SECTION X.
ON THE NATURE AND PROPERTIES OF TIMBER.
312. —Wood is that substance which forms the principal part of the roots, trunks, andbranches, of trees and shrubs; and its usefulness in buildings, ships, and the mechanicalarts, is well known.
The woods of different trees differ much in strength, hardness, durability, and beauty ;and, consequently, in their fitness for the various purposes to which they are applied. Thewood which is felled and seasoned for the purpose of building is called timber ; and instating the properties of woods, we shall consider those only which are fit for timber.*
313. —If the stem, or trunk of a tree be cut across, the wood is found to be made up ofnumerous concentric layers or rings; very distinct in some trees, but less so in others.One of these layers is commonly formed every year; consequently their number corres-ponds nearly with the age of the tree. Each layer consists, in general, of two parts ; theone solid, hard, heavy, and dark coloured; the other of a lighter colour, porous, and soft;which renders the lines of separation between the annual layers distinct. Scarcely any twolayers of the same tree are precisely alike; either in the proportion of the hard part, or inthe thickness of the layers; as the layers vary in thickness according to the degree ofvegetation which took place in the years of their formation: and also in the same tree theyvary in thickness, either according to the situation of the principal roots, or the aspect: theannual layers being always thicker on that side of the tree which has been most favourableto the growth of the roots, or that has had the advantage of a good aspect.
314. —The structure of wood, and the nature of the vessels through which the fluidsmove in the living plant, are not thoroughly understood : indeed the study of vegetableanatomy is attended with considerable difficulties; but some important facts have beenascertained which contribute materially towards a more perfect knowledge of the natureand properties of wood.