190 ox THE INTERACTION OF NATUEAL FORCES. 



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 is continually taking place 

 at the sun's surface. At all events, the law of the con- 

 servation 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 re- 

 gard 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 

 diminished 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 2,100 years. 

 So small a change it would be difficult to detect even by 

 the finest astronomical observations. 



Indeed, from the commencement of the period during 

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

 about 4,000 years, the temperature of the earth has not 

 sensibly diminished. From these old ages we have cer- 

 tainly no thermometric observations, but we have infor- 

 mation regarding the distribution of certain cultivated 

 plants, the vine, the olive tree, which are very sensitive 

 to changes of the mean annual temperature, and we find 

 that these plants at the present moment have the same 

 limits of distribution that they had in the times of 

 Abraham and Homer; from which we may infer back- 

 wards the constancy of the climate. 



