Professor W, Thomson on the 



Additions (May 9, 1854), No I. Conclusion of Physical Astro- 

 nomy against the Extra-planetary Meteoric Theory. 



Meteors which when at great distances possessed, relatively to 

 the centre of gravity of the solar system, velocities not incom- 

 parably smaller than the velocity due to gravitation to the sun^s 

 surface, must strike the surfaces of the earth and of the other 

 planets not incomparably less frequently than equal areas of the 

 sun's surface, and with not incomparably smaller velocities, and 

 consequently must generate heat at the surfaces of the earth and 

 other planets not incomparably less copiously than at equal areas 

 of the sun's surface. But the whole heat emitted from any part 

 of the sun's surface is incomparably greater than all that is gene- 

 rated by meteors on an equal area of the earth's surface, and 

 therefore is incomparably greater than all that can be generated 

 at his own surface by meteors coming in with velocities exceeding 

 considerably the velocity due to his attraction from an infinite 

 distance. Hence upon the extra-planetary meteoric theory of 

 solar heat the quantity of matter required to fall in cannot be 

 much, if at all, less than that required upon the hypothesis that 

 the work done by the sun's attraction is equal to the mechanical 

 value of the heat emitted from his surface, and must therefore 

 be, as found above, about -000060 of a pound per square foot 

 per second, or 1900 lbs. per square foot in a year. The mean 

 density of the sun being about 1^ times that of water, the matter 

 in a pyramidal portion from his centre to a square foot of^ his 

 surface is about 



^ X 441,000 X 5280 x 1^ x 64=62,100,000,000 lbs., 



and the whole annual addition of meteoric matter to the sun 

 would therefore be 



1900 _ 1 



62,100,000,000 ~ 32,400,000 



of his own mass. In about 6000 years the sun would there- 

 fore be augmented by j(/oo ^^ ^lass from extra-planetary space. 

 Since the time occupied by each meteor in falling to the sun 

 from any distance would be much less than the periodic time of 

 a planet revolving at that distance, and since the periodic times 

 of the most distant of the planets is but a small fraction of 6000 

 years, it follows that the chief eflfect on the motions of the planetary 

 system produced during such a period by the attraction of the 

 matter falling in would be that depending simply on the aug- 

 mentation of the central force. To determine this, let M be the 

 sun's mass at any time t, measured from an epoch 6000 years 

 ago; to the earth's mean angular velocity, and a its meftn 



