302 Mr. Knight on the inverted Action 
Du Hamel, and that I had one fact to communicate relative 
to the effects produced by the stagnation of the descending 
sap of resinous trees, which appeared to lead to important 
consequences. I have in my possession a piece of a fir-tree, 
from which a portion of bark, extending round its whole 
stem, had been taken off several years before the tree was 
felled ; and of this portion of wood one part grew above, and 
the other below, the decorticated space. Conceiving that, ac- 
cording to the theory I am endeavouring to support, the wood 
above the decorticated space ought to be much heavier than 
that below it, owing to the stagnation of the descending sap, 
I ascertained the specific gravity of both kinds, taking a 
wedge of each as nearly of the same form, as I could obtain, 
and I found the difference greatly more than I had anticipated, 
the specific gravity of the wood above the decorticated space 
being 0.590, and of that below only 0.491 : and having steeped 
pieces of each, which weighed a hundred grains, during twelve 
hours in water, I found the latter had absorbed 69 grains, and 
the former only 51. 
The increased solidity of the wood above the decorticated 
space, in this instance, must, I conceive have arisen from the 
stagnation of the true sap in its descent from the leaves ; and 
therefore in felling firs, or other resinous trees, considerable 
advantages may be expected from stripping off a portion of 
their bark all round their trunks, close to the surface of the 
ground, about the end of May or beginning of June, in the 
summer preceding the autumn in which they are to be felled. 
For much of the resinous matter contained in the roots of 
these is probably carried up by the ascending sap in the 
