514 Prof. Helmholtz on the Interaction of Natural Forces. 



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

 combustion ; and hence in a year a quantity equal to the com- 

 bustion of a layer of 17 miles. If this heat were drawn uni- 

 formly fi'om the entire mass of the sun^ its temperature would 

 only be diminished thereby lird of a. degree Centigrade per 

 year, assuming its capacity for heat to be equal to that of 

 water. These residts can give us an idea of the magnitude of 

 the emission, in relation to the surface and mass of the sun j 

 but they cannot inform us whether the sun radiates heat as a 

 glowing body, which since its formation has its heat accumu- 

 lated 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 eter- 

 nity 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 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 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 have information regarding the distribution of 

 certain cultivated plants, the vine, the olive tree, which are veiy 

 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 backwards the constancy of the climate. 



In opposition to this it has been urged, that here in Prussia 

 the German knights in former times cultivated the vine, cellared 

 their own wine and drank it, which is no longer possible. From 

 this the conclusion has been drawn, that the heat of our climate 

 has diminished since the time referred to. Against this, how- 

 ever. Dove has cited the reports of ancient chroniclers, according 

 to which, in some peculiarly hot years, the Prussian grape pos- 

 sessed somewhat less than its usual quantity of acid. The fact 



