1885.] METEOROLOGY. 301 



while the roots at a less depth experience the daily changes of 

 temperature in the superficial layers. A long, large, deeply- 

 penetrating tap-root will produce in the warm months a diminution, 

 and in the cold months an increase, in the temperature of the 

 superior roots. Humidity exercises a very great influence on the 

 thermometric conditions of the soil and roots. From this it may be 

 inferred that the influence of the heat of the soil is not confined to 

 the roots, but through the ascending sap makes itself he felt to a 

 certain height in the trunk. To determine by experiment this 

 hyjjothesis, Hartig made some observations on two oaks of the same 

 magnitude and same age, 200 years. At the height of a metre from 

 the ground, he introduced thermometers to three different depths, 

 and noted the thermometric condition of both trunks during the 

 winter's lethargy, as well as during the season of active vegetation. 



The aerial jjarts of a tree are exposed to the direct action of the 

 temperature of the air, and to the occasional sunshine, so that the 

 degree to which it is heated or cooled down, depends, when the con- 

 ditions are the same, on the degree and duration of the sunshine or 

 the shade. And in this respect the trees which are in a clump are 

 differently affected from those which are isolated. The rapidity 

 with which, other things being equal, the mass of a tree becomes 

 warmed, depends on the surface and thickness of the bark, and on 

 the conducting power of the roots and the wood in regard to heat. 

 Wood in general is a bad conductor of heat, but all species are not 

 alike in this respect. The influence of the specific heat of the bark 

 and wood, as well as the chemico-physiological functions of the tree, 

 however, we shall not here take into account. 



Differences in the size and diameter of the tree produce differences 

 in its temperature. The daily oscillations and the maximum 

 temjjerature of the trunk are so much the greater as the diameter of 

 the trunk is less or as the j)ouit of examination is nearer to the surface. 

 The thermometer shows a higher temperature in the branches than 

 in the trunk ; and so much is it so as to show the temperature 

 indicated by thermometers introduced into the trunk branch and 

 branchlet to be in inverse proportion to their respective diameters. 



From this, then, it is deduced that the internal temperature of a 

 tree takes a character related to and is produced by a great many 

 circumstances having a reaction upon' one another. It is therefore 

 necessary, in order to similar observations being comparable together, 

 that they should be made under identical conditions, and exact 

 accordance in the location of the thermometers, in the height and depth 

 of their position, the diameter of the trunk at the point at which the 

 temperature is observed, and finally in the measure of sunshine or shade. 



All of these particulars serve as a basis to determine at three 



