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. How magnificent are the operations of nature! The air is not much warmed by the radiations of caloric passing from the sun to the earth; but the surface soil is heated by its power of absorbing these rays. The temperature of the air next the earth is raised, and we thus have the circulation of those beneficial currents which are so remarkably regular in the Trade Winds. The air heated within the tropics would ascend directly to the poles, were the earth at rest, but being in motion, those great aërial currents--the Trade Winds--are produced, and the periodical monsoons are due to the same cause. A similar circulation, quite independent of the ordinary tidal movement, takes place also in the earth-girdling ocean. The water, warmed, by convection, from the hot surface of the tropical lands, sets across the Atlantic from the Gulf of Mexico; and being under the influence of the two forces--gravity and motion--it illustrates the parallelogram of forces, and flowing along the diagonal, reaches our own shores: the genial influences of the gulf stream produce that tempered climate which distinguishes our insular home. Here we have two immense influences produced by one agency, rendering those parts of the earth habitable and fertile, which but for these great results would sorrow in the cheerless aspect of an eternal winter.