472 POPULAR LECTURES AND ADDRESSES. 



propagated at the rate of about nine and a half 

 million million kilometres l per year, and therefore 

 twenty-eight and a half thousand million million 

 kilometres in three thousand years. We do not 

 know for certain whether the light which left the 

 sun three thousand years ago is still travelling out- 

 wards with almost undiminished energy, or whether 

 nearly all is already dissipated in heat, warming 

 the luminiferous ether, or ponderable bodies 

 which have obstructed its course. But we may, I 

 think, feel almost sure that it is partly still travel- 

 ling outwards as radiant heat, and partly spent (or 

 dissipated) in warming ponderable matter (or 

 ponderable matter and the luminiferous ether). 



The running down of the weight in the clock- 

 work has its perfect analogue, as Helmholtz was, 

 I believe, in reality the very first to point 'out, in 

 the shrinkage of the sun from century to century 

 under the influence of the mutual gravitational at- 

 tractions between its parts. The heat-producing 

 efficiency of the fire which then: would he if the sun 



, ] The "kilometre" is sixty two hundredths of the IJiitish statute 

 ; rather a lony half mile in fact. 



