463. An experiment to determine the comparative durability of different woods is related in Young's ' Annals of Agriculture,' which will be more satisfactory than any speculative opinion; and it is much to bo regretted that such experiments have not been oftener made.

"Inch-and-half planks of trees from thirty to forty-five years' growth, after ten years' standing in the weather, were examined and found to be in the following state and condition: -

Cedar, perfectly sound. Larch, the heart sound, but sap quite decayed. Spruce fir, sound. Silver fir, in decay. Scotch fir, much decayed. Pinaster, quite rotten.

Chestnut, perfectly sound. Abele, sound. Beech, sound. Walnut, in decay. Sycamore, much decayed. Birch, quite rotten." *

This shows at once the kinds that are best adapted to resist the weather; but even in the same kind of wood there is much difference in the durability, and the observation is as old as Pliny, "that the timber of those trees which grow in moist and shady places is not so good as that which comes from a more exposed situation, nor is it so close, substantial, and durable; "† and Vitruvius has made similar observations.‡

Also split timber is more durable than sawed timber, for the fissure in splitting follows the grain, and leaves it whole, whereas the saw divides the fibres, and moisture finds more ready access to the internal parts of the wood. Split timber is also stronger than sawed timber, because the fibres being continuous, they resist by means of their longitudinal strength, but when divided by the saw, the resistance often depends upon the lateral cohesion of the fibres, which is in some woods only one-twentieth of the direct cohesion of the same fibres. For the same reason whole trees are stronger than specimens, unless the specimens be selected of a straight rain, but the difference in large scantlings is so small as not to be deserving of notice in practice.

* 'Annals of Agriculture,' vol. vi., p. 256.

† Pliny, as quoted by Evelyn, ' Silva,' vol. i., p. 87.

‡' Vitruvius,' book ii., chap. ix.