Gravitation is now generally admitted to be the only conceivable source of the sun’s heat. But if we attribute the energy of the sun to gravitation as a source, we assign it to a cause the value of which can be accurately determined. Prodigious as is the energy of a single pound of matter falling into the sun, nevertheless a range of mountains, consisting of 176 cubic miles of solid rock, falling into the sun, would maintain his heat for only a single second. A mass equal to that of the earth would maintain the heat for only 93 years, and a mass equal to that of the sun itself falling into the sun would afford but 33,000,000 years’ sun-heat. It is quite possible, however, that a meteor may reach the sun with a velocity far greater than that which it could acquire by gravitation; for it might have been moving in a direct line towards the sun with an original velocity before coming under the sensible influence of the sun’s attraction. In this case a greater amount of heat would be generated by the meteor than would have resulted from its merely falling into the sun under the influence of gravitation. But then meteors of this sort must be of rare occurrence. The meteoric theory of the sun’s heat has now been pretty generally abandoned for the contraction theory advanced by Helmholtz. Suppose, with Helmholtz, that the sun originally existed as a nebulous mass, filling the entire space presently occupied by the solar system and extending into space indefinitely beyond the outermost planet. The total amount of work in foot-pounds performed by gravitation in the condensation of this mass to an orb of the sun’s present size can be found by means of the following formula given by Helmholtz,[202] 3 _r_^{2}M^{2} Work of condensation = — × ———————————— × _g_ 5 R_m_ M is the mass of the sun, _m_ the mass of the earth, R the sun’s radius, and _r_ the earth’s radius. Taking M = 4230 × 10^{27} lbs., _m_ = 11,920 × 10^{21} lbs., R = 2,328,500,000 feet, and _r_ = 20,889,272 feet; we have then for the total amount of work performed by gravitation in foot-pounds, 3 (20,889,272·5)^2 × (4230 × 10^{27})^2 Work = — × ————————————————————————————————————— 5 2,328,500,000 × 11,920 × 10^{21} = 168,790 × 10^{36} foot-pounds. The amount of heat thus produced by gravitation would suffice for nearly 20,237,500 years. These calculations are based upon the assumption that the density of the sun is uniform throughout. But it is highly probable that the sun’s density increases towards the centre, in which case the amount of work performed by gravitation would be somewhat more than the above.