Page:Handbook of Meteorology.djvu/28



The Nature of Heat.—The phenomena of heat and light are described, the one as “molecular motion,” the other as the “radiation of solar energy.” Roughly, either definition will apply to either phenomenon. Not much is known about the real essence of either, except that they are forms of energy which have been measured, and of which certain magnitudes have been established under the name of “wave lengths.”

Radiant Heat.—It is assumed that heat and light traverse space in “waves” or vibrations of the ether. Positive knowledge is confined to the fact that heat is radiated by the sun and stars in every direction. Practically all the heat received by the earth comes from the sun; and of the whole amount radiated by the sun, the earth intercepts less than one two-billionth part. Nevertheless, this small fraction of the sun's radiant heat produces all the results upon the earth which are manifested by life and its activities.

Some of the ether waves stimulate the nerves of the eye, producing the phenomena of light and vision. Others do not affect the nerves of sight; as they fall on the body they produce the sensation of warmth, thereby stimulating the growth of living matter. Meteorology is concerned chiefly with radiant energy of this character; they are conveniently called heat waves.

Perhaps our nearest approach to actual knowledge of heat is the recognition of the fact that when heat waves fall upon matter—say, a piece of metal—they set up a motion in the molecules composing it. If the intensity of the waves increases, molecular attraction little by little is overcome; the solid becomes a liquid and, finally, a vapor. This important change is explained as being due to increasing and to wider amplitude of the oscillations of the molecules. Perhaps the theory may not be satisfactorily established, but the facts cannot be denied;