Let us assume that by the time that the mass of the sun had condensed to within the space encircled by the orbit of the planet Mercury (that is, to a sphere having, say, a radius of 18,000,000 miles) the earth’s crust began to form; and let this be the time when the geological history of our globe dates its commencement. The total amount of heat generated by the condensation of the sun’s mass from a sphere of this size to its present volume would equal 19,740,000 years’ sun-heat. The amount of original heat given out during that time would equal 48,625,000 years’ sun-heat,—thus giving a total of 68,365,000 years’ sun-heat enjoyed by our globe since that period. The total quantity may possibly, of course, be considerably more than that, owing to the fact that the sun’s density may increase greatly towards his centre. But we should require to make extravagant assumptions regarding the interior density of the sun and the proportion of original heat to that produced by condensation before we could manage to account for anything like the period that geological phenomena are supposed by some to demand. The question now arises, by what conceivable means could the mass of the sun have become possessed of such a prodigious amount of energy in the form of heat previous to condensation? What power could have communicated to the mass 50,000,000 years’ heat before condensation began to take place? _The Sun’s Energy may have originally been derived from Motion in Space._—There is nothing at all absurd or improbable in the supposition that such an amount of energy might have been communicated to the mass. The Dynamical Theory of Heat affords an easy explanation of at least _how_ such an amount of energy _may_ have been communicated. Two bodies, each one-half the mass of the sun, moving directly towards each other with a velocity of 476 miles per second, would by their concussion generate in a _single moment_ the 50,000,000 years’ heat. For two bodies of that mass moving with a velocity of 476 miles per second would possess 4149 × 10^{38} foot-pounds of energy in the form of _vis viva_; and this, converted into heat by the stoppage of their motion, would give an amount of heat which would cover the present rate of the sun’s radiation, for a period of 50,000,000 years.