Proximate Causes of certain Winds and. Storms. 267 



motion of rotation : thus, to an observer who thinks himself immov- 

 able, the wind seems to blow in a direction opposite to the rotation of 

 the earth, that is, from west to east, which in fact is the direction of 

 the trade winds."* 



As Laplace speaks doubtingly of this theory, remarking merely 

 respecting it, that it "seems to be the most probable" we may, with- 

 out subjecting ourselves to the charge of overweening and unreasona- 

 ble presumption, proceed to discuss its claims to accuracy, and state 

 our objections to it — our objections to it as a full, complete, and sat- 

 isfactory theory. The cause assigned by Laplace, has unquestiona- 

 bly a concurrent influence in the production of these winds* 



The trade winds are here represented as a secondary result of the 

 movement of the air overhanging the higher latitudes towards the 

 equator, that movement being caused by the more elevated tempera- 

 ture of the tract towards which the current is directed,- We are led 

 to inquire why it is, that this current and the resulting wind are con- 

 fined within the limits of thirty degrees on each side of the line. 

 Why does not the air rush with as great velocity from the parallel of 

 60° towards that of 30°, as from the parallel of 30° towards the 

 equator, and produce a trade wind within the former, as well as with- 

 in the latter limits, especially as both of the causes upon which the 

 trade winds are made by Hadley to depend, operate with greater 

 energy in the higher than in the lower latitudes. 



(a.) The first of these causes is the excess of the temperature of 

 the equatorial regions over that of the countries lying nearer to the 



poles — of the tract under die equator, above that under the parallel 

 of 30°. But the heat at the parallel of 30° exceeds that of the par- 

 allel of 60° more than it is itself exceeded by the heat at the equa- 

 tor. Both theory and observation lead us to this conclusion. See 

 Halley's paper in the Philosophical Transactions, and Emerson's 

 Miscellanies, for a mathematical determination of the amount of heat 

 communicated by the sun's rays in different latitudes. Supposing 

 the sun to remain on the equator, it varies as the cosine of the lati- 

 tude. But the cosine diminishing more rapidly for ft given number 

 of degrees in the high than in the low latitudes, so must also the 

 heat ; or the mean temperature at 60° must differ more from that of 

 30° than this last does from that of the equator. With this agrees 

 the remark of one who had ample opportunity for observation. 



• PomTs Translation of the System of the World 



