INFLUENCE OF WINDS. 11 



As the heated earth, and everything upon its surface, 

 does not cool so fast when surrounded by moist as by 

 dry air, it follows, that even if the quantity and inten- 

 sity of the solar rays falling upon two given portions of 

 the earth's surface are exactly equal, yet the sensible and 

 effective heat produced in the two localities may be very 

 different according as the atmosphere contains much or 

 little vapour. In the one case the heat is absorbed 

 more rapidly than it can escape by radiation ; in the 

 other case it radiates away into space, and is lost, more 

 rapidly than it is being absorbed. In both cases an 

 equilibrium will be arrived at, but in the one case the 

 resulting mean temperature will be much higher than in 

 the other. 



Influence of Winds on the Temperature of the 

 Equator. The distance from the northern to the 

 southern tropics being considerably more than three 

 thousand miles, and the area of the intertropical zone 

 more than one-third the whole area of the globe, it 

 becomes hardly possible for any currents of air to reach 

 the equatorial belt without being previously warmed by 

 contact with the earth or ocean, or by mixture with the 

 heated surface-air which is found in all intertropical and 

 sub-tropical lands. This warming of the air is rendered 

 more certain and more effective by the circumstance, that 

 all currents of air coming from the north or south have 

 their direction changed owing to the increasing rapidity 

 of the earth's rotational velocity, so that they reach 

 the equator as easterly winds, and thus pass obliquely 

 over a great extent of the heated surface of the globe. 

 The causes that produce the westerly monsoons act in a 

 similar manner, so that on the equator direct north or 



