Page:Physical Geography of the Sea and its Meteorology.djvu/191

Rh true that R S carry the vapour 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 S T 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 footprints of an agent whose character I could not comprehend. Can it be the magnetism that resides in the oxygen of the air? 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 drought the most excessive, and then again seasons of rains the most destructive; 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. 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 having shown that there is reason for supposing that the air of each current, after descending, continues on in the direction towards which it was travelling 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, continues on thence, each current towards the pole which it was approaching while on the surface. 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 lights of reason; and in all cases in which positive proof cannot be adduced, it is permitted to bring in circumstantial evidence; and the circumstantial evidence afforded by my investigations goes to show that the winds represented by O Q, § 215, do become those represented by R S T U V A, and A B C D E F respectively. In the first place, O Q represents the south-east trade-winds—i. e., all the