英文科學(xué)讀本 第四冊(cè)·Lesson 19 Timber—Its Uses

Lesson 19 Timber—Its Uses

The trunks of all our trees yield wood. The only distinction between timber and wood is that the timber trees are so large that their trunks may be sawn up into pieces not only long, but wide and thick as well. Such pieces of wood we call logs or balks of timber. They may be used in that state, or they may be sawn up into planks. In any case, by the term timber we mean great balks or planks of wood, which, from their size and quality, are fit to be used in engineering and building. For all such purposes strength and durability are, of course, the primary objects.

Amongst the chief of the timber trees are the ash, beech, cedar, elm, fir, hornbeam, larch, lime, mahogany, oak, poplar, teak, etc. The various ornamental woods used by the cabinet-maker form a class by themselves, and are not included under the head of timber. Among these are the birch, box, cherry, ebony, maple, rosewood, satin-wood, walnut, etc.

The special qualities of each individual wood render it specially fitted for some particular kind of work. Let us examine a few varieties of wood.

1. The Ash. —This wood is prized for its great toughness and elasticity. It is coarse in texture and of considerable strength. It is much used in building and engineering. It is the principal wood used by the wheelwright, on account of its elasticity. He makes the spokes of wheels and the shafts of carriages of this wood. For the same reason it is specially fitted for all purposes where severe shocks and wrenches have to be provided against. It is the best wood for making hammer-shafts and the handles of tools generally.

2. The Beech. —This wood is almost as strong as oak, but it is specially distinguished for its closeness of grain and the even smoothness of its surface. It polishes readily, and is a beautiful wood when it is so cut as to skillfully expose the silver grain. It is much used for furniture, and it is also largely employed, because of its closeness, toughness, and strength, for making cogs for mill-wheels. For purposes such as these it requires to be kept very dry, for it quickly decays in lamp situations. It is, on the other hand, are markable fact that when it is used for purposes in which it is kept constantly in water, beech-wood shows considerable endurance.

3. The Elm. —This is a rough, cross-grained wood of remarkable strength and toughness. It is specially suitable for all sorts of rough purposes, as it is less liable to split by the driving of bolts than other woods.

Elm is very durable under water, but it rapidly decays if it is subjected to frequent changes from wet to dry. Hence it is not a good wood for exposure to the atmosphere in a climate such as ours. Its great defect is its great liability to warp and twist and get out of shape, while as regards strength, rigidity, toughness, and endurance, it is certainly inferior to oak.

The wheelwright makes the nave of his wheel of elm-wood; it is also used for making pulley-blocks, and heavy, naval gun-carriages.