HERMANN VON HELMHOLTZ 301 



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 21CXD 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 observations, but we have in- 

 formation 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 mo- 

 ment have the same limits of distribution that they had in the times of 

 Abraham and Homer ; from which we may infer backwards the con- 

 stancy 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 conclu- 

 sion has been drawn, that the heat of our climate has diminished since 

 the time referred to. Against this, however. Dove has cited the re- 

 ports of ancient chroniclers, according 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 im- 

 mensely 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 planetary system occurs, is totally 

 incapable of measurement, still the inexorable laws of mechanics in- 

 dicate that this store of force, which can only suffer loss and not gain, 

 must be finally exhausted. Shall we terrify ourselves by this thought? 

 Men are in the habit of measuring the greatness and the wisdom of 

 the universe by the duration and the profit which it promises to their 

 own race; but the past history of the earth already shows what an 

 insignificant moment the duration of the existence of our race upon 



