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Upon the Extent of the Expansion and Contraction of Timber in dif- 

 ferent 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. [Phil. Trans. 1817, p. 269.] 



Most of the attempts which have been made by writers on vege- 

 table physiology, to account for the force with which the sap of trees 

 ascends during the spring, having proved unsatisfactory and inade- 

 quate, Mr. Knight was induced some years ago to suggest the ex- 

 pansion and contraction of the cellular processes proceeding from 

 the bark to the medulla, and which he called the true or silver grain 

 of the wood, as concerned in this process. 



The present paper contains further experiments, showing this power 

 to be active in living trees, and were made on many kinds of timber 

 with nearly similar results. Some boards of ash and beech wood 

 were cut in opposite directions relative to their medulla, so that the 

 convergent cellular processes crossed the surfaces of some of them at 

 right angles, and were parallel with the surfaces of others. These 

 were placed, under similar circumstances, in a warm room, and the 

 former warped about ten times more than the latter, contracting 

 nearly 14 per cent, in breadth, while the others only contracted 3^ 

 per cent. During his experiment Mr. Knight was led to infer that 

 the medullary canal must be liable to considerable changes of dia- 

 meter, as the moisture of wood increases or diminishes. To ascertain 

 this, parts of the stems of young trees were carefully dried, the me- 

 dulla was removed, and metal cylinders driven with force into the 

 empty space. The pieces of wood were then suffered to absorb 

 moisture, and the medullary canal became so much enlarged as to 

 suffer the cylinders to fall out. 



Mr. Knight conceives that this kind of expansion often produces 

 those rifts in trees referred to wind or frost. That winds cannot be 

 the cause, seems obvious from the circumstance of pollard-oak-trees 

 being almost always rifted, upon which they can have little power ; 

 and the frost of this climate is seldom sufficiently intense to congeal 

 the winter sap in trees. 



The force with which this cellular substance of timber expands, is 

 more than adequate to such effects, and often overcomes a pressure 

 of many tons ; and as it is in action in the living tree, Mr. Knight is 

 of opinion that it is the agent by which the powerful propulsion of 

 the sap observed by Hales is effected. 



Observations on the Temperature of the Ocean and Atmosphere, and on 

 the Density of Sea-water, made during a Voyage to Ceylon. In a 

 Letter to Sir Humphry Davy, LL.D. F.R.S. By John Davy, M.D. 

 F.R.S. Read May 22, 1817. [Phil. Trans. 1817,^. 275.] 



The experiments on the specific gravity of sea-water, detailed in 

 this paper, were partly conducted at sea, and partly after the author's 



