different temperatures, the 

 cool coming down, the warm going up. They do not readily com- 

 mingle, for the astronomer, long after nightfall, when he turns his 

 telescope upon the heavens, perceives and laments the unsteadi- 

 ness they produce in the sky. 



353. If the air brought down by the northeast trade-winds dif- 

 fer 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. Proof is daily afford- 

 ed 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 



