PHYSICAL GEOGRAPHY. 307 



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 276,000 feet (or more than 

 four miles) ; whilst 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 sur- 

 face forms a moving base on which rest the lower, denser, 

 and more saturated strata of air. 



Proceeding upwards and downwards 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 tempera- 

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

 sea, which has a tendency under all latitudes to maintain its 

 temperature in the strata of water 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 

 ordinary 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 animals. Moreover, the depths at which fishes 



* [See Wilson's Paper, On Wollaston' s Argument from the Limi- 

 tation of the Atmosphere as to the finite Divisibility of Matter. Trans, 

 of the Royal Society of Edinb., vol. xvi. p. 1, 1845.] Tr. 



t Humboldt, Relation hist., t. iii. chap. xxix. p. 514-530. 

 x 2 



