relative Conductibility for Caloric of different Woods, <§-c. 133 



geneousness render the results less regular than in the experi- 

 ments which have been made on other substances ; but there is 

 a considerable difference according to the directions of the ca- 

 loric, with reference to the woody layers. Woods are much 

 worse conductors in the direction contrary to the fibres of which 

 they are composed, than in that of their length. The diffe- 

 rence which results from these directions of the caloric is so 

 much the greatei*, the smaller the degree of conducting power 

 which the wood possesses. Thus, referring to the second ther- 

 mometer, and taking in each wood the differences resulting from 

 the direction of the fibres, we find 16° in the chesnut, 22° in 

 the oak, and 28° in the fir. In the oak, the conductibility in 

 the direction of the fibres is to that in the perpendicular direc- 

 tion as 5 to 3. 



The curve formed by the heights of the thermometers, which 

 is a logarithmic curve in the bodies that are very good conduc- 

 tors, is not so regular in the substances which conduct ill. It 

 decreases at first very quickly, and then becomes nearly parallel 

 to the line of the abscisses. Thus, in the oak, the second ther- 

 mometer being at a height six times less than the first, the last 

 is very little different from the one next to it ; it is at 1°, and 

 the next to the last at 1° 5&, while in the hornbeam the quo- 

 tients are nearly equal. These numbers, which are directly 

 given by experiment, do not express the conducting powers in 

 an absolute manner, for they are the result of the combination 

 of several elements, such as the dimensions of the body, their 

 radiating power, &c., which elements would require to be cal- 

 culated, were it wished to compare exactly the conductibility of 

 woods with tliat of other substances. 



The great difference which results from the direction, accord- 

 ing to which the woody layers present themselves to the caloric, 

 may in part explain how trees preserve so well in the interior 

 of their trunk the temperature of the soil, from which they ex- 

 tract their nourishment. On the one hand, this temperature 

 transmits itself by the ascent of the fluids, and by its propaga- 

 tion in the solid tissue of the wood, while the little conductibi- 

 lity in the transverse direction form.s a great obstacle to its 

 coming into equilibrium with the external temperature. 



