470 rnYSiCAL geography of the sea, and its meteorology. 



883. The annual sup'ply of solar lieat uniform. — The flow of heat 

 from the sun is held to be uniform, and the quantity that is 

 annually impressed npon the earth is considered as a constant. 

 The smi spots may make this "constant" a variable, hut the 

 amount annually received by the earth is. so nearly imiform, that 

 our best instruments have not been able to show us any variation 

 in its uniformit3^ Some maintain that climates arc undergoing 

 a gradual change as to temperature. However this may be as to 

 certain localities. Baron Fourier, after a long and laborious cal- 

 culation, claims to have shown that if the earth had been once 

 heated, and after having been brought to any given temperature, 

 if it had then been plunged into a colder medium, it would not 

 in the space of 1,280,000 3- ears be reduced in temperature more 

 than would a 12-inch globe of like materials in one second of 

 tiijie if placed under like conditions. It may be assumed that 

 for the whole earth, there has not been since the invention of the 

 thermometer any appreciable change in the temperature of the 

 crust of our planet. 



884. Quantity of heat daily imp-cssed u])07i the earth. — The earth 

 receives from the sun heat enough daily, it has been said (§ 271), 

 to melt a quantity of ice sufficient to incase it in a film 1 J inch 

 thick. What becomes of this heat after it is so impressed, 

 how is it dispersed by the land? how by the sea? Let us 

 inquire. 



885. Hoio far heloio the surface does the lieat of the sun j>enetrafe ? — 

 The solar ray penetrates the solid parts of the earth's crust only 

 to the depth of a few inches, but striking its fluid parts with its 

 light and heat, it penetrates the sea to depths more or less pro- 

 found, according to the transparency of the waters. Let us, in 

 imagination, divide these depths, whatever they may be, into 

 any number of stratifications or layers of equal thickness. The 

 direct heat of the sun is supposed to be extinguished in the lowest 

 layer; the bottom Isijev, then, Avill receive and absorb the 

 minimum amount of heat, the top the maximum ; consequently, 

 each layer, as we go from the top to the bottom, will receive less 

 and less of the sun's heat. 



886. The stratum of warmest water. — Now, which will retain 

 most heat and reach the highest temperature? Not the top 

 layer, or that to which most heat is imparted, because by eva- 

 poration heat is carried off from the surface of the sea almost as 

 fast as by the sun it is impressed upon the surface of the sea ; 



