Mechanical Energies of the Solar System. 4J1 



I^ alluding to theories of solar heat in former communica^ 

 tions to the Royal Society, I pointed out that the first hypo- 

 thesis is quite untenable. In fact it is demonstrable, that, unless 

 the sun be of matter inconceivably more conductive for heat, and 

 less volatile, than any terrestrial meteoric matter we know, he 

 would become dark in two or three minutes, or days, or months, 

 or years, at his present rate of emission, if he had no source of 

 energy to draw from but primitive heat*. The second has been 

 not only held by the Fire-worshipers, but has probably been 

 conceived of by all men in all times, and considered as more or less 

 probable by every philosopher who has ever speculated on the 

 subject. The third may have occurred at any time to ingenious 

 minds, and may have occurred and been set aside as not worth 

 considering ; but was never brought forward in any definite form, 

 so far as I am aware, until Mr. Waterston communicated to the 

 British Association, during its meeting at Hull, a remarkable 

 speculation on cosmical dynamics, in which he proposed the 

 theory that solar heat is produced by the impact of meteors 

 falling from extra-planetary space, and striking his surface with 

 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 capabilities of the second and third hypothesis to account 

 for the phsenomena ; to examine 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 phsenomena. 



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 f proves the existence of such bodies 

 moving about in space ; and even if the motions of these bodies are 



* This assertion is founded on the supposition that conduction is the 

 only means by which heat could reach the sun's surface from the interior, 

 and perhaps requires limitation. For it might be supposed that, as the 

 sun is no doubt a melted mass, the brightness of his surface is constantly 

 refreshed by incandescent fluid rushing from below to take the place of 

 matter falling upon the surface after becoming somewhat cooled and con- 

 sequently denser — a process which might go on for many years without 

 any sensible loss of brightness. If we consider, however, the whole annual 

 emission at the present actual rate, we find, even if the sun's thermal capa- 

 city were as great as that of an equal mass of water, that his mean tempe- 

 rature would be lowered by about 3° Cent, in two years. We may, I think, 

 safely conclude that primitive heat within the sun is not a sufficient source 

 for the emission which has continued without sensible (if any) abatement 

 for 6000 years.— (May 4, 1864.) 



t To make the argument perfectly conclusive, it would have to be as- 

 sumed that meteors not only are, but have been, always falling to the 



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