1284

THEORY AND PRACTICE OP ENGINEERING.

Book IT.

the bark and the wood, and in process of time converted into cellular and woody tissue, andthat the alburnum derives its origin from the bark, and not from the wood ; whilst othersimagine that wood and bark arc independent formations out of cambium. The central partof the tree is the best timber, for the last layers or rings have their vessels larger and lesscompressed, being full of juices, and consequently of less density.

Different Species of Timber used in Construction. — Acacia , (llobinia pseudo-Acacia, orLocust Tree,') forms heartwood at an early stage ; when it arrives at maturity it measuresnearly 2 feet in diameter ; in the pleasure-ground it is a very ornamental tree, and of rapidgrowth. When used for construction it is durable, and well adapted for sills and wall-plates, its lateral strength being greater than that of oak, in the proportion of 1 to m l5 : itis heavier, harder, and tougher than the best oak, and more elastic, not readily breaking toany strain : for posts placed in the ground it will endure sound a long time ; it is valuablefor fences and treenails for ship-building. The weight of acacia is one-sixth greater thanthat of oak.

lbs. oz.

It weighs when newly cut - - - - 63 3 per cube foot.

half dry - - - 56 4 —

quite dry - - - - 48 4 —

Its strength is as, compared to oak, 1427 to 820, and its elasticity as 21 to 9 : its lateralstrength has perhaps been too highly rated.

Alder (Alnus). — This tree has the property of enduring a long time under water, whichrenders it valuable for piles and water-pipes : it has a fine and close texture, is of abeautiful colour, works easily and well, and has been much used by the joiner and cabinet-maker.

lbs. oz.

It weighs when green - - - 62 4 per cube foot.

half dry - - - - 48 8 —

quite dry - - - - 39 4 —

The charcoal obtained from it is valuable for gunpowder, and 1000 lbs. of its ashesyield 65 lbs. of potash.

Ash (Fraxinus excelsior ) grows to a large size, with a straight trunk ; its wood is veryelastic, and joists cut out of it bear before breaking a greater weight than almost any othervariety of British timber. The wood when young is very flexible, and easy to work ; butafter it has undergone the process of seasoning, it is both tough and hard, and is thenmore used by the wheelwright than the carpenter. When subject to alternate dryness andmoisture it is not durable, and if cut down when full of sap the worm soon attacks it: itsquality is much altered in different soils.

lbs oz.

When green it weighs - - - - 64 9 per cube foot,

dry it weighs - - - - 49 8 —

Beech (Fagus sylvatica ) is described by Pliny as one of the thirteen species of treesbearing mast or acorns; on a chalky soil it acquires a prodigious size, and is the finesttree of the forest: its timber is smooth and hard, its transverse fibres very obvious, some-times forming distinct and dark lines, and it is highly useful for various purposes. Theworm attacks it speedily, if not freed from sap, which is often done by steeping it in water,and then exposing it to smoke: when beech is constantly under water, it is very lasting,and therefore well adapted for piles; in a dry state and much exposed, it will split andcrack.

lbs. oz.

When green it weighs - - - 65 13 per cube foot,

half dry - - - - 56 6 —

quite dry - - - - - 50 3 —

Its ashes are productive of potash, 100 lbs. cf green wood producing 1 lb. 10 oz.

Birch (Betula alba) has its wood tinged with red, and a moderately fine grain; whenthe tree has acquired its full growth, the timber is exceedingly hard, but not very durable;it works better in a green state than when perfectly seasoned and dry.

lbs. oz.

When green it weighs - - - - 65 4 per cube foot.

half dry - - - - - 56 6 _

dry - - - - - -45 1 —

In a tree 60 years old, perfectly dry, a cube foot only weighed 36 lbs. 13 oz. : 1000 lbs.weight of this wood burnt in a green state yields 1 lb. 4 oz. of potash, which is most abun-dant in the bark and spray.

Box (Buxus sempercirens) is a valuable wood, of a yellowish colour, close grained, veryhard and heavy ; it cuts better than most others, is susceptible of a fine polish, and is verydurable ; it is much used by turners and mathematical instrument-makers, and by the wood-engraver almost exclusively.