SECT. XV. CLOUD RING MONSOONS. 123 



that from the S. to the tropic of Cancer. But while travelling 

 in these lofty regions they become cold and heavy, and, sinking 

 to the surface at the tropics, each proceeds to the opposite pole 

 from which it set out. Now, however, they have a greater rota- 

 tory motion than the places they successively arrive at, so the 

 bias is to the W., and they become the N.W. and S.W. extra- 

 tropical winds. 



If on arriving at the poles the air were to accumulate there, 

 the circulation of the winds would cease ; but currents rise into 

 the upper regions, and flow back again to the tropics, where 

 they sink down to fill the vacuum caused by the great precipita- 

 tion of vapour in these regions, and then flow to the equator as 

 trade- winds (N. 177). So the currents of air cross again at 

 the tropics and produce two belts of calms which surround 

 the globe, named by Lieutenant Maury the Calms of Cancer 

 and the Calms of Capricorn, but generally know to sailors as 

 the Doldrums. Thus the winds go from pole to pole and back 

 again, alternately as under and upper currents. In their 

 circuits the winds cross each other five times, producing regions 

 of calms at the poles, the tropics, and equator. The trade-winds 

 generally extend for about 28 on each side of the equator, but, 

 on account of the greater quantity of land in the northern hemi- 

 sphere, the N.E. trade-wind is narrower than the S.E. 



The sun is perpetually raising enormous quantities of vapour 

 from the ocean which the trade- winds carry to the equator : 

 it is condensed when it rises with the air into the higher 

 strata, and forms a ring of clouds along the southern side of the 

 belt of equatorial calms that surrounds the earth, which, during 

 the day, is perpetually pouring down torrents of rain, while the 

 sun continually beating upon its upper surface dissolves the clouds 

 into invisible vapour which is carried by the winds and condensed 

 into rain on the extra-tropical regions. The whole system of 

 trade-winds, equatorial and tropical calms, with the cloud ring, 

 follow the sun in declination ; consequently in its journeys back 

 and forwards it annually travels over 1000 miles 'of latitude, and 

 regulates the dry and rainy season in the tropical parts of the 

 earth. 



The monsoons, which are periodic winds in the Indian Ocean, 

 in part depend upon this movement. For when the sun is in the 

 northern hemisphere the trade-winds come northward with him ; 



G 2 



