2G6 Proximate Causes of certain Winds and Star/' : 



Now it is well known, that even within the limits of the trade 

 winds, and in the seas where they blow with great violence, an alter- 

 nation of land and sea breezes is experienced in islands of very mod- 

 erate extent — in the Sandwich islands for example. Where does 

 the land wind come from ? The atmosphere overhanging the island 

 would soon be exhausted. It must evidently be poured down from 

 above, and its great coldness is at once accounted for. But it reach- 

 es an inconsiderable distance only, seaward ; where does it go to ? 

 It must ascend, and having traced it through three-fourths of its en- 

 tire route, the remaining quarter, which we cannot reach to observe 

 it, may safely be inferred. When the sea breezes prevail, the mo- 

 tion is reversed, and probably also extends through a greater space. 

 An ellipsis, whose longer diameter. is parallel to the horizon, or some 

 other figure of the kind, may be described. 



Of the Causes of the Trade Winds. 



With the above facts and arguments before us, we are prepared 

 for an investigation of the proximate causes of the trade winds. Two 

 theories have, as is well known, been advanced upon this subject. 

 The earliest is contained in a paper of Dr. Halley's, read before the 

 Royal Society in 1686. The other, that of Hadley, was brought 

 forward in 1735, and as it is that which is generally adopted by the 

 oldest philosophers of the present age, it may be regarded as pre- 

 senting the strongest claim to our particular and continued attention. 

 It may be stated in the words of Laplace. 



" The sun, which we will suppose, for the sake of simplicity, in 

 the plane of the equator, there rarefies by its heat the columns of 

 air, and elevates them above their natural level ; they should then 

 re-descend by their weight, and be carried towards the poles in the 

 superior part of the atmosphere ; but at the same time a current of 

 cool air should arrive from the climates near the poles, to replace 

 that which has been rarefied at the equator. Thus two opposite cur- 

 rents of air are established, one in the inferior, the other in the supe- 

 rior part of the atmosphere. But the real velocity of the air, due to 

 the rotation of the earth, is so much the less, as it is nearer the pole ; 

 it ought, therefore, in advancing towards the equator, to turn slower 

 than the corresponding parts of the earth, and bodies placed at the 

 terrestrial surface, should strike against it with the excess of their ve- 

 locity, and experience by its reaction a resistance contrary to their 



