ON THE AGE OF THE SUN'S HEAT. 365 



the length of the year, as known for the last 2,000 

 years, may not have been sensibly disturbed by the 

 accessions which the sun's mass must have had 

 during that period, if the heat radiated away has 

 been always compensated by heat generated by 

 meteoric influx. 



For reasons mentioned in the first part of the 

 present article, we may now believe that all 

 theories of complete, or nearly complete, contem- 

 poraneous meteoric compensation, must be rejected ; 

 but we may still hold that 



" Meteoric action . . . . is . . . . 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" 1 



The form of meteoric theory which now seems 

 most probable, and which was first discussed on 

 true thermodynamic principles by Helmholtz, 2 

 consists in supposing the sun and his heat to 

 have originated in a coalition of smaller bodies, 

 falling together by mutual gravitation, and 



1 " Mechanical Energies of the Solar System." See note p. 351. 



2 Popular lecture delivered on the yth February, 1854, at Konigs- 

 berg, on the occasion of the Kant commemoration. 



