96 PHYSICAL GEOGRAPHY OF THE SEA, AXD ITS lilETEOEOLOGY. 



placed air weigh 1000 ounces, the vapour will weigh 623 ; conse- 

 quently, when air is surcharged with vapour, the atmosphere is 

 bulged out above, and the barometric pressure is diminished in 

 proportion to the volume which flows off above in consequence of 

 this bulging out. Thus, if we imagine the air over the Atlantic 

 Ocean to be all in a state of rest, and that suddenly dming this 

 calm, columns of vapour were to commence rising from the middle 

 of this ocean, we can understand how the wind would commence to 

 flow into this central space from all around. Now, if we imagine 

 no other disturbing cause to arise, but suppose the evaporation 

 from this central area to go on with ceaseless activity, we can see 

 that there would be a system of winds in the Atlantic as steady, 

 but perhfips not so strong as the trades, yet owing their existence, 

 nevertheless, merely to the formation of aqueous vapour. But this 

 is not all. 



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

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



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

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

 dered latent, without raising the temperatm^e of the vapour in the 

 least — to raise the temperature of 1000 such measm-es of water 

 1° ; when this vapour 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 

 sufficient to raise the temperatm-e of 140 additional measm-es of 

 water 1°. 



261. In this heat rendered latent by the processes of evapora- 

 Tbeiatentheat trans- tion, and transported hither and thither by the 

 ported ill vapour. wiuds, rcsidcs the chief source of i\\Q dpiamical 

 power which gives them motion. In some aspects vapom' is to the 

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

 torial calm belt ; there it rises, entangling the air, and carrjdng 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 vapoiu: is condensed, 

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

 tiu-e 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 inpom-ing trade-winds below. 



262. Thus the vapours uniting with the direct solar ray would, 

 * Black's law. It is an important one, and should be ieni?mb( red. 



