3^4 Professor Mitchell wi tlie Trade-Winds. 



from the lower towards the higher latitudes. For it will hardly 

 be contended, that the air rushing towards the poles, might oc- 

 cupy the higher regions of the atmosphere, and communicate to 

 the strata below its motion eastward, without at the same time 

 communicating its motion northward or southward. Is the ex- 

 istence of such a current probable ? We have already seen that 

 the causes by which the trade-winds are produced (according 

 to the theory whose merits we are now endeavouring to estimate), 

 act with less energy within the parallel of 30° than without it. 

 Are we to embrace the opinion, nevertheless, that these causes 

 produce the trade-winds within the parallel, and also counteract 

 the operation of stronger causes, and determine the movements of 

 the atmosphere to the distance of 30° beyond, forcing back the air 

 of the temperate zone, notwithstanding its tendency to approach 

 the equator, into the neighbourhood of the pole ? This doctrine 

 is very improbable, and has no evidence to support it. If there 

 be an under current from the equator towards the poles, between 

 the latitudes of 30° and 60°, the air that is transferred by it must 

 be returned from the poles through the upper regions of the at- 

 mosphere, and the circulation be carried on in a curve, return- 

 ing twice into itself, or resembling a figure 8, placed horizontally, 

 and bent so as to apply itself to the arc of a circle. But we are 

 not left to argument and conjecture in the case. It has been 

 already shown that within the limits specified, there is no predo- 

 minance of wind directed from the equator towards the poles ; 

 the current is in the opposite direction. Of course, the westerly 

 winds of the temperate zones cannot be produced by winds blow- 

 ing from the equator ; and the objection to Hadley's theory, 

 drawn from the predominance of westerly winds, between the 

 latitude of 30° and 60°, remains unanswered, and it is believed 

 unanswerable. Other objections may be found in the writings 

 of Kirwan *, but it seems unnecessary to notice them. 



The account of the origin and cause of the trade- winds, given 

 by Dr Halley, was characterized by D'Alembert as obscure. Kir- 

 wan undertook to illustrate it, but does not appear to have been 

 altogether successful. It seems to have been misunderstood by 

 Playfair, by whom it is stated in the following terms : — " The 

 cause usually assigned for the trade-wind, is the constant motion 

 • See Philosophical Magazine, vol. xv. 



