THE ATMOSPHERE. 93 



252. There are at least two forces concerned in drmng the 

 Forces T^hich propel wind throiigh its circuits. We have seen (§ 207) 

 the wiiKi. 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 soui'ce whence they derive their northing and 

 their southing. Philosophers foi-merly held that the trade-winds 

 are dra^Ti 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 the south ; and as the solar played with 

 greatest effect at the equator, there the ascent of the air and the 

 meeting of the two winds would naturally be. So it was held, and 

 such was the doctrine. 



253. But the direct rays of the sun, instead of being most 

 Effect of the direct powcrful upoH the air at the equator, is most 



heat of the sun upon ■•■ r» i i j i • i • i i • p . i 



the trade-winds. powertul whcre the sun is vertical; and if the 

 trade-winds were produced by direct heat alone ffom the sun, 

 the place of meeting would follow the sun in declination much 

 more regularly than it does. But, instead of so follo^\ang the sun, 

 tiie 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 12° 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 received from the vertical ray 

 in a day there is very much greater (jV) than it is when she is 

 in aphelion, as in our midsummer. For this reason the southern 

 summer is really hotter than the northern ; yet, notwith- 

 standing 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 days,* whereas 

 the north-east trades have rarely the force to reach the south side of 

 the equator at all. 



254. By examining the log-books of vessels while sailing 

 The two sj-stems ..f tln'ough tlio uorth-cast and south-east trade-wind 

 b'Jthi^'force^Xr^- bclts, and Comparing their rate of sailing, it has 

 tion. and stability. -^^^^ asccrtaincd that ships sail faster with the 

 south-east than they do with the north-east trade-winds, and that 



* " The Winds of the Sea," Maury's Nautical Monograplis, No. 1. 



