An easy experiment pleasingly illustrates this remarkable change. If a blackened card is placed upon snow or ice in the sunshine, the frozen mass underneath it will be gradually thawed, and the card sink into it, while that by which it is surrounded, although exposed to the full power of solar heat, is but little disturbed. If, however, we reflect the sun’s rays from a metal surface, an exactly contrary result takes place; the uncovered parts are the first to melt, and the blackened card stands high above the surrounding portion.
The evidences of science all indicate the sun as the source, not only of that heat which we receive directly through our atmosphere, but even of that which has been stored by our planet, and which we can, by several methods, develope. We have not to inquire if the earth was ever an intensely heated sphere;--this concerns not our question; as we should, even were this admitted, still have to speculate on the origin--the primitive source of this caloric.
Before, however, we proceed to the examination of the phenomena of terrestrial heat, a few of the great results of the laws of radiation and convection claim our attention.
Nearly all the heat which the sun pours upon the ocean is employed in converting its water into vapour at the very surface, or is radiated back from it, to perform the important office of producing those disturbing influences in the atmosphere, which are essential to the preservation of the healthful condition of the great aërial envelope in which we live.
Currents of air are generally due to the unequal degrees in which the atmosphere is warmed. Heat, by expanding, increases the elasticity, and lessens the density, of a given mass. Consequently, the air heated by the high temperature of the tropics, ascends charged with aqueous vapours, whilst the colder air of the temperate and the frigid zones flows towards the equator to supply its place. These great currents of the atmosphere are, independent of the minor disturbances produced by local causes, in constant flow, and by them a uniformity of temperature is produced, which could not in any other way be accomplished. By these currents, too, the equalisation of the constituents of the “breath of life” is effected, and the purer oxygen of the “land of the sunny south” is diffused in healthful gales over the colder climes of the north. The waters, too, evaporated from the great central Atlantic Ocean, or the far Pacific, are thus carried over the wide-spread continents, and poured in fertilising showers upon distant lands.