132 THE MICROSCOPE. 



equilibrium. 1 Two systems of currents, an up- 

 per and an under, are thus ever travelling be- 

 tween the equator and the poles, and the causes 

 above-mentioned, in connexion with the rotary 

 motion of the earth from west to east, produce 

 permanent north-easterly or south-easterly cur- 

 rents, forming the " magnificent phenomena" 

 of the Trade Winds, perpetually extending 

 round the earth, from the parallels of about 30 

 north and south, nearly to the equator. 



It is evident that these two currents of air, 

 ever pouring from the poles to the equator, must 

 return by some channel to the poles, otherwise, 

 as Maury observes, the wind of the polar regions 

 would soon be exhausted, and it would all be 

 piled up about the equator. But if they return 

 from the equator to the poles, two things follow ; 

 first, this return current must be in a direction 

 opposite to that wind the place of which it goes 

 to supply ; and, secondly, this return current 

 must be in tlie upper regions of the atmosphere, 

 at least until it has passed over those parallels 

 where the trade-winds are always blowing on 

 the surface, and in a direction contrary to them. 



1 Johnston's Physical Atlas, p. 61,. 



