1 86 POPULAR LECTURKX AND ADDRESSES. 



really to showing that the Sun's thermal capacity 

 is enormously greater, in virtue of the mutual 

 gravitation between the parts of so enormous a 

 mass, than the sum of the thermal capacities of 

 separate and smaller bodies of the same material 

 and same total mass. Reasons for adopting this 

 theory, and the consequences which follow from it, 

 are discussed in an article " On the Age of the 

 Sun's Heat," published in Macmillaits Magazine 

 for March 1862. 



For a few years Mayer's theory of solar heat 

 had seemed to me probable ; but I had been led to 

 regard it as no longer tenable, because I had been 

 in the first place driven, by consideration of the 

 very approximate constancy of the Earth's period 

 of revolution round the Sun for the last 2000 

 years, to conclude that " The principal source, 

 " perhaps the sole appreciably effective source of 

 " Sun-heat, is in bodies circulating round the Sun 

 at present inside the Earth's orbit"; 1 and because 



1 " ( )n the Mechaim-al KIKT^K-S of the Solar System," 7' 

 lions of the Royal Society of /^liit/>nr-/i, 1854; and Phil. .JA',v~. 

 1854, second half year. 



