C REPORT — 1871. 



been other than fiery in the beginning. Mayer first suggested that the heat 

 of the Sun may be due to gravitation : but he supposed meteors falling in 

 to keep always generating the heat which is radiated year by year from the 

 Sun. Helmholtz, on the other hand, adopting the nebular hypothesis, showed 

 in 1854 that it was not necessary to suppose the nebulous matter to have 

 been originally fiery, but that mutual gravitation between its parts may 

 have generated the heat to which the present high temperature of the Sun is 

 due. Turther he made the important observations that the potential energy 

 of gravitation in the Sun is even now far from exhausted ; but that with 

 further and further shrinking more and more heat is to bo generated, and 

 that thus we can conceive the Sun even now to possess a sufficient store of 

 energy to produce heat and light, almost as at present, for several million 

 years of time future. It ought, however, to be added that this condensation 

 can only follow from cooling, and therefore that Helmholtz's gravitational 

 explanation of future Sun-heat amounts really to showing that the Sun's 

 thermal capacity is enormously greater, in virtue of the mutual gravitation 

 between the parts of so enormous a mass, than the sum of the thermal capa- 

 cities of separate and smaller bodies of the same material and same total 

 mass. Eeasons for adopting this theory, and the consequences which follow 

 from it, are discussed in an article " On the Age of the Sun's Heat," published 

 in ' Macmillan's Magazine ' for March 1862. 



For a few years Mayer's theor}- of solar heat had seemed to me probable ; 

 but I had been led to regard it as no longer tenable, because I had been in 

 the first place driven, by consideration of the very approximate constancy of 

 the Earth's period of revolution round the Sun for the last 2000 years, to 

 conclude that " The principal source, perhaps the sole appreciably eftective 

 " source of Sun-heat, is in bodies circulating round the Sun at present inside 

 " the Earth's orbit"* ; and because Le Verricr's researches on the motion of 

 the planet Mercury, though giving evidence of a sensible influence attributable 

 to matter circulating as a great number of small planets within his orbit 

 round the Sun, showed that the amoimt of matter that could possibly be as- 

 sumed to circulate at any considerable distance from the Sun must be very 

 small ; and therefore " if the meteoric influx taking place at present is 

 " enough to pi-oduce any appreciable portion of the heat radiated away, it 

 " must be supposed to be from matter circulating round the Sun, within very 

 " short distances of his surface. The density of this meteoric cloud would 

 " have to be siipposed so great that comets could scarcely have escaped as 

 " comets actually have escaped, showing no discoverable effects of resistance, 

 " after passing his surface within a distance equal to one-eighth of his radius. 

 " All things considered, there seems little probability in the hypothesis that 

 " solar radiation is compensated to any appreciable degree, by heat generated 

 " by meteors falling in, at pi-esent ; and, as it can be shown that no chemical 

 " theory is tenablef, it must be concluded as most probable that the Sun is 

 " at present mere an incandescent liquid mass cooling "J. 



Thus on purelj- astronomical grounds was I long ago led to abandon as 

 very improbable the hypothesis that the Sun's heat is supplied dynamically 

 from year to year by the influx of meteors. Eut now spectrum analysis gives 

 proof finally conclusive against it. 



Each meteor circulating round the Sun must fall in along a very gradual 



* "On the meclianical energies of the Solar System." Transactions of the Eoyal Society 

 of Edinburgh, 1854 ; and Phil. Mag. 1854, second half year, 

 •j- " Mechanical Energies " &c. 

 X "Age of the Sun's Heat" (MacmilLan's Magazine, March 1862). 



