THE ATMOSPHERE 99 



2 J per cent. ; that is, a column of atmosphere 100 feet high will, 

 after its temperature has been raised 12°, he 102^ feet high. 

 However, only about one-third of the direct heat of the sun is 

 absorbed in its passage down through the atmosphere. The other 

 two-thirds are employed in lifting vapour up from the sea, or in 

 warming the ci-ust of the earth, thence to be radiated off again, 

 or to raise the temperature of sea and air by conduction. The 

 air at the surface of the earth receives most heat directly from 

 the sun ; as you ascend, it receives less and less, and the con- 

 sequent temperature becomes more and more uniform ; so that 

 the height within the tropics to which the direct rays of the sun 

 ascend is not, as reason suggests, and as the snow-lines of Chim- 

 borazoand other mountains show, very great or very variable. 



256. Hurricanes not due to direct heat of the sun. — Moreover, 

 daily observations show most conclusively that the strong winds 

 and the great winds, the hurricanes and tornadoes, do not arise 

 from the direct heat of the sun, for they do not come in the 

 hottest weather or in the clearest skies. 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 

 sunshine f — accompanj^ 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 which 

 it makes by playing upon the water. 



257. The influence of other agents required. — From these facts it 

 is made obvious that other agents besides the direct and reflected 

 heat of the sun are concerned in producing the trade-winds. Let 

 us inquire into the nature of these agents. 



258. Wliere found. — They are to be found in the unequal dis- 

 tribution of land and sea, and rains, as between the two hemi- 

 spheres. They derive their power from heat, it is true, but it is 

 chiefly from the latent heat of vapour which is set free during 

 the processes of precipitation. The vapour itself, as it rises from 



* Grales of the Atlantic, Observatory, Washington, 1856. 

 t Storm and IJain Charts. 



If 2 



