274 Mr. Knight on the extent of the expansion , &c. 
which is observable only in lifeless vrood ; and which has 
apparently no connection with the power by which the sap is 
raised in the living tree. The interior and older layers of wood 
are much more solid and specifically heavy, than the external 
layers in the same tree ; and the latter, consequently, con- 
tract more longitudinally in drying than the former, and the 
edge of every board (that has been cut with surfaces nearly 
parallel with the line of the convergent cellular processes) 
which lay nearest the medulla in the tree, will therefore in 
drying become convex, whilst the opposite edge will become 
concave. The ill effects of this are often felt when oak tim- 
ber is employed to form joists, part of these in drying always 
rising above, and others sinking below the first and proper 
position. The cause of some musical and other instruments 
being put out of order by changes of weather, whilst others, 
apparently similarly constructed, are free from such de- 
fects, may probably be traced to one of the sources above- 
mentioned. 
I am, my dear Sir, &c. 
T. A. KNIGHT. 
Downton, April z 6 , 1817. 
The Right Hon. Sir Joseph Banks, Bart. G. C. B. P.R. S. 
