100 PHYSICAL GEOGRArm' OF THE SEA, AND ITS METEOROLOGY. 



the sea, is (§ 232) no feeble agent * in the production of wind, 

 nor is it inconsiderable in its influence upon the trade-winds. 



259. Vapour as one of the causes of the trade-winds. — Let us con- 

 sider this influence. A cubic foot of water, being convei-ted 

 into vapour, occupies the space of 1800 cubic feet. | This 

 vapour is also lighter than the 1800 cubic feet of air which it 

 displaces. Thus, if the displaced air weigh 1000 ounces, the 

 vapour will weigh 023 ; consequently, when air is surcharged 

 with vapour, the atmosphere is bulged out ab"ove, 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 during 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 

 perhaps not so strong as the trades, yet owing their existence, 

 nevertheless, merely to the formation of aqueous vapour. But 

 this is not all. 



260. Black's law. — " During the conversion of solids into 

 liquids, or of liquids into vapours, heat is absorbed, which is 

 again given out on their recondensation." J In the process of 

 converting one measure of water into vapour, heat enough is 

 absorbed — i. e., rendered latent, without raising the temperature 

 of the vapour in the least — to raise the temperature of 1000 

 *such measures 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 tempera- 

 ture of 140 additional measures of water 1°. 



261. Tlie latent heat transported in vapour. — In this heat rendered 

 latent by the processes of evaporation, and transported hither and 



* I am sustained in this view hy a recent paper on " tlie forces that produce 

 the great currents of the air, and of the ocean," recently read before the Royal 

 Society by Tlionias Hopkins. 



t Black and Watt's Experiments on Heat. 



t Black's law. It is an important one, and should be rcmcml)ered. 



