INADEQUACY OF TERRESTRIALLY-KNOWN SOURCES.

We have Been that each square centimetre of the sun's surface emits sufficient energy to drive an eight-horse-power engine continuously; the output from each square centimetre of an O or B type star, such as Plaskett's star or V Puppis, which is at least 200 times as great, is sufficient to drive an express locomotive at full speed year after year and century after century for millions of years. Since the full implications of the doctrine of conservation of energy have been understood, efforts have been made to discover the origin of the energy which is poured out with such terrific profusion by the sun and stars.

A priori there are two general possibilities open. Either the stream of energy liberated from a star's surface may be continually fed to the star from outside, or it may be generated in the star's interior, and driven out through its surface, as the only means of preventing an intolerable heating of the interior. An illustration of the former mode of liberation of energy is provided by a meteorite falling through the earth's atmosphere, the energy of its radiation being provided by the impact of molecules of air on its surface ; an illustration of the latter is provided by an ordinary coal fire.

The only serious effort to explain the sun's energy as being supplied from outside was that of Robert Mayer, who conceived solar energy as arising from a continuous fall of meteors into the solar atmosphere.