CONSTANCY OF THE EABTH ? S TEMPERATURE. 245 



bly diminished. From these old ages we have certainly no 

 thermometric observations, but we have information regard- 

 ing 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 pres- 

 ent moment have the same limits of distribution that they had 

 in the times of Abraham and Homer ; from which we may in- 

 fer 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 pos- 

 sible. From this the conclusion has been drawn, that the 

 heat of our climate has diminished since the time referred to. 

 Against this, however, Dove has cited the reports 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 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 planetary system occurs, is totally incapable of measure- 

 ment, still the inexorable laws of mechanics indicate 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 it constitutes. A 

 Nineveh vessel, a Roman sword awakes in us the conception 

 of grey antiquity. What the museums of Europe show us of 



