ON THE INTERACTION OF NATURAL FORCES. 169 



ture, 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 gene- 

 rated to cover the total emission for 2,100 years. So small a 

 change 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 4,000 

 years, the temperature of the earth has not sensibly diminished. 

 From these old ages we have certainly 110 thermornetric observa- 

 tions, but we have information 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 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, accord- 

 ing to which, in some peculiarly hot years, the Prussian grape" 

 possessed somewhat less than its usual quantity of acid. The 

 fact also speaks not so much for the climate of the country as 

 for the throats of the German drinkers. 



But even though the force store of our planetary system is 

 so immensely great, that by the incessant emission which has 

 occurred during the period of human history it has not been 

 sensibly diminished, even though the length of the time which 

 must flow by before a sensible change in the state of our plane- 

 tary system occurs ia totally incapable of measurement, still the 



