§ 246-248. THE ATMOSPHERE. ^-^ 



2-16. But the direct rays of the sun, instead of being most pow- 

 Effect of the direct eiful UDOU the aif at the equator, is most powerful 



heat of the sun upon ^ , , , ^ ^ 



the trade-winds. 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 12° K It is in the northern hemisphere, not- 

 withstanding the fact that in the southern summer, when the sun 

 is on the other side of the line, the earth is in perihelion, cmd the 

 amount of heat received from the vertical ray in a day there is 

 very much greater (yV) 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, notwithstanding this, the southeast 

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

 ern sun, and bring it over into the northern hemisphere. They 

 cross over into the northern hemisjDhere annually, and blow be- 

 tween 0" and 5° N. for 198 days,^ whereas the northeast trades 

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



2-17. By examining the log-books of vessels while sailing 

 The two systems cf through the northcast and southeast trade-wind 

 Sh'rn'fo^nural ^elts, and comparing their rate of sailing, it has 

 tion,andstabmt7. ^^^^ asccrtaiucd that ships sail faster with the 

 southeast than they do with the northeast trade-winds, and that 

 the southeast blow more days during the year than do the north- 

 east trades.f The logs of vessels that spent no less than 166,000 

 days in sailing through these two belts of wind show that the aver- 

 age sailing speed through the southeast trade- wind belt, which lies 

 between the equator and 80° S., is about eight miles an hour, and 

 the average number of uninterrupted southeast trade-wind days 

 in the year is 227. For the northeast it is 183 days, with strength 

 enough to give ships an average speed of only 5.6 miles an hour. 

 Hence it appears that the two systems of trade-winds are very 

 unequal both as to force and stability, the southeast surpassing in 

 both. 



248. Moreover, the hottest place within the trade-wind regions 



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



t See Maun's Wind and Current Charts, vol. ii., 8th edition, Sailing Directions 



