Of the different kinds of timber-trees, such as Oak, Ash, Beech, etc. we have already treated in the progress of this work, and stated the most appropriate methods of rearing them : hence, we shall at present confine our attention to the proper times for felling, seasoning, and preserving timber.
The age at which timber-trees ought to be cut down, varies according their nature, and the purposes for which they are designed. - Mr. Marshall observes, that poplars may stand from 30 to 50 years; ash and elm-trees, from 50 to 100 years ; but oaks should on no account be felled, till they have attained the age of from 100 to 200 years. The durability of timber, however, depends on the soil and sub-soil, on which the trees have been produced: thus, in loose or absorbent lauds, the oak and elm decay at an earlier period than those grown in cool and more retentive ground; but, in dry loams with a rocky sub-soil, the oak rapidly decays, after it has attained the age of two centuries. - Farther, the Cypress, Cedar, and Mahogany, are supposed to be indestructible by time, or the depredations of insects, in whatever exposure they may have been raised; and the Alder endures the action of water, for many years longer than any other species of timber : hence it is deservedly employed for piles In constructing dams; in order to secure the banks of rivers.
The season for felling oak, or fir-timber, commences toward the end of April; though the winter is often preferred for that purpose, and likewise for cutting down ash and elm-trees ; as it furuishes labour to persons who would otherwise be unemployed. All branches or limbs, that may injure the trees in their fall, ought previously to be lopped, and the trunk cut down close to the ground, unless it be grubbed or rooted up ; which latter method is preferable, as well as more profitable ; because timber, produced from old stocks, is of inferior value.
After the trees are felled, it will be advisable to season them, previously to working the timber : it has, therefore, been recommended to pile them, one upon another, with small blocks of wood between each, in an open but not too exposed situation ; so that the ; sun and air may penetrate every part ; the surface, or uppermost trees, being plastered with cow-dung, to prevent them from cracking. Another method is that of burying the timber in the earth; but the most effectual mode of seasoning it, by heat, is performed at Venice, where the trees are exposed to a strong fire, at which they are continually turned round by means of an engine, till they become charred, or covered with a black crust: by this expedient, the internal part of the wood is so hardened, as to resist equally the effects of earth and water, for a long series of years, without receiving any injury.
When boards or planks have been properly dried, additional care becomes necessary to preserve them against the depredations of worms, the effects of air, moisture, etc. For this purpose, Mr. Evelyn directs common sulphur to be put into a glass retort, with such a portion of aqua-fortis, as will cover it, "to the depth of three fingers :" the whole must be distilled to dryness, and reclined two or three times. The remaining sulphur is then to be exposed to the open air on a marble, or in a shallow glass-vessel, where it will liquefy into a kind of oil, with which the timber must be anointed: this mixture, he asserts, will not only infallibly prevent the attacks of worms, but also preserve every kind of wood from decay or putrefaction, either in air or water. - Timber may also be defended from the influence of air, or moisture, by coating it two or three times with linseed oil; and some builders have advised the wood-work to be painted: the latter practice, however, ought to be adopted with great caution; because, though it may in some eases be proper, it should always be deferred, till the planks and similar articles have become perfectly dry. Lastly, no green timber, must be employed for any purpose; as it is apt to crack and splinter, when the work is completed; and will thus disfigure the most expensive buildings. Where such deformity occurs, it has been recommended to anoint the wood repeatedly with a solution of beef-suet: some carpenters, indeed, close the crevices with a composition of grease and line saw-dust; but the former method, in the opinion of M. Cho-mel, deserves the preference; as he has seen riven or split timber so perfectly closed by such expedient, that the defect was scarcely perceptible: this operation, however, ought to be performed while the wood is green. - See also BoaRD, and DRy-rot.
Timber used for buildings, especially for ships, bridges, canals, granaries, and stables, may be effectually preserved from decay, and particularly the rot, by repeatedly impregnating the wood with strong brine, or a solution of common salt. This simple process is attended with such decided advantage, that wood, thus prepared, Will remain perfectly sound for ages. An instance of this fact lately occurred in the theatre at Copenhagen, where the lower part of the planks and joists formerly required to be replaced in a few years, till Mr. VolmEister, an architect of that city, discovered and employed the process above stated. Since that period (which includes the term exceeding 12 years) the wood, on removing one of the boards, was found in such a state of preservation, that he could not observe the least appearance of decay.
In October, 1795, a patent was granted to Mr. ChistoPher Wilson, for a method of combining timber; which may be applied to the improvement of naval architecture, and likewise to all large erections of wood. - For a minute account of this invention, and of the principles on which the patentee proceeds, we are obliged to refer the curious reader to the 9th vol. of the "Repertory of Arts, " etc. where the whole is illustrated by an engraving.