SOURCES Or ENERGY; NATTER; ETIIER. Of the ultimate nature of energ,y, like that of matter, we are ignorant; nor do we know of energy by direct observation, except as associated with mat ter. It is the change of energy, either as to quantity or form, that is scrutinized and meas• tired, and such change is continually occurring as energy passes from body to body; but the condition of a body totally devoid of energy is a subject of speculation only. Natter, on the other hand, is to the physieist only a vehicle for energy; to hold or to convey energy is its sole physical function. Any substance which performs this function, whatever else it may do, is, in so far, of the nature of matter, and this function is performed by the luminiferous ether through which radiant energy is conveyed. In a radiating body of ordinary matter and ether the total energy must lie shared by both. Terrestrial energy available to man may be classi fied as energy of fuel, food, head of water. wind. tides, chemical action, solar radiation. These, with a partial exception in the case of the fifth and sixth, are all traceable to the sun. The earth receives part of the total solar radiation, and the rate at which radiant energy from the sun is expended upon a unit area of the earth's surface is called the 'solar constant.' From tests conducted above the clouds, notably at Mont Blanc and at Mount 'Whitney. this constant has been determined.
From the latter observations Professor Langley concludes "that in view of the large limits of error we can adopt three calories as the most probable value of the solar constant; by which I mean that at the earth's mean distance, in the ab sence of its absorbing atmosphere, the solar rays would raise one gram of water three degrees centigrade per minute for each normally ex posed square centimeter of its surface." From
a careful discussion of the various determina tions of the solar constant, Professor F. W. Very, in the Monthly Weather Review of the United States Weather Bureau for August, 1901, deduces a mean value of 3.127 calories per square centimeter per minute. This is 2.18 X 10° ergs, or 22.18 gram-meters per second to the square centimeter of surface. As this is transmitted at the rate of 300,000,000 meters per second, it represents the energy of solar radiation present in a column of ether near the earth one square centimeter in cross-section and 300,000,000 me ters in length. Owing to absorption by clouds and the atmosphere, the portion actually reach ing the ground surface of the earth is reduced to about one-half the above value.
Besides papers cited above, the following works, among many others. are devoted wholly or in part to energetics: Maxwell. Matter and Motion: Theory of Heat (London and New York, 1891) ; Tait, Recent Adrances in Physical Science (London, 1876) ; Tait, Properties of Matter (Edinburgh. 1885); Thomson (Lord Kelvin), Popular Lectures and Addresses, vol. ii. (London and New York, 1891) ;. 1)olbear, Matter, Ether, and Motion (Boston, 1892) ; .Toule, Seientifle Papers (2 vols.. London. 1884-87) : Bowla nd • Seirntifir Papers (Baltimore, 1902) ; Tait, Thermodynamirs (Ed inburgh. 1877) ; Popper, Dir physikalisehen Grundsiitz:e der elektrisehen Kraft iibertragung Vienna. 1883) : Die Lehr,. von der EMT Or (Leipzig. 1877) ; Planck. Pas Prineip der Erhallung der •nergir (Leipzig. 1877) : Clausius. Die in( chanisehr 1876 91) ; Energy, Force, and Work (New York, 189S).