H. HelmhoUz on the Interaction of Natural Forces. 213 



nation. As 



has but little influence 



on the temperature of the surface, the heat of the sun is the o ily 

 thing which essentMj aflects the question. The quantity of 

 heat falling from the sun during a given time upon a given p )r- 

 tion of the earth's surface may be measured, and from th'S it 



can 



given lime is sent out irom 



the entire sun. Such measurements 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 Avhich a layer 

 of the densest coal 10 feet thick would give out by its combus- 

 tion ; and hence in a year a quantity equal to the combustion of 

 a layer of 17 miles. If this heat were drawn uniformly from 

 the entire mass of the sun, its temperature would only be dimin- 

 ished thereby l^d of a degree Centigrade per year, assuming its 

 capacity for heat to be equal to that. of water. These results 

 can give us an idea of the magnitude of the emissionj 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 sur- 

 face 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 a^ 

 what may become heat, is sufficient for an immeasurable time. 

 With rerard to the store of chemical force in the sun, we can 



can 



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 suflScient 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 astronomical 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 sensibly diminished. 

 From these old ages we have certainly no thermometric observa- 

 tions, but we ha\? information regarding the distribution of cer- 

 tain 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 lim- 

 its of distribution that they had in the times of Abraham and 

 Homer; from which we may infer backwards the constancy of 

 the climate. 



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