Page:Physical Geography of the Sea and its Meteorology.djvu/190

164 polar side, is clearly indicative of a crossing. Upon no other supposition can we account for the barrenness on one side, the fertility on the other. The following are also links in the chain of facts and circumstances which give strength to the supposition that the rains for the Lena and the Missouri are brought across the calm belt of Cancer by those currents of air which flow thence towards the pole as the prevailing counter-trades or south-westerly winds of the extra-tropical north. We have already seen (§ 353) that, on the north side of this calm zone of Cancer, the prevailing winds on the surface are from this zone towards the pole, and (Plate I., § 215) that these winds return as A B C through the upper regions from the pole; that, arriving at the calms of Cancer, this upper current, A B C, meets another upper current, S R, from the equator, where they neutralize each other, produce a calm, descend, and come out as surface winds, D E, or the trade-winds; and as T U, or the counter-trades. Now observations have shown that the winds represented by T U are rain winds; those represented by D E, dry winds; and it is evident that A B C could not bring any vapours to these calms to serve for T U to make rains of; for the winds represented by A B C have already performed the circuit of surface winds as far as the pole, during which journey they parted with all their moisture, and, returning through the upper regions of the air to the calm belt of Cancer, they arrived there as dry winds. The winds represented by D E are dry winds; therefore it was supposed that these are, for the most part, but a continuation of the winds A B C. On the other hand, if the winds A B C, after descending, do turn about and become the surface winds T U, they would first have to remain a long time in contact with the sea, in order to be supplied with vapour enough to feed the great rivers, and supply the rains for the whole earth between us and the north pole. In this case, we should have an evaporating region at sea and a rainless region ashore on the north as well as on the south side of this zone of Cancer; but investigation shows no such region. Hence it was inferred that B C and R S do come out on the surface as represented by Plate I. But what is the agent that should lead them out by such opposite paths? According to this mode of reasoning, the vapours which supply the rains for T U would be taken up in the south-east trade-wind region by O Q, and conveyed thence along the route Q R S to T. And if this mode of reasoning be admitted as plausible—if it be