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XX. Upon the extent of the expansion and contraction of timber 
in different directions relative to the position of the medulla of 
the tree. By Thomas Andrew Knight, Esq. F. R. S. In a 
Letter addressed to the Right Hon. Sir Joseph Banks, Bart. 
G.C. B. P.R. S. 
Read May 8, 1817. 
My Dear Sir, 
M any attempts have been made by writers on vegetable 
physiology, to account for the force with which the sap of 
trees has been proved by Hale to ascend during the spring, 
without any hypothesis having been offered, which has been 
thought satisfactory : and almost all, which have been offered, 
have been justly rejected as wholly inadequate. I have sug- 
gested in the Philosophical Transactions of 1801, sd Part, 
page 533, the expansion and contraction of those cellular 
processes, which proceed from the bark to the medulla, 
which I have there called the true, or silver grain of the 
wood; and which have, generally, though most improperly, 
been called medullary processes. I have there shown, that 
this substance expands and contracts very considerably under 
changes of temperatyre and moisture ; and I have stated that 
a board of oak, which has been formed by cutting across the 
supposed medullary processes, can scarcely be made, by any 
means, to retain the same form and position when subjected 
to various degrees of heat and moisture. I had not at that 
time ascertained, with accuracy, the comparative expansion 
