THE NON-LIVING WORLD 151 



the earth it is evident that while at the equator the sur- 

 face of the earth has a rotatory motion of 1042 miles an 

 hour, that of a circle very near one of the poles would 

 travel at the rate of only ten miles an hour, and others still 

 nearer at a less and less rate. Therefore polar currents 

 (from the equator, towards the poles) must acquire an east- 

 ward direction, as their velocity east will grow more and 

 more in excess of that of each tract of the earth's surface 

 they pass over. On the other hand, the currents from the 

 poles towards the equator will more and more fall below 

 the velocity of each part of the rotating earth they succes- 

 sively come to. They will therefore lag behind it, and so 

 appear to blow in a westerly direction and this constantly. 

 These currents constitute the trade winds of the Northern 

 and Southern Hemispheres. Near the equator they 

 become neutralised, and the heated air ascends, and so 

 we have an equatorial band of calms and occasional 

 storms between the north and south trade winds. The 

 trade winds are modified by the form of the continents 

 they traverse, while they disappear north and south of 

 latitude 30, They disappear by giving place to the 

 variable winds of the temperate regions with which we 

 are familiar, and the conditions determining which are 

 too complex to be further noticed here, the reader being 

 referred for such knowledge to works on Meteorology. 



The winds of the Indian Ocean known as " monsoons " 

 are modifications of the trade winds due to the influence 

 of vast masses of land. Land and sea breezes are due to 

 the more equable temperature of the sea and the more 

 ready heating and cooling of the surface of the land. 

 During the day the tendency is for the air over the 

 cooler sea to come in as a sea breeze, and replace the 

 ascending current of air produced by the action of the 

 sun's heat on the land. After sunset, on the contrary, the 



