MECHANICAL ENERGIES OF THE SOLAR SYSTEM. 69 



perienced such an augmentation ; for according to the form of the gravitation theory 

 which I have proposed, the added matter is drawn from a space where it acts on 

 the planets with very nearly the same forces as when incorporated in the Sun. 

 This form of the gravitation theory then, which may be proved to require a 

 greater mass of meteoric matter to produce the solar heat than would be required 

 on any other assumption that could be made regarding the previous positions 

 and motions of the meteors, requires not more than it is perfectly possible does 

 fall in to the Sun. Hence I think we may regard the adequacy of the meteoric 

 theory to be fully established. 



Let us now consider how much chemical action would be required to produce 

 the same effects, with a view both to test the adequacy of the theory that the 

 Sun is merely a burning mass without a supply of either fuel or dynamical energy 

 from without, and to ascertain the extent to which, in the third theory, the com- 

 bustion of meteors may contribute, along with their dynamical energies, to the 

 supply of solar heat. Taking the former estimate, 2781 thermal units centigrade, 

 or 3,869,000 foot-lbs. as the rate per second of emission of energy from a square 

 foot of the Sun's surface, equivalent to 7000 horse power, we find that more than 

 •42 of a lb. of coal per second, or 1500 lbs. per hour would be required to produce 

 heat at the same rate. Now if all the fires of the whole Baltic fleet were heaped 

 up and kept in full combustion, over one or two square yards of surface, and if 

 the surface of a globe all round had every square yard so occupied, where could 

 a sufficient supply of air come from to sustain the combustion ? yet such is the 

 condition we must suppose the Sun to be in, according to the hypothesis now 

 under consideration, at least if one of the combining elements be oxygen or any 

 other gas drawn from the surrounding atmosphere. If the products of combus- 

 tion were gaseous, they would in rising check the necessary supply of fresh air ; 

 or if they be solid or liquid (as they might be wholly or partly if the fuel be 

 metallic) they would interfere with the supply of the elements from below. In 

 either or in both ways the fire would be choked, and I think it may be safely 

 affirmed that no such fire could keep alight for more than a few minutes, by any 

 conceivable adaptation of air and fuel. If then the Sun be a burning mass, it 

 must be more analogous to burning gunpowder than to a fire burning in air ; and 

 it is quite conceivable that a solid mass, containing within itself all the elements 

 required for combustion, provided the products of combustion are permanently 

 gaseous,* could burn off at its surface all round, and actually emit heat as co- 

 piously as the Sun. Thus an enormous globe of gun-cotton might, if at first cold, 

 and once set on fire round its surface, get to a permanent rate of burning, in 

 which any internal part would become heated by conduction, sufficiently to 

 ignite, only when nearly approached by the diminishing surface. It is highly 

 probable indeed that such a body might for a time be as large as the Sun, and 

 give out luminous heat as copiously, to be freely radiated into space, without suf- 



* On this account gunpowder would not do. 



