§ 34,35. THE SEA AND THE ATMOSPHERE. n 



still more so wlien combined. One cubic inch of water, wlien 

 invested with a sufficiency of heat, will form one cubic foot of 

 steam — the water before its evaporation, and the vapor which it 

 forms, being exactly of the same temperature ; though in reality, 

 in the process of conversion, 1100 degrees of heat have been ab- 

 sorbed or carried away from the vicinage, and rendered latent or 

 imperceptible ; this heat is returned in a sensible and j^erceptible 

 form the moment the vapor is converted once more into water. 

 The general fact is the same in the case of vapor carried off by 

 dry air at any temperature that may be imagined ; for, down far 

 below the freezing point, evaporation proceeds uninterruptedly. 



81:. ''The air, heated and dried as it sweeps over the arid surface 

 Latent heat. of the soil, driuks Up by day myriads of tons of 

 moisture from the sea — as much, indeed, as would, were no moist- 

 ure restored to it, depress its whole surface at the rate of eight or 

 ten feet annually. The quantity of heat thus converted from a 

 sensible or perceptible to an insensible or latent state is almost in- 

 credible. The action equally goes on, and with the like results, 

 over the surface of the earth, where there is moisture to be with- 

 drawn. But night and the seasons of the year come around, and 

 the surplus temperature, thus withdrawn and stored away at the 

 time it might have proved superfluous or inconvenient, is render- 

 ed back so soon as it is required ; thus the cold of night and the 

 rigor of winter arc modified by the heat given out at the point of 

 condensation by dew, rain, hail, and snow. 



So. " The earth is a bad conductor of heat; the rays of the sun, 

 Effects upon the which cutcr its surfacc and raise the temperature 

 ^^^^- to 100° or 150°, scarcely penetrate a foot into the 



ground ; a few feet down, the warmth of the ground is nearly the 

 same night and day. The moisture which is there preserved free 

 from the influence of currents of air is never raised into vapor ; 

 so soon as the upper stratum of qarth becomes thoroughly dried, 

 capillary action, by means of which all excess of water was with- 

 drawn, ceases ; so that, even under the heats of the tropics, the 

 soil two feet down will be found, on the approach of the rains, 

 sufficiently moist for the nourishment of plants. The splendid 

 flowers and vigorous foliage which burst forth in May, when the 

 parched soil would lead us to look for nothing but sterility, need 

 in no way surprise us ; fountains of water, boundless in extent 



