108 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
supply the rains for H would be taken up in the southeast trade- 
wind region by F, and conveyed thence by G, and delivered to H. 
And if this mode of reasoning be admitted as plausible — if it be 
true that G have the vapor which, by condensation, is to water 
with showers the extra-tropical regions of the northern hemisphere, 
Nature, we may be sure, has provided a guide for conducting G 
across this belt of calms, and for sending it on in the right way. 
Here it was, then, at this crossing of the winds, that I thought I 
first saw the foot-prints of an agent whose character I could not 
comprehend. Could it be the magnetism that resides in the oxy- 
gen of the air ? 
188. Heat and cold, the early and the latter rain, clouds and 
sunshine, are not, we may rely upon it, distributed over the earth 
by chance ; they are distributed in obedience to laws that are as 
certain and as sure in their operations as the seasons in their 
rounds. If it depended upon chance whether the dry air should 
come out on this side or on that of this calm belt, or whether 
the moist air should return or not whence it came — if such were 
the case in nature, we perceive that, so far from any regularity as 
to seasons, we should have, or might have, years of droughts the 
most excessive, and then again seasons of rains the most destruct- 
ive ; but, so far from this, we find for each place a mean annual 
proportion of both, and that so regulated withal, that year after 
year the quantity is preserved with remarkable regularity. 
189. Having thus shown that there is no reason for supposing 
that the upper currents of air, when they meet over the calms of 
Cancer and Capricorn, are turned back to the equator, but hav- 
ing shown that there is reason for supposing that the air of each 
current, after descending, continues on in the direction toward 
which it was traveling before it descended, we may go farther, 
and, by a similar train of circumstantial evidence, afforded by these 
researches and other sources of information, show that the air, 
kept in motion on the surface by the two systems of trade-winds, 
when it arrives at the belt of equatorial calms, and ascends, con- 
tinues on thence, each current toward the pole which it was ap- 
proaching while on the surface. 
190. In a problem like this, demonstration in the positive way 
is difficult, if not impossible. We must rely for our proof upon 
philosophical deduction, guided by the hghts of reason ; and in all 
