PHYSICAL GEOGRAPHY. 303 



oxygenous and nitrogenous contents of their swimming blail- 

 deis. 



As fresh and salt water do not attain the maximum of 

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

 saltness of the sea lowers the thermornetrical degree corre- 

 sponding to this point, we can understand how the watei 

 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 3G-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 toward 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 radi- 

 ating and cooled surface of the tropical sea. In the Mediter- 

 ranean, the cause of the absence of such a refrigeration of the 

 lower strata is ingeniously explained by Arago, on the as- 

 sumption that the entrance of the deeper polar currents into 

 the Straits of Gibraltar, 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 moder- 

 ator of climates, exhibits a most remarkable uniformity and 

 constancy 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, therefore, been 

 justly observed, that an exact and long-continued investiga- 

 tion of these thermic relations of the tropical seas might most 

 easily afford a solution to the great and much-contested prob- 

 lem of the permanence of climates and terrestrial tempera 

 tures.f Great changes in the luminous disk of the sun would, 



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

 5' to 13 16' N. lat., in my Asie Centrals, 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 tern 

 peratures, without taking into account the very circumscribed local 

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

 mountains, and the drying up of lakes and marshes. Each age might 

 easily transmit to the succeeding one some few data, which would per- 

 haps furnish the most simple, exact, and direct means of deciding whetn- 

 er the sun, which is almost tho sole and exclusive source of the heat of 



