308 COSMOS. 



live, modify, by the increase of pressure, their cutaneous 

 respiration, and the oxygenous and nitrogenous contents of 

 their swimming bladders. 



As fresh and salt water do not attain the maximum of their 

 density at the same degree of temperature, and as the saltnesg 

 of the sea lowers the thermometrical degree corresponding to 

 this point, we can understand how the water drawn from 

 great depths of the sea during the voyages of Kotzebue and 

 Dupetit-Thouars could have been found to have only the 

 temperature of 37 and 36'5. This icy temperature of sea 

 water, which is likewise manifested at the depths of tropical 

 seas, first led to a study of the lower polar currents, which 

 move from both poles towards the equator. Without these 

 submarine currents, the tropical seas at those depths could 

 only have a temperature, equal to the local maximum of cold 

 possessed by the falling particles of water at the radiating and 

 cooled surface of the tropical sea. In the Mediterranean the 

 cause of the absence of such a refrigeration of the lower strata 

 is ingeniously explained by Arago, on the assumption that the 

 entrance of the deeper polar currents into the Straits of Gib- 

 raltar, where the water at the surface flows in from the 

 Atlantic ocean from west to east, is hindered by the submarine 

 counter currents which move from east to west, from the 

 Mediterranean into the Atlantic. 



The ocean, which acts as a general equalizer and moderator 

 of climates, exhibits a most remarkable uniformity and con- 

 stancy of temperature especially between 10 north and 10 

 south latitude,* over spaces of many thousands of square 

 miles, at a distance from land where it is not penetrated 

 by currents of cold and heated water. It has, there- 

 fore, been justly observed, that an exact and long continued 

 investigation of these thermic relations of the tropical 

 seas, might most easily afford a solution to the great and 

 much contested problem of the permanence of climates and 

 terrestrial temperatures. f Great changes in the luminous disc 



* See the series of observations made by me in the South Sea, from 

 5' to 13 16' N". lat., in my Asie centrale, t. iii. p. 234. 



t " We might (by means of the temperature of the ocean under the 

 tropics) enter into the consideration of a question which has hitherto 

 remained unanswered, namely, that of the constancy of terrestrial tem- 

 peratures, without taking into account the very circumscribed local 

 influences arising from the diminution of wood in the plains and oa 



