106 THE PHYSICAL GEOGRAPHY OF THE SEA. 



179. Moreover, these researches afforded grounds for the sup- 

 position that the air of which the northeast trade-winds are com- 

 posed, and which comes out of the same zone of calms as do these 

 southwesterly winds, so far from being saturated with vapor at its 

 exodus, is dry ; for near their polar edge, the northeast trade- 

 winds are, for the most part, dry winds. Reason suggests, and 

 philosophy teaches, that, going from a lower to a higher tempera- 

 ture, the evaporating powers of these wdnds are increased ; that 

 they have to travel, in their oblique course toward the equator, 

 a distance of nearly three thousand miles ; that, as a general rule, 

 they evaporate all the time, and all the w^ay, and precipitate little 

 or none on their route ; investigations have proved that they are 

 not saturated with moisture until they have arrived fully up to the 

 regions of equatorial calms, a zone of constant precipitation. 



This calm zone of Cancer borders also, it was perceived, upon 

 a rainy region. 



180. Where does the vapor which here, on the northern edge 

 of this zone of Cancer, is condensed into rains, come from? — 

 and where, also — was the oft-repeated question — does the vapor 

 which is condensed into rains for the extra-tropical regions of 

 the north generally come from ? By what agency is it convey- 

 ed across this calm belt from its birth-place between the trop- 

 ics ? 



' 181. I know of no law of nature or rule of philosophy which 

 would forbid the supposition that the air which has been brought 

 along as the northeast trade-winds to the equatorial calms does, 

 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 northeast trade-winds again. I know of no agent 

 in nature which would jweve^it it from taking this circuit, nor do 

 I know of any which would compel it to take this circuit ; but 

 while I know of no agent in nature that would prevent it from 

 taking this circuit, I know, on the other hand, of circumstances 

 which rendered it probable that such, in general, is not the course 

 of atmospherical circulation — that it does 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. 



182. And I moreover knew of facts which go to strengthen 

 the supposition that the winds which have come in the upper 



