138 THE MONSOONS. [SECT. xv. 



rally much nearer to the surface of the earth. They are 

 owing to currents of air running upon each other in hori- 

 zontal strata, and differing in their electric state, in tempera- 

 ture and moisture, as well as in velocity and direction. 



The monsoons are steady currents six months in duration, 

 owing to diminished atmospheric pressure at each tropic 

 alternately from the heat of the sun, thereby producing a 

 regular alternation of north and south winds which, combining 

 their motion with that of the earth on its axis, become a 

 north-east wind in the northern hemisphere, and a south-west 

 in the southern ; the former blows from April to October, 

 and the latter from October to April. The change from one 

 to the other is attended by violent rains, with storms of 

 thunder and lightning. From some peculiar conformation 

 of the land and water, these winds are confined to the 

 Arabian Gulf, the Indian Ocean, and the China Sea. 



When north and south winds blow alternately, the wind 

 at any place will veer in one uniform direction through every 

 point of the compass, provided the one begins before the 

 other has ceased. In the northern hemisphere a north wind 

 sets out with a smaller degree of rotatory motion than the 

 places have at which it successively arrives, consequently it 

 passes through all the points of the compass from K to N.E. 

 and E. A current from the south, on the contrary, sets out 

 with a greater rotatory velocity than the places have at which 

 it successively arrives, so by the rotation of the earth it is 

 deflected from S. to S.W. and W. Now, if the vane at any 

 place should have veered from the N. through N.E. to E., 

 and a south wind should spring up, it would combine its mo- 

 tion with the former and cause the vane to turn successively 

 from the E. to S.E. and S. But by the earth's rotation 

 this south wind will veer to the S.W. and W., and, if a north 

 wind should now arise, it would combine its motion with that 

 of the west, and cause it to veer to the N.W. and N. Thus 

 two alternations of north and south wind will cause the vane 

 at any place to go completely round the compass, from N. 



