412 Professor W. Thomson on the 



at any instant such as to correspond to elliptical or circular orbits 

 round the sun, the effects of the resisting medium would gradually 

 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 atmo- 

 sphere, 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 shown 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 "fall- 

 ing 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 pro- 

 bable, 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 because 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. Waterston brought forward his theory, to neglect meteoric 

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

 we certainly cannot do, that its influence is insensible. It is in 

 fact not only proved to exist as a cause of solar heat, but it is 

 the only one of all conceivable causes which we know to exist 

 from independent evidence. 



To test the possibility of this being the principal or the sole 

 cause of the phaenomenon, let us estimate at what rate meteoric 

 matter would have to fall on the sun, to generate as much heat 



earth for some immense period of time. The conclusion, however, appears 

 sufficiently probable with the facts we know. 



* See Philosoj)hical 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 atmos])here 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." 



