883. The annual supply of solar heat uniform.—The flow of heat from the sun is held to be uniform, and the quantity that is annually impressed upon the earth is considered as a constant. The sun spots may make this "constant" a variable, but the amount annually received by the earth is. so nearly uniform, that our best instruments have not been able to show us any variation in its uniformity Some maintain that climates are undergoing a gradual change as to temperature. However this may be as to certain localities. Baron Fourier, after a long and laborious calculation, 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 years be reduced in temperature more than would a 12-inch globe of like materials in one second of time 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 impressed upon 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 1½ 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. How far below the surface does the heat of the sun penetrate?—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 profound, 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 layer, then, will 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 evaporation 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;
