THE ATMOSPHERE. 97 



this latitude. Have I not, therefore, very good grounds for the 

 opinion (§ 200) that the " wind in his circuits," though ap- 

 parently to us never so wa3^ward, is as obedient to law and as 

 subservient to order as were the morning stars when first they 

 *'sang together?" 



252. Forces which propel the wind. — There are at least two forces 

 concerned in driving the wind through its circuits. We have 

 seen (§ 207) whence that force is derived which gives easting to 

 the winds as they approach the equator, and westing as they 

 approach the poles ; and allusion, without explanation, has been 

 made (§ 212) to the source whence they derive their northing 

 and their southing. Philosophers formerly held that the trade- 

 Avinds are drawn towards the equator by the influence of the 

 direct rays of the sun upon the atmosphere there. They heated 

 it, expanded it, and produced rarefaction, thereby causing a rush 

 of the wind both from the north and south ; and as the solar rays 

 plaj-ed with greatest effect at the equator, there the ascent of the 

 air and the meeting of the two winds -svould naturally be. So it 

 was held, and such was the doctrine. 



253. Effect of the direct heat of the sun upon the trade-icinds. — 

 But the direct rays of the sun, instead of being most powerful 

 upon the air at the equator, are most powerful where the sun is 

 vertical ; and if the trade-winds were produced by direct heat 

 alone from the sun, the place of meeting would follow the sun 

 in declination much more regularly than it does. But, instead 

 of so following the sun, the usual place of meeting between the 

 trade-winds is neither at the equator nor where the sun is 

 vertical. It is at a mean between the parallels of 5'^ and 10° or 

 1 2'^ N. It is in the northern hemisphere, notwithstanding the 

 fact that in the southern summer, when the sun is on the other 

 side of the line, the earth is in perihelion, and the amount of 

 heat Teceived from the vertical ray in a day there is ver}^ much 

 greater ( J3-) than it is when she is in aphelion, as in our mid- 

 summer. For this reason the southern summer is reall}- hotter 

 than the northern ; yet, notwithstanding this, the south-east 

 trade- winds actually blow the air away from under this hot 

 southern sun, and bring it over into the northern hemisphere. 

 They cross over into the northern hemisphere annually, and blow 

 between 0°and 5" N. for 193 daj's,* whereas the north-east trades 

 have rarely the force to reach the south side of the equator at all. 



* " The Winds of the Sea," Maury's Xautical Monographs, No. 1. 



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