Magnetism and the Trade Winds. 273 



ascending column of air there, and which residuum was 

 brought back in the upper regions of the atmosphere by the 

 current which we know perpetually blows up there, counter 

 to the trade winds. 



We know that there is an upper current of perpetual 

 winds from the equatorial to the tropical calms, — that the 

 volume of air moved by these two upper currents, north and 

 south, to Cancer and Capricorn, is equal to the volume that 

 is felt on the surface, as the NE. and SE. trade winds. 



I knew of no law of nature which ^vould forbid the suppo- 

 sition that the air which has been brought down as the NE. 

 trade winds to the equatorial calms, should, after ascending 

 there, return by the counter and upper currents to the calm 

 zone of Cancer, here descend and reappear on the surface as 

 the NE. trade winds again. I knew of no agent in nature 

 which would prevent it from taking this circuit ; but while I 

 knew of no agent in nature that would prevent it from taking 

 this circuit, I knew on the other hand, of circumstances which 

 rendered it probable that such in general is not the course of 

 atmospherical circulation — that it did not take this circuit. 

 I speak of the rule, not of the exceptions ; these are infinite, 

 and for the most part are caused by the land. 



And I, moreover, knew of facts which greatly strengthen 

 the supposition, that after arriving at the calms of Cancer 

 and descending, the winds which have come in the upper 

 regions of the atmosphere from the equator, do not return 

 to the equator on the surface, but continue on the surface 

 towards the pole. 



And these are the circumstances which favour the conjec- 

 ture that the winds which flow in the upper regions of the 

 atmosphere from the equator to the calms of Cancer, do not, 

 after arriving and descending in the midst of these calms, 

 turn about and go back to the equator with the NE. trades ; 

 on the contrary, these are the facts and circumstances which 

 give strength to the supposition that they continue on to- 

 wards the pole, as the prevailing south-westerly winds of the 

 extra tropical north : 



We have seen that on the north side of this calm zone of 

 Cancer, the prevailing winds on the surface are from this 



