366 niYSICAL GEOGRArUY or THE SEA, AND ITS METEOROLOGY. 



instead of blowing from that quarter for twelve months, as in other 

 seas, they blow only for six. During the remaining six months 

 they are turned back, as it were ; for, instead of blowing towards 

 the equator, they blow away from it, and instead of N.E. trades 

 we have S.W. monsoons. 



683. A low barometer in Northern India. — If the N.E. trade-winds 

 blow towards the equator by reason (§ 657) of the lower baro- 

 meter of the calm belt there, we should — seeing them turned 

 back and blowing in«the opposite direction as the S.W. monsoon 

 — expect to find towards the north, and at the place where they 

 cease to blow, a lower barometer than that of the equatorial calm 

 belt. The circumstances which indicate the existence of a lower 

 summer barometer — the period of the S.W. monsoon — in the 

 regions about northern India are developed by the law which 

 (§ 657) requires the wind to blow towards that place where there 

 is least atmospheric pressure. 



684. The S.W. monsoons " baching down." — The S.W. monsoons 

 commence at the north, and " back down," or work their way 

 towards the south. Thus they set in earlier at Calcutta than they 

 do at Ceylon, and earlier at Ceylon than they do at the equator. 

 The average rate of travel, or '' backing down to the south," as 

 seamen express it, is from fifteen to twenty miles a day. It 

 takes the S.W. monsoons six or eight weeks to "back down" 

 from the tropic of Cancer to the equator. During this period 

 there is a sort of barometric ridge in the air over this region, 

 wbicli we may call the monsoon wave. In this time it passes 

 from the northern to the southern edge of the monsoon belt, and 

 as it rolls along in its invisible but stately march, the air beneath 

 its pressure flows out from imder it both ways — on the polar side 

 as the S.W. monsoon, on the equatorial as the N.E. 



686. How they begin. — As the vernal equinox approaches, the 

 heat of the sun begins to play upon the steppes and deserts of 

 Asia with power enough to rarefy the air, and cause an uprising 

 sufficient to produce an indraught thitherward from the surround- 

 ing regions. The air that is now about to set off to the south as 

 the N.E. monsoon is thus arrested, turned bac*k, and drawn into 

 this place of low barometer as the S.W. monsoon. These plains 

 become daily more and more heated, the sun more and more 

 powerful, and the ascending columns more and more active ; the 

 area of inrushing air, like a circle on the water, is widened, and 

 thus the S.W. monsoons, "backing down" towards the equator, 



