The winter solstice has come, with its long night and shortened day. The time during which radiation is going on is nearly twice that in which absorption takes place, and the earth is in her worst position for receiving heat, as that half which has the largest surface of water is towards the sun.
These are the causes which lead to the variations of the seasons, and through these we learn why we are colder when near the sun than when at a considerably greater distance.
An analysis of the spectrum shows us that there are some changes regularly taking place in the state of the solar beam, which cannot be referred to the mere alteration of position. It may be inferred, from facts by long-continued observations, that the three classes of phenomena—light, heat, and chemical power, distinguished by the term Actinism—which we detect in the sun’s rays, are constantly changing their relative proportions. In spring, the chemical agency prevails; in summer, the luminous principle is the most powerful; and in the autumn, the calorific forces are in a state of the greatest activity.[52] The importance of these variations, to the great economy of vegetable life, will be shown when we come to examine the phenomena connected with organisation.
A remarkable change takes place in the character of heat in being reflected from material substances. In nature we often see this fact curiously illustrated. Snow which lies near the trunks of trees or wooden poles melts much quicker than that which is at a distance from them, the sun shining equally on both—the liquefaction commencing on the side facing the sun, and gradually extending. We see, therefore, that the direct rays of solar heat produce less effect upon the snow than those which are radiated from coloured surfaces. By numerous experiments, it has been shown that these secondary radiations are more abundantly absorbed by snow or white bodies than the direct solar rays themselves. Here is one of the many very curious evidences, which science lays open to us, of the intimate connection between the most ethereal and the grosser forms of matter. Heat, by touching the earth, becomes more earth-like. The subtile principle which, like the spirit of superstition, has the power of passing, unfelt, through the crystal mass, is robbed of its might by embracing the things of earth; and although it still retains the evidences of its refined origin, its movements are shackled as by a clog of clay, and its wings are heavy with the dust of this rolling ball. It has, however, acquired new properties, which fit it for the requirements of creation, and by which its great tasks are facilitated. Matter and heat unite in a common bond, and, harmoniously pursuing the necessities of some universal law, the result is the extension of beautiful forms in every kingdom of nature.
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.