joists; and is particularly useful when the timber is warped

or twisted.

In order that the reader may the more fully understand the preceding description of the joinings of timbers, we have annexed a plate (to which the subjoined description refers,) of the best methods now in practice.

Fig. 457. No. 1 and 2, and 3 and 4, exhibit two methods of a simple joint, where the two pieces are halved upon each other; in both of which the end of one piece does not pass the outer surface of the other. No. 3 and 4 represent the two pieces before put together.

Fig. 568, is a method of joining timber, when the end of one piece passes the end of the other at a small distance. No. 1 represents the pieces before joined.

Fig. 569 shews how two pieces may be joined by what is termed a mitre. In this case, the two pieces should be fixed to another by a bolt at right angles to the mitre joint.

Fig. 570. How one piece of timber may be joined to another, when one of the pieces is extended on both sides of the other piece. Nos. 1 and 2 show the pieces before put together.

Fig. 571 shows the manner of joining the binding joists and girders. No. 1. The binding joist prepared for being joined to the girder.

Fig. 572 is the general and most approved method of framing the rafter foot into the girder.

Fig. 573 is a section of the beam, shewing the different shoulders of the rafter foot.

Fig. 574 is another example, preferable to the former, because the abutment of the inner part is better supported. In this the beam, when no broader than the rafter is thick, may be weakened, in which case, it would require a much deeper socket than is here given; and perhaps an advantage would be gained by introducing a joint like fig. 575.

Fig. 576 is the method of introducing iron straps to confine the foot of the rafter to the tie-beam.

When it is found necessary to employ iron straps for strengthening a joint, considerable attention is required to place them properly. The first thing to be ascertained is the direction of the strain. We must then endeavour, as near as we can, to resolve this strain into a strain parallel to each piece, and another perpendicular to it. Then the strap which is to be made fast to any of the pieces, must be so fixed that it shall resist in the direction parallel to the piece.

The strap which is generally misplaced, is that which connects the foot of the rafter with the tie-beam. It binds down the rafter; but does not act against its horizontal thrust. It should be placed farther back on the beam, and have a bolt through it, to allow it to turn round; and should embrace the rafter almost horizontally near the foot, and be notched square with the back of the rafter. The example given in No. 10 combines these requisites. By

moving round the eye-bolt, it follows the rafter, and can not pinch and cripple it, which it always does in its ordinary form. Straps which have eye-bolts on the very angles, and allow motion round them, are considered the most perfect.

Fig. 577 exhibits two methods of connecting the struts of a roof, or partition, &c. with the king-post.

If the action of a piece of timber on another does not extend, but compress, the same, there is no difficulty whatever in the joint, indeed joining is unnecessary: it is enough that the pieces abut on each other; and we have only to take care that the mutual pressure be equally borne by all the parts, and that no lateral pressure, which may cause one of the pieces to slide on the butting joint, be produced. At the joggle of a king-post, a very slight mortise and tenon, with a rafter, or straining beam, is sufficient. It is generally best to make the butting plain, bisecting the angle formed by the sides, or else perpendicular to one of the pieces. For instance, the joint a is preferable to b, and, indeed, to any uneven joints, which never fail to produce very unequal pressures, by which some of the parts are crippled, and others splintered off.

Fig. 578 is the method of securing the tie-beam and principals, when the king-post is made of an iron rod..

Fig. 579 shows a method of joining the principals with the king-post by means of an iron dove-tail, which is received in a mortise at the head of each principal.

Trusting that the reader will be able, from the above description, to comprehend the best methods of joining timbers, we shall next proceed to describe the modes of connecting several timbers, in order to complete the design, and to effect certain powers respectively required by each individual piece.

In framing centres for groins, the boarding which forms the interior surface is supported by transverse ribs of timber, which are either constructed simply, or with trusses, according to the magnitude of the work; and, as a groin consists generally of two vaults intersecting each other, one of them is always boarded over the same as a plain vault, without any respect to the other, which is afterwards ribbed and boarded so as to make out the regular surface.

Timbers inserted in walls, and at returns, or angles, are joined together where the magnitude of the building or exposure to strain may require. There are three denominations, viz. bond timber, lintels, and wall-plates.

Flooring is supported by one or more rows of parallel beams, called naked, or carcase flooring, and is denominated either single or double. During the construction of the building, the flooring, if not supported by walls or partitions, must be shored. The framing of flooring, whether single or double, depends upon the magnitude of the building, the horizontal dimensions of the apartments, or the stress with which the surface of the boarding is likely to be affected. When the flooring is intended to be very stiff and firm, it is necessary to introduce truss girders. Naked flooring, for ball-rooms, should be framed very strong, and the upper part contrived with a spring, to bend with the impression of the force, while the lower part, which sustains the ceiling, remains immovable.

Partitions are constructed of a number of pieces of timber, called scantling, placed vertically, at a specified distance from each other, dependent on the purposes for which it is intended to answer. If to support girders, they should be trussed, and afterwards filled in with parallel pieces, called studs.

The framing ought to be so contrived, as to supersede the necessity of hanging up the floor, in whatever situation the doors may be placed. Truss partitions are also of the greatest utility in supporting floors which are above them.

The rafters which support the covering in a roof are sustained by one, two, or several pieces of framing, called a pair of principals, placed at right angles to the ridge of the roof. In roofing, many ingenious contrivances are resorted to, their application depending upon the pitch of the roof, the number of compartments into which it may be divided, and the introduction of tie-beams. In cases where apartments are required to be within the framing of the roof, and it is inconvenient to introduce tie-beams, the sides of the roof may be prevented from descending, by arching them with cast-iron, or trussing them with wood in the inclined planes of their sides. To restrain the pressure of the rafters, which would be discharged at the extremities of the building, a strong wall-plate, well connected in all its parts, must be introduced, to act as a tie, and prevent the lateral pressure from forcing out the walls.

In this construction, as well as in the former, the rafters would have a tendency to become hollow, so that it is necessary, in order to counteract this tendency, to introduce straining beams at convenient heights; and if it be requisite to occupy very little space by the wood-work, cast-iron

arches, abutting upon each other, and screwed with their planes upon the upper sides of the rafters, are best adapted for the purpose. If this and the former principle were adopted, the combined effect would be very great.

We shall now present the reader with a few practical observations.

Timber, except it stand perpendicular to the horizon, is much weakened by its own weight. The bending of timber is nearly in proportion to the weight laid on it. No beam, ought to be trusted for any long time, with above one-third or one-fourth part of the weight it will absolutely carry; for experiments prove, that a far less weight will break a piece of timber when hung to it a considerable time, than is sufficient to break it when first applied.

The strain occasioned by pulling timber in the direction of its length, is called tension. It frequently occurs in roofs, and is therefore worthy of consideration.

The absolute strength of a fibre, or small thread of timber, is the force by which every part of it is held together, and is equal to the force that would be required to pull it asunder. The force required to tear any number of threads asunder, is proportional to that of their sum; but the areas of the sections of two pieces of timber, composed of fibres of the same kind, are as the number of fibres in each; therefore, the strength of the timber is as the areas of the sections. Hence all prismatic bodies are equally strong; that is, they will not break in one part rather than in another. Bodies which have unequal sections, will break at their smallest part; therefore if the absolute strength required to tear a square inch of each kind of timber be known, we shall be able to determine the strength of any other quantity whatever.

The wood next to the bark, commonly called white or blea, is also weaker than the rest and the wood gradually increases in strength as we recede from the centre to the blea.

The heart of a tree is never in its centre, but always nearer to the north side, and on that side the annual coats of wood are thinner. In conformity to this, it is a general opinion among carpenters, that that timber is strongest whose annual plates are thickest. The Trachea, or airvessels, are weaker than the simple ligneous fibres. These air-vessels make the separations between the annual plates, and are the same in diameter, and number of rows, in all trees of the same species; consequently, when these