228 
DR. TYNDALL ON MOLECULAR INFLUENCES. 
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— . , -r 
Wood possesses three axes of fluid permeability which coincide with those of calorijic 
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 dis- 
covered by Savart. The experiments also furnish an illustration of the theory of Pro- 
fessor 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 acquires 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 from within and the sudden accession 
of it from without. But nature has gone further, and clothes the tree with a sheath- 
ing of worse-conducting material than the wood itself, even in its worst direction. 
The following are the deflections obtained by submitting a number of cubes of bark 
of the same size as the cubes of wood to the same conditions of experiment . 
Deflection. Corresponding deflection 
^ produced by the wood. 
Beech-tree Bark 7 
Oak-tree Bark 7 H‘0 
Elm-tree Bark ..... 7 
Pine-tree Bark 7 ^ 
The direction of transmission, in these cases, was from the interior suiface of the 
bark outwards. 
The average deflection produced by a cube of wood, when the flux is lateral, may 
be taken at 
12 °; 
* Cambridge and Dublin Mathematical Journal, November 1851. 
