MONSOONS. 351 



the S.W. monsoons of the Indian Ocean, for example : a force 

 is exerted upon the N.E. trade-winds of that sea by the dis- 

 turbance which the heat of summer creates in the atmosphere 

 over the interior plains of Asia, which is more than sufficient 

 to neutralize the forces which cause those winds to blow as 

 trade-winds ; it arrests them and turns them back ; but, were- 

 it not for the peculiar conditions of the land about that ocean, 

 what are now called the N.E. monsoons would blow the year 

 round ; there would be no S.W. monsoons there ; and the 

 N.E. winds, being perpetual, would become all the year what 

 in reality for several months they are, viz., N.E. trade-winds. 



682. Upon India and its seas the monsoon phenomena are 

 The region of. devclopcd ou the grandest scale. These remark- 

 able winds blow over all that expanse of northern w^ater that lies 

 between Africa and the Philippine Islands. Throughout this vast 

 expanse, the winds that are known in other parts of the world as 

 the N.E. trades, are here called N.E. monsoons, because, 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 ^Ye 

 have S.W. monsoons. 



683. If the N.E. trade-winds blow towards the equator by reason 

 A low barometer iu (§ 657) of the lowcr baromcter of the calm belt 

 Northern India. thorc, WO should — scoing 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 commence at the north, and "back 

 The S.W. monsoons dowu," or work their way towards the south. Thus 

 "backing down." ^^gy gg^. [^ 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 tw^enty 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 



