The surface of the earth during the sunshine—and, though in a less degree, even when the sun is obscured by clouds—is constantly receiving heat; but the rate of its absorption varies. Benjamin Franklin showed, by a set of simple but most conclusive experiments, that a piece of black cloth was warmed much sooner than cloth of a lighter colour;[49] and we know, from observations of a similar class, that the bare brown soil receives heat more readily than the bright green grassy carpet of the earth. Consequently, during the winter season, relatively to the quantity poured from its source, more heat penetrates the uncovered soil, than during the spring or summer.
There is a constant tendency to an equilibrium; and, during the night, the surface is robbed of more heat, by the colder air, than by day; as, when the earth is not receiving heat, it is constantly radiating it back into space. Even in these processes of convection and radiation, a similar law prevails to that which is discovered in examining into the rate of calorific absorption.
Every tree spreading its green leaves to the sunshine, or exposing its brown branches to the air—every flower which lends its beauty to the earth—possesses different absorbing and radiating powers. The chalice-like cup of the pure white lily floating on the lake—the variegated tulip—the brilliant anemony—the delicate rose—and the intensely coloured peony or dahlia—have each powers peculiar to themselves for drinking in the warming life-stream of the sun, and for radiating it back again to the thirsting atmosphere. These are no conceits of a scientific dreamer; they are the truths of direct induction; and, by experiments of a simple character, they may be put to a searching test.[50]
A thermometric examination of the various coloured flowers, by enclosing a delicate thermometer amongst their leaves, will readily establish the correctness of the one; and by a discovery of recent date, connected with calorific radiation, which must be particularly described presently, we can, with equal ease and certainty, test the truth of the other;[51] the absorption and radiation of heat being directly regulated by the colours of the surfaces upon which the sun rays fall.
It follows, as a natural consequence of the position of the sun, as it regards any particular spot on the earth at a given time, that the amount of heat is constantly varying during the year. This variation regulates the seasons.
When it is remembered that the earth is, in the winter, nearly three millions of miles nearer the sun than in the summer, some explanation is required to account for our suffering more cold when nearer the source of heat, than when at the remotest distance.
The earth in her path around the sun describes an ellipse, the sun’s place being one of its foci. In obedience to the law, already described, of the conservation of the axis of rotation, the axis of the earth constantly points towards the star in the constellation of the Little Bear. Recollecting this, and also the two facts, that a dense solid body absorbs heat more readily than a fluid one, and that radiation from the surface is constantly going on when absorption is not taking place, let us follow the earth in her orbit.
It is the time of the vernal equinox—we have equal day and night—therefore the periods of absorption and radiation of heat are alike. But at this time of the year the southern hemisphere is opposite to the sun, consequently the degree of absorption by the wide-spread oceans small.
It is the summer solstice—we have sixteen hours of daylight, when the absorption of heat is going on—and but eight hours of night, during which heat is passing off. The northern hemisphere is now presented to the sun, and as here we have the largest portion of dry land, the powers of absorption are at their maximum.
The autumnal equinox has arrived, with its equal day and night, as in the spring, but now the whole northern hemisphere is opposite the sun; hence, according to the laws already explained, we see the causes of the increased heat of the autumnal season.