THE ATMOSPHEPiE. 95 



ascend, it receives less and less, and the consequent temperature 

 becomes more and more uniibrm ; so that the height within the 

 tropics to which the direct rays of the sun is not, as reason sug- 

 gests, and as the snow-lines of Chimborazo and other mountains 

 show, very great or very variable. 



256. Moreover, daily observations show most conclusively that 

 Hurricanes not due tlic strouo; winds and the £?reat winds, the hurricanes 



to direct heat of the -, ^ -, , • r- jit j_i • p 



sun. and tornacioes, do not arise from the direct heat of 



the sun, for they do not come in the hottest weather or in the 

 clearest sides. On the contrary, winter is the stormy period in the 

 extra-tropical regions of the north ;* and in the south, rains and 

 gales — not gales and simshinej — 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 ; whereas 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 v/hich it makes by 

 playing upon the w^ater. 



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

 The influence of sidcs the diroct aiid reflected heat of the sun are 



otlier agents re- -, . -, . n , i • ^ t ■ 



quired. coucemed m producing the trade-winds. Let us 



inquire into the nature of these agents. 



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

 AHiere found, aud sca, aiid raiiis, as between the two hemispheres. 



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

 the latent heat of vapour wdiich is set free during the processes of 

 precipitation. The vapour itself, as it rises from the sea, is (§ 232) 

 no feeble agentj in the production of wind, nor is it inconsiderable 

 -in its influence upon the trade-winds. 



259. Let us consider this influence. A cubic foot of water, 

 vapr.ur as one of the bein.<^ couvcrted into vapour, occupies the space of 



causes of the trade- -.Qf.;? -. • o , c m- • l T i / i.i 



winds. ibUU cubic icet.§ 1ms vapour is also lighter than 



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



* Gales of the Atlantic, Observatory, Y/asliington, 185G. 



t Storm and Kaiu Charts. 



X I am sustained in this view by a recent paper on " tlie forces that produce 

 the great currents of the air, and of the ocean," recentl) read before the Eoyal 

 Society by Thomas Hopkins. 



§ Black and Watt's Experiments on Heat. 



