94 THE PHYSICAL GEOGRAPHY OF THE SEA. 



less, merely to the formation of aqueous vapor. But this is not 

 all. 



253. " During the conversion of solids into liquids, or of liquids 

 Black's law. into vapors, heat is absorbed, which is again given 



out on their recondensation."* In the process of converting one 

 measure of water into vapor, heat enough is absorbed — i. e., ren- 

 dered latent, without raising the temperature of the vapor in the 

 least — to raise the temperature of 1000 such measures of water 

 1° ; when this vapor is condensed again into water, wherever the 

 place of recondensation may be, this heat is set free again. If it 

 be still further condensed, as into hail or snow, the latent heat 

 rendered sensible during the process of congelation would be suf- 

 ficient to raise the temperature of 140 additional measures of wa- 

 ter 1°. 



254. In this heat rendered latent by the processes of evapo- 

 Theiatentheattrans- ^ation, aud transported hither and thither by the 

 ported in vapor. wiuds, rcsidcs the chicf source of the dynamical 

 power which gives them motion. In some aspects vapor is to 

 the winds what fuel is to the steam-engine : they carry it to the 

 equatorial calm belt ; there it rises, entangling the air, and carry- 

 ing it up along with it as it goes. As it ascends it expands, as it 

 expands it grows cool ; and as it does this its vapor is condensed, 

 the latent heat of which is thus liberated ; this raises the tempera- 

 ture of the upper air, causing it to be rarefied and to ascend still 

 higher. This increased rarefaction calls for increased velocity on 

 the part of the inpouring trade-winds below. 



255. Thus the vapors imiting with the direct solar ray would. 

 The effect of the des- wcrc thcrc uo countcractiug influences, cause the 

 wi^d"!"'^ northeast and southeast trade- winds to rush in with 

 equal force. But there is on the polar side of the northeast trade- 

 winds an immense area of arid plains for the heat of the solar 

 ray to beat down upon, also an area of immense precipitation. 

 These two sources of heat hold back the northeast trade-winds, as 

 it were, and, when the two are united, as they are in India, they 

 are sufficient not only to hold back the northeast trade- wind, but 

 to reverse it, causing the southwest monsoon to blow for half the 

 year instead of the northeast trade. 



256. We have, in this difference as to strength and stability 

 * Black's law. It is an important one, and should be remembered. 



