THE SEA AND TPIE ATMOSPHERE. 11 



vested with a sufficiency of heat, will form one cubic foot of steam 

 — the water before its evaporation, and the vapoui' which it forms, 

 being exactly of the same temperatm-e ; though in reality, in the 

 process of conversion, 1100 degrees of heat have been absorbed or 

 carried away fi'om the -vicinage, and rendered latent or imper- 

 ceptible ; this heat, is retiu^ned in a sensible and perceptible form 

 the moment the vapour is converted once more into water. The 

 general fact is the same in the case of vapom- carried off by dry air 

 at any temperature that may be imagined ; for, down far belovv' the 

 freezing-point, evaporation proceeds uninterruptedly. 



34. " The ah', heated and dried as it sweeps over the aridsmface 

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



moistm'e fi-om the sea — as much, indeed, as w(Md, were no 

 moisture restored to it, depress its whole smface 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 moistm'e to be vdth- 

 di'awn. But night and the seasons of the year come round, and 

 the sm^plus temperatm'e, thus withdi-a^Mi and stored away at the 

 time it might have proved superfluous or inconvenient, is rendered 

 back so soon as it is requu'ed ; thus the cold of night and the rigour 

 of winter are modified by the heat given out at the point of con- 

 densation by dew, rain, hail, and snow. 



35. '.' The earth is a bad conductor of heat ; the rays of the sun. 

 Effects upon the which enter its smface and raise the temperature 

 ^^'^^- to 100^ or 150"", scarcely penetrate a fbot into the 

 groimd ; a few feet d.ovm, the warmth of the ground is nearly the 

 same night and day. The moistm-e which is there preserved free 

 from the influence of currents of air is never raised into vapom^ ; 

 so soon as the upper stratum of earth becomes thoroughb,^ dried, 

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

 dra^^Ti, ceases ; so that, even under the heats of the tropics, the 

 soil two feet doAvn wiU be found, on the approach of the rains, 

 sufficiently moist for the nomishment of plants. The splendid 

 flowers and vigorous foliage which bm'st forth in May, when the 

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

 no way sm-prise us ; fountains of water, bomidless in extent and 

 limited in depth only by the thiclmess of the soil which contains 

 them, have been set aside and sealed up for their use, beyond the 

 reach of those thirsty wmds or burning rays which are saflered to 



