To preserve timber is next in importance to obtaining it; for, unless properly cared for, however good may be the material, all previous precautions avail but little. Wood is liable at any time to change its nature, and part with its most valuable properties. In all timber, even if well seasoned, there remains a certain quantity of sap and vegetable matter, which, when the piece is shut up in stagnant air liable to be heated in summer, will mould or ferment, and an acid be formed, decomposing the wood.

In no instance should a piece of framing be so enclosed that fresh air cannot at all times come in contact with it. To every roof, church-spire, dome, etc, there should be air-holes at such points above and below as will insure a continual circulation of air about the wood.

The next consideration is to protect it from the action of alternate moisture and dryness. If a stick of timber be exposed to a continued heat, - as, for instance, over a baker's oven, - it will in time lose its elastic power, and become brittle. If, on the other hand, a piece of the same timber, in all respects identical in properties and nature, be immersed in water, and remain submerged, it will retain the larger part of its properties for centuries. In fact, if not injured by insects or acids in the water, it may be considered as almost indestructible; but the reverse of this is the case if it be subjected to the alternate action of water and air.

As soon as a piece of wood is exposed to the action of moisture, the vegetable and saccharine matter begins to dissolve; and a slimy coating is formed wherever the solution exists. When exposed to the air, this slime ferments; and from it grows a sort of fungus, which lives on the vital parts of the wood itself. A continual series of exposures of this kind soon induces a visible decay, which ends in the entire destruction of the timber.

Every part of a frame should in some way be protected. No work should, therefore, be placed on or very near the ground, where earth can in any way come into immediate contact with it; nor should any be subjected to the direct action of steam or vapor, without being in some way shielded. The artificial methods of preserving timber are numerous. Covering the work with boards is in many instances effectual. Sometimes, however, this is no protection, but, on the contrary, serves to retain moisture that would evapo-porate if left exposed, as is often the case with the sills of buildings, timber-bridges, &c

In every instance where the timber must be covered, but is necessarily exposed to moisture, also when it is exposed to the action of the weather without a covering, artificial means for preservation must be resorted to.

The first step to be taken is to season the timber thoroughly. When this has been done, the whole of the exposed surfaces should be covered with some preparation that will strike into the wood, and, as much as possible, harden the outside. Thus, by closing the pores, the piece is made impervious to the weather.

A valuable practice for the preservation of timber is to heat it by burning charcoal, or something of like nature, beneath; and, while it is in a heated state, applying a hot solution of an ounce and a half of corrosive sublimate, or one of aqua fords (nitric acid), in a gallon of water. After this is done, and the work well dried, it should be painted with the best quality of white lead and oil.

A thin solution of hot coal-tar and whale-oil is of great benefit to all timber that is to be placed near the ground. If this operation be repeated two or three times, and finely pulverized clinkers from a blacksmith's forge, or the dust made from the scales of iron which lie about an anvil, be sifted upon the timber while the tar is newly put on, it will possess great durability, since wood prepared in this manner is scarcely susceptible of decay.

Pyroligneous acid, or the liquor that drips from stove-pipes in which vapor condenses, also strong decoctions of soot applied hot, are recommended as good preservative agents. In all framing exposed to the weather, every mortise that can hold water, and their tenons, together with all the wood about them, should have a good coating of one of the preparations first named, before the work is put together.

It not unfrequently occurs that the wood at the lower end of posts, rafters, etc, of church-steeples, is found to be entirely decayed, while other parts of the structure are perfectly sound. In almost all such cases, some part of the work, being imperfect, has admitted water, which, following down the post to its end, has -filled the mortise, and thus rotted the wood.

When a piece of framing is to be permanently exposed to the weather, it should be treated with one of the solutions first described, and then thoroughly painted and sanded. Wood in a proper condition when felled, afterwards thoroughly seasoned, well saturated with diluted corrosive sublimate, and, finally, kept properly painted and sanded, is as durable as it ever can be.

One caution it would be well to remember; namely, to refrain from applying paint or any preparation to wood before it is thoroughly seasoned: for, should the outside be coated so as effectually to prevent impenetration from without, evaporation will also be prevented from within; therefore, all moisture that may be in the wood will be retained, and rot the piece.

Another suggestion is to use timbers as small as the nature of the work will permit. It is a mistake to suppose that large timbers will continue good longer than small ones. We may see an exemplification of this at any New-England farmhouse. The light spokes of a wheel will remain sound and strong for years after the tongue of the cart to which they belonged has entirely decayed.

If a timber is sufficiently strong when first used (the requisite allowance being made for permanent strain), all has been done that prudence would dictate, since no increase of the dimensions of the piece will insure its longer duration. And, finally, it should be scrupulously remembered, that timber will not certainly remain sound because a large portion, or even most of it, is in proper condition, and well cared for. Only so much as is actually protected will retain its qualities; and any part so exposed as to injure the whole stick will as surely injure or destroy the unprotected part. Therefore, the ends of timbers which are built into walls, also all surfaces in contact, - as where the side or edge of one stick rests upon another, tenons in mortises, etc, - should be supplied with air, kept dry, and in every way properly protected.