270 



>f 



by it must be returned from the poles through the upper regions of the 

 atmosphere, and the circulation be carried on in a curve, returning 

 twice into itself, or resembling a figure eight, 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 predominance of winds di- 

 rected from the equator towards the poles ; the current is in the op- 

 posite direction. Of course, the westerly winds of the temperate 

 zones cannot be produced by winds blowing from the equator ; and 

 the objection to Hadley's theory, drawn from the predominance of 

 westerly winds, between the latitudes of 30° and 60°, remains unan- 

 swered, 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. Kirwan 

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

 successful. It seems to have been misunderstood by Playfair, by 



: ' The cause usually as- 

 signed for the trade wind, is the constant motion toward the west of 

 the spot to which the sun is vertical, and where of course the rare- 

 faction is greatest. This, it is supposed, draws along with it the air 



from the east. This, however, is by no means a satisfactory explan- 

 ation, etc."f 



Halley's theory, as here presented, verges so closely on the absurd 

 and ridiculous, that we cannot, without doing injustice to its very acute 

 and able author, accept of it as a correct exhibition of his views— 

 which receive illustration from that part of his paper where he treats 

 of the monsoons, and in the accurate conception of which we may 

 derive aid from a diagram. c 



Let LAM be a part 

 of the equator, or of 

 an adjacent parallel of 

 latitude, the spectator 



whom it is stated in the following terms : 



being 



on the north 

 side of it. Let BIG A L 



M 



* Sec Philosophical Magazine. Vol. 



xv 



i Outlines, Vol. i, P. 307. 



