§ GS3-GSG. MONSOONS. 36,5 



688. If the N.E. trade-winds blow toward the equator bj reason 

 A low barometer in (§ ^5'^) of the lowcr barometer of the calm belt there, 

 Northern India. ^^^q gjiould — Seeing them turned back and blowing 

 in the opposite direction as the S.W. monsoon — expect to find to- 

 ward the north, and at the place where they c^ase to blow, a low- 

 er barometer than that of the equatorial calm belt. The circum- 

 stances which indicate the existence of a low 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 toward that place where there is least atmospheric press- 

 ure. 



684. The S.W. monsoons commence at the north, and "back 

 The s.w. monpoons dowu," or work their way toward the south. Thus 

 -backing down." ^-^^^ g^^ ^^ earlier at Calcutta than they do at Cey. 

 Ion, and earlier at Ceylon than they do at the equator. The aver- 

 age rate of travel, or '' backing down to the south," as seamen ex- 

 press 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 bar- 

 ometric ridge in the air over this region, which 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 in- 

 visible but stately march, the air beneath its pressure flows out 

 from under it both ways — on the polar side as the S.W. monsoon, 

 on the equatorial as the N.E. 



685. As the vernal equinox approaches, the heat of the sun 

 How they begin, bcgius to play "upon the steppes and deserts of Asia 



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

 cient to produce an indraught thitherward from the surrounding 

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

 N.B. -monsoon is thus arrested, turned back, and drawn into this 

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

 come daily more and more heated, the sun more and more pow- 

 erful, 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" toward the equator, drive 

 the N.E. monsoons from the land, replace them, and gradually ex- 

 tend themselves out to sea. 



686. Coming now from the water, they bring vapor, which, 



