FORESTS AND THEIR CLIMATIC INFLUENCE. 403 



and its bark a more considerable absorbing power; so true is this that 

 when the trunk of a phini tree was enveloped to the height of 2 metres 

 (7 feet) with tin, which possesses strong reflecting i^roperties, the tem- 

 perature of the air being perceptibly the same as in the preceding case, 

 the difference between the maximum and minimum descended from 

 130.07 to 50,2. (23° to 1(P.) It Avill be seen from this that the tempera- 

 ture had become more uniform in the plum tree. On removing the en- 

 velope, the difference between the maxima and minima increased and 

 became what it had been before. 



Metallic envelopes or those of straw diminishing in trees the varia- 

 tions of temperature and rendering the movement of heat more regu- 

 lar, it will readily be conceived that the nature and thickness of the 

 bark must exert a great influence on the calorific state of trees. Ex- 

 periments made on the Opuntia and on other plants tend to show that 

 the leaves and small branches are ]n'omptly brought into an equilibrium 

 of temperature with the ambient air. On comparing the mean temper- 

 ature of the air with that of the interior of a chestnut tree having a 

 diameter of 0"\5, (20 inches,) it has been found that the mean of the 

 temperatures observed in the tree, during a period of thirteen months, 

 was superior by 0°.3G to that of the air at its surface, and by 0°.83 to 

 the temperature of the air at the north and in the sliade ; this difference 

 is owing probably to the fact that the thermometer was placed to the 

 north and sheltered from the sun, while the tree was shielded from the 

 norih winds by a neighboring building, and was exposed moreover to 

 the solar radiation. Experiments made on other trees show that the 

 principle of equilibrium of temperature between the air and the trees 

 shifts with the lapse of more or less time, and so much the more rapidly 

 as the variations in the air are less frequent. In winter and autumn 

 the difference is at a minimum, and in spring and summer it is at a maxi- 

 mum. The maximum of temperature in the air takes place, according 

 to the season, between 2 and 3 o'clock in the afternoon, while in 

 the tree it occurs after sunset. If regard be had to the seasons, it will 

 be found that it is in summer especially that the maximum is more 

 marked ; it does not occur till about nine o'clock in the evening. 



The heat disengaged in the organs and tissues of plants interposes 

 but very feebly as regards their i)roper temperature, which is almost 

 "wholly of extrinsic derivation ; for its principal cause, we must look to 

 the solar radiation and the temi^erature of the air. The diurnal varia- 

 tion of temperature in the air is easy to determine, since it is the differ- 

 ence between the maximum and minimum of the day. To find this 

 variation in a tree is a matter of difficulty, but we may arrive at it, in 

 at least an approximate manner, by the following means : 



Observations on temperature were made at Geneva, from 179G to ISOO, 

 at the rising and setting of the sun, and at 2 o'clock p. m., in the air to 

 the north and in the interior of a chestnut tree 0'".G (24 inches) in 

 diameter; the maxima and minima could be obtained by combining the 

 temperatures at 2 o'clock and at the rising and setting of the sun, the 

 maximum taking place about or after the setting of the sun, as was said 

 above, and the minimum about the time of its rising; the difference 

 obviously gives the variations within the tree. By discussing the varia- 

 tions thus obtained in the air and in the tree, it was seen that, during 

 the years 1790, 1797, and 1798, the variations were, on a mean, more than 

 five times greater in the air than in the tree. 



In the observations made at the Jardiu des Plantes, from December, 

 1858, to July, 1859, it was ascertained that the means of the variations 

 of temperature in the air and in the tree were in the ratio of 3^.80 to 



