Bain. 339 



slightly greater (so far as our present information extends), 

 than the downfall south of the medial line. And, therefore, 

 since the area of the northern region is less than the area of 

 the southern, it is clear that the annual downfall over any- 

 northern zone is, in general, considerably heavier than the 

 downfall over the corresponding southern zone. Now, if we 

 remember that the amount of aqueous vapour raised by evapo- 

 ration over the southern or watery hemisphere must neces- 

 sarily be much greater than the amount raised over the., 

 northern hemisphere, this result will appear a remarkable one. 

 One would expect to find a difference — and a very marked 

 difference — between the two hemispheres; but instead of 

 the excess of rainfall being in favour of the northern hemis- 

 phere, one would expect it to have been in favour of the 

 southern. 



If we assume with Maury that the north-easterly and south- 

 easterly trade-winds which meet near the equator merge, 

 respectively, into the north-westerly and south-westerly 

 counter-trades ; that is, that they cross over to the opposite 

 hemisphere to that in which they were generated, the difficulty 

 seems to vanish. For in this case, the downfall over the 

 northern hemisphere is due to evaporation over the southern 

 hemisphere, and vice versa. Maury adduces other arguments 

 in favour of his theory of an intercrossing of this sort. Sir 

 John Hers ch el, however, will not listen to Maury's views. 

 He " declines adopting the doctrine recently propounded of a 

 systematic crossing of the south-east and north-east trades at 

 the medial line. In so doing/' he is " in no way disturbed by 

 the phenomenon of infusorial dust of South American origin 

 which occasionally falls on the north-east of Africa," and so 

 on. I must confess that the balance of evidence seems to me 

 to lie on Maury's side in this instance. 



It may be asked, however, whether there is any occasion 

 to adopt either view as a systematic account of the laws 

 affecting the trades and counter-trades. May not Maury and 

 Herschel be like the two knights who saw opposite sides of 

 the same shield, and who — both right and both wrong — were 

 persuaded, one that the shield was silvern, the other that it 

 was golden. 



If we remember that the medial line marks a zone of* calm 

 towards which, from either hemisphere, immense masses of 

 moisture-laden air are continually being swept in, why should 

 we arbitrarily assign to the masses of air passing away above 

 from this calm zone, such a law of motion that every particle of 

 air which has originally come from the northern hemisphere 

 shall take one course, and every pai-ticle which has come from 

 the southern shall take an opposite one. It appears to me, on 



