Professor Mitchell an the Trade-Winds. 33 



rallels over which it passes in its progress towards the equator, and 

 the rapidity of the current flowing westward, will be greater in 

 proportion as the differences in the circumferences of the succes- 

 sive parallels is greater. But these differences depending upon 

 the differences of the cosines of the latitudes, must be greatest 

 in the high latitudes ; and supposing the movement of the air 

 towards the equator to be the same as within the parallel of 30^, 

 the trade- winds should not only exist, but blow more violently 

 there. 



It appears, therefore, that both of the causes on which the trade- 

 winds are made by Hadley's theory to depend, operate with 

 greater energy between the parallels of 30° and 60°, than within 

 the actual limits of the trades, and yet fail of producing any 

 wind. Not only is there no trade-wind there,\but there is in 

 both the northern and southern hemispheres, a decided predomi- 

 nance of winds from the west. It is generally regarded as a 

 sound maxim in philosophy, that when a particular effect is at- 

 tributed to the action of a certain cause, if, on the reproduction 

 of the cause, the effect fails to follow, we are to conclude there 

 was an error in the first instance, and the original effect is to be 

 traced to some other source. 



An attempt is however made by some of the philosophers who 

 reject altogether the theory of Halley, and embrace the views of 

 Hadley, to account for the fact that the trade-winds are limited 

 by the 30th parallel, and that the westerly winds prevail in the 

 regions lying beyond it. It is said that the air which is rarefied 

 and ascends about the equator, flows off towards the poles, that 

 being cooled and condensed, it at length descends to the earth, 

 and retaining its original velocity, moves eastward faster than 

 the parallel over which it is incumbent, producing a wind from 

 the west ♦. He (Mr Daniell) remarks that the restriction of the 

 trade-winds within the 30th degree of latitude, can be accounted 

 for on no other hypothesis. Not upon his principles. It, how- 

 ever, may be accounted for on different grounds. Now, accord- 

 ing to this hypothesis, the westerly winds of the temperate zones 

 are a secondary result of a current flowing from the equator to- 

 wards the poles. They prevail at the surface of the earth, and 

 can therefore he generated (mly by a ground current, directed 

 * See Daniell's Meteorological Essays, p. 104. 



OCT0JJ£K— DECEMBER 1831. C 



