Page:Physical Geography of the Sea and its Meteorology.djvu/123

Rh this latitude. Have I not, therefore, very good grounds for the opinion (§ 200) that the "wind in his circuits," though apparently to us never so wayward, 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-winds 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 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. Effect of the direct heat of the sun upon the trade-winds.—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 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 (1/15) than it is when she is in aphelion, as in our mid-summer. For this reason the southern summer is really 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 days, whereas the north-east trades have rarely the force to reach the south side of the equator at all.