NATURAL PHILOSOPHY. 139 



perature, 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 observa- 

 tions. 



Indeed, from the commencement of the period during which we possess 

 historic accounts, that is, for a period of about 4UOO years, the temperature 

 of the earth has not sensibly diminished. From these old ages we have 

 certainly no thermometric observations, 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 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 possible. 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 pos- 

 sessed 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 measurement, 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 our- 

 selves by this thought 1 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 concep- 

 tion of gray antiquity. What the museums of Europe show us of the re- 

 mains of Egypt and Assyria we gaze upon with silent astonishment, and 

 despair of being able to carry our thoughts back to a period so remote. 

 Still must the human race have existed for ages, and multiplied itself be- 

 fore the pyramids of Nineveh could have been erected. We estimate the 

 duration of human history at 6000 years ; but immeasurable as this time may 

 appear to us, what is it in comparison with the time during which the earth 

 carried successive series of rank plants and mighty animals, and no men ; 

 during which in our neighborhood the amber-tree bloomed, and dropped its 

 costly gum on the earth and in the sea ; when in Siberia, Europe and North 

 America groves of tropical palms flourished ; where gigantic lizards, and 



