208 PROGRESS OF SCIENCE IN THE CENTURY. 



other suggestions. About 1848, Mayer, who shared 

 in stating the idea of the conservation of energy, 

 brought forward a '' meteoric hypothesis '' according 

 to which it was supposed that the meteorites swarm- 

 ing around the sun engendered heat by impact with 

 it, — thus furnishing a supply of heat many thousand 

 times greater than if they underwent complete com- 

 bustion. This view, also suggested by Waterston, was 

 developed in 1853 by Sir William Thomson (Lord 

 Kelvin) and was supported by Tyndall and Tait. 

 The latter says : '^ We find, by calculations in which 

 there is no possibility of large error, that this hypoth- 

 esis is thoroughly competent to explain 100,000,000 

 of years' solar radiation at the present rate, perhaps 

 more ; and it is capable of showing us how it is 

 that the sun, for thousands of years together, can 

 part with energy at the enormous rate at which it 

 does still part with it, and yet not apparently cool 

 by perhaps any measurable quantity." * 



On the other hand, while the infall of meteorites 

 and the heat they produce by impact may be re- 

 garded as certain, it is urged by competent au- 

 thorities that the " intra-planetary " supply is too 

 scanty to be more than a makeshift, while Lord 

 Kelvin himself excluded an " extra-planetary '' 

 supply on the ground that if it were true the year 

 would be shorter now by six weeks than at the open- 

 ing of the Christian era.f 



In 1854, Helmholtz gave the answer which is 

 now generally accepted. If we start with the reason- 

 able assumption of a once larger and less condensed 

 sun, we can understand that as the sun shrank 

 there was thereby accumulated a great thermal store 

 * Recent Advances, 1876, pp. 153-54. 

 f See Miss Clerke's History, p. 352. 



