134 Dr. Tyndall on Molecular Influences. 



direction. The cohesion in the direction parallel to the layers 

 and perpendicular to the fibre is therefore a minimum. In the 

 same way, it will be found that of all lines perpendicular to the 

 fibre the line of greatest cohesion is perpendicular to the ligneous 

 layers; while the cohesion in the direction of the fibre is far 

 greater than along either of the lines just referred to. Hence — 



Wood possesses three axes of cohesion which coincide with the 

 axes of calorific conduction — the greatest with the greatest, and 

 the least with the least. 



It would have also added interest to the inquiry to have ex- 

 amined the permeability of wood to fluids in various directions. 

 Here, again, however, the experimental knowledge already 

 amassed by housewives and cask-makers comes to our aid. It is 

 well known that fluids would ooze with facility through a stave 

 cut perpendicular to the fibre ; a wooden plate, for instance, cut 

 perpendicular to the axis of a tree would be totally unfit for 

 the bottom of a vessel destined to hold spirits, water, or brine. 

 Further precautions, however, must be taken in choosing staves 

 for casks. If the surface of the stave be parallel to the ligneous 

 layers, the liquid, though with greater difficulty than in the case 

 just mentioned, will still make its way through. The stave must 

 be cut perpendicular to the layers ; for, in crossing such a stave, 

 the resistance offered to the passage of the fluid is a maximum. 

 Hence — 



Wood possesses three axes of fluid permeability which coincide 

 with those of calorific conduction, — the greatest with the greatest, 

 and the least with the least. 



To sum up : — -In this single substance we have pointed out 

 the existence of three new systems of axis ; the axes of calorific 

 conduction, of cohesion, and of fluid permeability ; all of which 

 coincide with a fourth system of axes of elasticity discovered by 

 Savart. The experiments also furnish an illustration of the 

 theory of Professor Stokes, who proves that the flux of heat 

 through any body may be referred to three rectangular axes, 

 which he calls the thermic axes of the body*. 



MM. De la Rive and DeCandolle have remarked upon the 

 influence which its feeble conducting power in a lateral direction 

 must exert in preserving within a tree the warmth which it ac- 

 quires from the soil. In virtue of this property a tree is able to 

 resist sudden changes of temperature which would probably be 

 prejudicial to it ; it resists alike the sudden abstraction of heat 



* Cambridge and Dublin Mathematical Journal, November 1851. 



