302 COSMOS. 



The two envelopes of the solid surface of our planet— th© 

 h'quid and the aeriform — exhibit, owing t ) the mobility of 

 their particles, their currents, and their atmospheric relations, 

 many analogies combined with the contrasts which arise from 

 the great difference in the condition of their aggregation and 

 elasticity. The depths of ocean and of air are alike unknown 

 to us. At some few places under the tropics no bottom has 

 been found with soundings of 270,000 feet (or more than four 

 miles), while in the air, if, according to Wollaston, we may 

 assume that it has a limit from which waves of sound may 

 be reverberated, the phenomenon of twilight would incline 

 us to assume a height at least nine times as great.* The 

 aerial ocean rests partly on the solid earth, whose mountain 

 chains and elevated plateaux rise, as we have already seen, 

 like green wooded shoals, and partly on the sea, whose surface 

 forms a moving base, on wdiicli rest the lower, denser, and 

 more saturated strata of air. 



Proceedino: unward and downward from the common limit of 

 the aerial and liquid oceans, we find that the strata of air 

 and water are subject to determinate laws of decrease of tem- 

 perature. This decrease is much less rapid in the air than 

 in the sea, wdiich has a tendency under all latitudes to main- 

 tain its temperature in the strata of w^ater most contiguous to 

 the atmosphere, owing to the sinking of the heavier and more 

 cooled particles. A large series of the most carefully con- 

 ducted observations on temperature shows us that in the or- 

 dinary and mean condition of its surface, the ocean from the 

 equator to the forty-eighth degree of north and south latitude 

 is somewhat warmer than the adjacent strata of air.f Owing 

 to this decrease of temperature at increasing depths, fishes and 

 other inhabitants of the sea, the nature of whose digestive and 

 respiratory organs fits them for living in deep water, may even, 

 under the tropics, find the low degree of temperature and the 

 coolness of climate characteristic of more temperate and more 

 northern latitudes. This circumstance, which is analogous 

 to the prevalence of a mild and even cold air on the elevated 

 plains of the torrid zone, exercises a special influence on the 

 migration and geographical distribution of many marine ani- 

 mals. Moreover, the depths at which fishes live, modify, by 

 the increase of pressure, their cutaneous respiration, and the 



* [See Wilson's Paper, Oa Wollastori's Argument from the Limitali^n 

 ff the Atmosphere as to the fiiii'e Divisibility of Matter. — Trans, oj tht 

 Royal S or iety of Edinb., vdl. xvi., p. 1, 1815.] — Tr. 



t llLi'nhoMt, llelation llii't , . iii., chap, xxix., p. 511-530. 



