MECHANICAL ENERGIES OF THE SOLAR SYSTEM. 65 



velocities which they have acquired by his attraction. This is a form of what may 

 be called the Gravitation Theory of Solar Heat, which is itself included in the 

 general meteoric theory. 



The objects of the present communication are to consider the relative capa- 

 bilities of the second and third hypothesis to account for the phenomena ; to ex- 

 amine the relation of the gravitation theory to the meteoric theory in general ; and 

 to determine what form of the gravitation theory is required to explain solar 

 heat consistently with other astronomical phenomena. 



In the first place, it may be remarked, that in all probability there must always 

 be meteors falling into the Sun, since the fact of meteors coming to the earth* 

 proves the existence of such bodies moving about in space ; and even if the mo- 

 tions of these bodies are at any instant such as to correspond to elliptical or 

 circular orbits round the Sun, the effects of the resisting medium would gra- 

 dually bring them in to strike his surface. Also, it is easy to prove dynamically 

 that meteors falling in to the Sun, whatever may have been their previous state 

 of motion, must enter his atmosphere, or strike his surface, with, on the whole, 

 immensely greater relative velocities than those with which meteors falling to the 

 earth enter the earth's atmosphere, or strike the earth's surface. Now, Joule has 

 shewn what enormous quantities of heat must be generated from this relative 

 motion in the case of meteors coming to the earth ; and by his explanation! of 

 " falling stars," has made it all but certain that, in a vast majority of cases, this 

 generation of heat is so intense as to raise the body in temperature gradually 

 up to an intense white heat, and cause it ultimately to burst into sparks in the 

 air (and burn if it be of metallic iron) before it reaches the surface. Such effects 

 must be experienced to an enormously greater degree before reaching his surface, 

 by meteors falling to the Sun, if, as is highly probable, he has a dense atmosphere ; 

 or they would take place yet more intensely on striking his solid or liquid surface, 

 were they to reach it still possessing great velocities. Hence, it is certain that 

 some heat and light radiating from the Sun is due to meteors. It is excessively 

 probable that there is much more of this from any part of the Sun's surface than 

 from an equal area of the earth's, because of the enormously greater action that 

 an equal amount of meteoric matter would produce in entering the Sun, and be- 

 cause the Sun, by his greater attraction, must draw in meteoric matter much more 

 copiously with reference to equal areas of surface. We would have no right then, 

 as was done till Mr Watekston brought forward his theory, to neglect meteoric 

 action in speculating on solar heat, unless we could prove, which we certainly 



* To make the argument perfectly conclusive, it would have to be assumed that meteors not 

 only are, hut have been, always falling to the earth for some immense period of time. The conclu- 

 sion, however, appears sufficiently probable with the facts we know. 



\ See Philosophical Magazine, May 1848, for reference to a lecture in Manchester, on the 28th 

 April 1847, in which Mr Joule said, that " the velocity of a meteoric stone is checked by the atmo- 

 sphere and its vis viva converted into heat, which at last becomes so intense, as to melt the body and 

 dissipate it in fragments too small probably to be noticed in their fall to the ground, in most cases/' 



