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 winds 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 way, 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 prevent 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 
