352 POPULAR LECTURES AND ADDRESSES 



position were true, the meteors by which the sun's 

 heat would have been produced during the last 

 2,000 or 3,000 years must have been all that time 

 much within the earth's distance from the sun, 

 and must therefore have approached the central 

 body in very gradual spirals ; because, if enough 

 of matter to produce the supposed thermal effect 

 fell in from space outside the earth's orbit, the 

 length of the year would have been very sensibly 

 shortened by the additions to the sun's mass which 

 must have been made: The quantity of matter 

 annually falling in must, on that supposition, 

 have amounted to 1/47 of the earth's mass, or to 

 1/15,000,000 of the sun's ; and therefore it would 

 be necessary to suppose the " Zodiacal Light " to 

 amount to at least 1/5,000 of the sun's mass, to 

 account in the same way for a future supply of 

 3,000 years' sun-heat. When these conclusions 

 were first published it was pointed out that 

 " disturbances in the motions of visible planets " 

 should be looked for, as affording us means for 



Philosophical Magazine, December, 1854 (Mathematical and 

 Physical Papers, Vol. II., Article LXVL). 





