§ 250-252. THE ATMOSPHERE. 93 



and gales — not gales and sunshine* — accompany each other. The 

 land and sea breezes express more than double the amount of 

 wind force which the direct heat of the sun is capable of exerting 

 upon the trade- winds. I say more than double, because in the 

 land and sea breezes the wind-producing power acts alternately on 

 the land and on the sea — in opposite scales of the balance ; where- 

 as in the trade-winds it acts all the time in one scale — in the sea 

 scale; and the thermal impression which the solar ray makes 

 through the land upon the air is much greater than that which it 

 makes by playing upon the water. 



250. From these facts it is made obvious that other agents be- 

 • The influence of oth- sidcs the dlrcct and reflected heat of the sun are 



er agent, required, conccmed lu produclug thc tradc-wluds. Let us 

 inquire into the nature of these agents. 



251. They are to be found in the unequal distribution of land 

 Where found, and sca, and rains, as between the two hemispheres, 



They derive their power from heat, it is true, but it is chiefly 

 from the latent heat of vapor which is set free during the proc- 

 esses of precipitation. The vapor itself, as it rises from the sea, is 

 (§ 232) no feeble agent in the production of wind, nor is it incon- 

 siderable in its influence upon the trade-winds. 



252. Let us consider this influence. A cubic foot of water, be- 

 vapor as one of the ing coiivcrted luto vaDor, occuplcs the space of 1800 



causes of the trade- ^ • n m^ • . 



winds. cubic leet.j Ihis vapor is also lighter than the 



1800 cubic feet of air which it displaces. Thus, if the displaced 

 air weigh 1000 ounces, the vapor will weigh 623 ; consequently, 

 when air is surcharged with vapor, 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 during this calm columns 

 of vapor 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. ISTow, 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, neverthe- 

 * Storm and Rain Charts. f Black and Watt's Experiments on Heat. 



