Professor Mitchell on the Trade-Winds. 35 



towards the west of the spot to which the sun is vertical, and 

 where of course the rarefaction is greatest. This, it is supposed, 

 draws along with it the air from the east. This, however, is by 

 no means a satisfactory explanation,*" &c.* 

 ipiHalley's theory, as here represented, 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 ex- 

 hibition of his views, which receive illustration from that part 

 of his paper where he treats of the monsoons, and in the accu- 

 rate conception of which we may derive aid from a diagram. 



Let L AM be a part of the equator, or of an adjacent paral- 

 lel of latitude, the spectator being on the north side of it. Let 

 B I G A be the lower stratum of the atmosphere, three or four 

 miles in thickness. Let the sun be vertical at A. The lower 

 part of the column A C will be heated and expanded, and the 

 portion B 6 lifted into the position & C, undergoing at the same 

 time a slight condensation -f*. The portion 6 C will therefore 

 have a tendency to flow over into the columns on each side of it. 

 But it cannot flow in the direction C E, because the sun, mov- 

 ing in the direction A C, at the rate of a thousand miles an hour, 

 and carrying the point of the greatest heat forward with the 

 same velocity : before the part 6 C has time to yield to the im- 

 pulse of elasticity and gravity, and flow into the columns west 

 of it, they will themselves have been heated and expanded, and 

 brought into the same condition with the column A C. It can- 

 not flow either north or south, or at least its tendency to escape 

 in those directions will be feeble, because all parts of the same 

 meridian will be heated at the same time. There remains there- 

 fore only the direction C D. But the sun having already passed 

 over the columns on the eastern side of A C, and they being cooled 

 * Outlines, voL i. p. 307. + See remarks connected with Fig. 1. 



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