"Here then we have an agency competent to restore his lost energy, and to maintain a temperature at his surface which transcends all terrestrial combustion. The very quality of the solar rays--their incomparable penetrating power--enables us to infer that the temperature of their origin must be enormous; but in the fall of asteroids we find the means of producing such a temperature. It may be contended that this showering down of matter must be accompanied by the growth of the sun in size; it is so; but the quantity necessary to produce the observed calorific emission, even if accumulated for 4,000 years, would defy the scrutiny of our best instruments. If the earth struck the sun it would utterly vanish from perception, but the heat developed by the shock would cover the expenditure of the sun for a century. "To the earth itself apply considerations similar to those which we have applied to the sun. Newton's theory of gravitation, which enables us, from the present form of the earth, to deduce its original state of aggregation, reveals to us, at the same time, a source of heat powerful enough to bring about the fluid state--powerful enough to fuse even worlds. It teaches us to regard the molten condition of a planet as resulting from mechanical union of cosmical masses, and thus reduces to the same homogeneous process the heat stored up in the body of the earth, and the heat emitted by the sun. Without doubt the whole surface of the sun displays an unbroken ocean of fiery fluid matter. On this ocean rests an atmosphere of flowing gas--a flame atmosphere, or photosphere. But gaseous substances, when compared with solid ones, emit, even when their temperature is very high, only a feeble and transparent light. Hence it is probable that the dazzling white light of the sun comes through the atmosphere from the more solid portions of the surface.... In conclusion, thus writes Professor Thomson: 'The source of energy from which the solar heat is derived is undoubtedly meteoric.... The principal source--perhaps the sole appreciable efficient source--is in the bodies circulating round the sun at present inside the earth's orbit seen in the sunlight by us called "Zodiacal light." The store of energy for future sunlight is at present partly dynamical--that of the motions of these bodies round the sun; and partly potential--that of their gravitation towards the sun. This latter is gradually being spent, half against the resisting medium, and half in causing a continuous increase of the former. Each meteor thus goes on moving faster and faster, and getting nearer and nearer the center, until some time, very suddenly, it gets so much entangled in the solar atmosphere as to begin to lose its velocity. In a few seconds more it is at rest on the sun's surface, and the energy given up is vibrated across the district where it was gathered during so many ages, ultimately to penetrate as light the remotest regions of space....