244 INTEKACTION OF NATUEAL FOKCES. 



have been made by the French physicist Pouillet, and it has 

 been found that the sun gives out a quantity of heat per hour 

 equal to that which a layer of the densest coal ten feet thick 

 would give out by its combustion ; and hence in a year a 

 quantity equal to the combustion of a layer of seventeen miles. 

 If this heat were drawn uniformly from the entire mass of the 

 sun, its temperature would only be diminished thereby one and 

 one third of a degree centigrade per year, assuming its capa- 

 city for heat to be equal to that of water. These results can 

 give us an idea of the magnitude of the emission, in relation 

 to the surface and mass of the sun ; but they cannot inform 

 us whether the sun radiates heat as a glowing body, which 

 since its formation has its heat accumulated within it, or 

 whether a new generation of heat by chemical processes 

 takes place at the sun's surface. At all events the law of the 

 conservation of force teaches us that no process analogous to 

 those known at the surface of the earth, can supply for eternity 

 an inexhaustible amount of light and heat to the sun. But 

 the same law also teaches that the store of force at present 

 existing, as heat, or as what may become heat, is sufficient for 

 an immeasurable time. With regard to the store of chemical 

 force in the sun, we can form no conjecture, and the store of 

 heat there existing can only be determined by very uncertain 

 estimations. If, however, we adopt the very probable view, 

 that the remarkably small density of so large a body is caused 

 by its high temperature, and may become greater in time, it 

 may be calculated that if the diameter of the sun were dimin- 

 ished only the ten-thousandth part of its present length, by 

 this act a sufficient quantity of heat would be generated to 

 cover the total emission for 2100 years. Such a small change 

 besides it would be difficult to detect even by the finest astro- 

 nomical observations. 



Indeed, from the commencement of the period during 

 which we possess historic accounts, that is, for a period of 

 about 4000 years, the temperature of the earth has not sensi- 



