EASTING OF THE TRADE-WINDS, ETC. 159 



•excessive, and then again seasons of rains the most destmctive ; 

 but, so far fi'om this, we find for each place a mean aminal pro- 

 portion 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 

 cm-rents of air, when they meet over the calms of Cancer and Ca- 

 IDricorn, are tmned back to the equator, but having shovm that 

 there is reason for supposing that the air of each current, after 

 descending, continues on in the direction towards which it was tra- 

 velling before it descended, we may go farther, and, by a similar 

 train of circumstantial evidence, afforded by these researches and 

 other som-ces of information, show that the air, kept in motion on 

 the sm'face by the two systems of trade-winds, when it arrives at 

 the belt of equatorial calms and ascends, continues on thence, each 

 cmTent towards the pole which it was approaching while on the 

 smface. In a problem like this, demonstration in the positive 

 way is difficult, if not impossible. We must rely for our proot 

 upon philosophical deduction, guided by the lights of reason ; and 

 in all cases in which positive proof cannot be adduced, it is per- 

 mitted to bring in cii'cumstantial evidence ; and the circumstan- 

 tial e^ddence afforded by my investigations goes to show that the 

 winds represented by Q, § 215, tZo become those represented by 

 E S T U Y A, and A B C D E F respectively. In the fii^t 

 place, Q represents the south-east trade- winds — i. e., all the winds 

 of the southern hemisphere as they approach the equator ; and is 

 there any reason for supposing that the atmosphere does not pass 

 freely from one hemisphere to another ? On the contrary, many 

 reasons present themselves for supposing that it does. If it did 

 not, the proportion of land and water, and consequently of plants 

 and warm-blooded animals, being so different m the two hemi- 

 spheres, we might imagine that the constituents of the atmosphere 

 in them would, in the com'se of ages, probably become different 

 also, and that consequently, in such a case, man could not safely 

 pass from one hemisphere to the other. Consider the manifold 

 beauties in the whole system of terrestrial adaptations ; remember 

 what a perfect and wonderful machine (§ 268) is this atmosphere ; 

 how exquisitely balanced and beautifully compensated it is in all 

 its parts. We know that it is perfect ; that in the performance of 

 its various offices it is never left to the guidance of chance — no, 

 not for a moment. Wherefore I was led to ask myself why the 

 air of the south-east trades, when arrived at the zone of equatorial 



