CARPENTRY.

This branch of building comprises the art of employing timber in the construction of edifices.

The art of employing timber in building may be classed under two distinct branches, Carpentry and Joinery.

Carpentry comprehends the large and rough description of work, or that which is requisite in the construction and stability of an edifice; and Joinery, the fittings up and decorative work, so necessary to the completion of a building. Carpentry is, in general, valued by the cubical foot; and joinery by the superficial foot.

The principal operations which timbers have to undergo, from the time of their arrival in the carpenter's yard to their final destination in an edifice, may be classed under two general heads; those which respect individual prices, and those which respect their dependence on others.

Under the former of these heads is the pit-saw, by means of which, whole pieces of timber are divided, and reduced into their respective sized scantlings.

The term scantling implies dimensions in breadth and thickness, without any regard to length.

Planing, is the operation by which wood is reduced to a smooth and uniform surface, by means of an instrument called a plane, which takes a thin shaving off the surface of the wood, as it is moved backwards and forwards in a straight line by the hands of the workmen. There are, however, other operations of the plane besides that of reducing timber to an uniform and smooth surface, termed grooving, rebating, and moulding.

Grooving is forming a channel on the surface of a piece of wood, by taking away so much of the solid as is of the shape and size of the groove required.

Rabating or rebating, is reducing a piece by taking away from the angles a prism of the shape and size of the rabate required, so as to form an internal angle, and generally a right angle. This operation is frequently required in constructing door cases, and the frames of casement windows: the rabate, or groove, being intended as a ledge for the door or casement to rest in.

The pieces being cut into their proper scantlings, the next operation is the joining them together.

In this department we shall treat first, of the most approved methods of lengthening beams, by what is termed scarfing, or joining them in pieces; secondly, of the strengthening of beams by trussing; thirdly, of the methods of joining two timbers at angles, in any given direction; and lastly, of the mode of connecting several timbers in order to complete the design, and to effect certain powers respectively required by each individual piece.

To lengthen a piece of timber implies the act of joining or fastening two distinct pieces, so that a part of the end of one shall lap upon the end of another, and the surfaces of both, being one continued plane, form a close joint, called by workmen a scarf. It is manifest, that two bodies, joined together and intended to act as one continued piece, in a state of tension, or compression, cannot, by any possible means, be so strong as either pieces taken separately. It, therefore, requires much attention, and careful discrimination, in the choice and selection of such methods as are the most applicable to the peculiar circunstances o. the case. Every two pieces of timber joined in the manner thus described, and, indeed, in most other cases, require some force to compress them equally on each side, and more particularly when the pieces are light; for this pur póse iron bolts are used, which act as a tie, and possess the same effect as two equal and opposite forces would have in compressing the beam on each side the joint: and as the cohesive power of iron is very great, the hole, which is made to receive the bolt, may be of such dimensions as will not, in the least degree, tend to diminish the strength of the timber. When wooden pins are used, the bore is larger, and the joints weaker; consequently the two pieces, thus connected, are not held together by any compression of the pin, but merely by the friction of the individual pieces.

No specific distance can be laid down for the length of the scarf, though, in general, it may be observed, that, a long scarf has but little effect in diminishing the cohesive strength of a compound piece of timber; on the contrary, it affords an opportunity of increasing the number of bolts.

Fig. 558 shows the method of joining two pieces of timber by means of a single step on each piece.

By this method more than one-half the power is lost; and this scarf is not calculated to resist the force of tension equal to a single piece sawed half through its thickness from the opposite side, at a distance equal to the length of the

scarf; by the application of straps, however, it may be made to resist a much greater force.

Fig. 559 represents a scarf with parallel joints, and a single table upon each piece.

In this the cohesive strength is decreased in a greater degree than the preceding example, by the projection of the table; but this affords an opportunity of driving a wedge through the joint between the ends of the tables, and thereby forcing the abutting parts to a joint.

A scarf of this description to be longer than those which have no tables, and the transverse parts of the scarf, must be strapped and bolted.

Fig. 560 presents us with the same opportunity of wedging as before. In this figure, if the parts LM and NO be compressed together by bolts as firmly as if they were but one piece, and if the projection of the tables be equal to the transverse parts of the joints L and O, the loss of strength, compared with that of a solid piece, will be no more than what it would be at L and O.

Strapping across the transverse part of the joint is much the best and most effectual way of preventing the pieces from being drawn from each other, by the sliding of the longitudinal parts of the scarf, and, therefore, giving to the bolts an oblique position.

Fig. 561 is a scarf formed by several steps.

In this, if all the transverse parts of the steps be equal, and the longitudinal parts strongly compressed by bolts, the loss of strength will only be a fourth, compared to that of a solid piece, there being four transverse parts, that is, the part which the end of the steps is of the whole.

Fig. 562 is a scarf with a bevel joint, and equally as eligible for ordinary purposes as any in use.

Figs. 561 and 563. Scarfs intended for longer bearings than the preceding one.

Fig. 564 represents the method of constructing a compound timber, when two pieces are not of adequate length to allow them to lap, by means of a third piece joined to both by a double scarf, formed by several gradations or steps, the pieces abutting upon each other with the middle of the connecting piece over their abutment.

That which shall next claim our attention is a consideration of the principles and the best methods of strengthening beams by trussing.

When girders are extended beyond a certain length, they bend under their own weight, and the degree of curvature increases in a proportion far greater than that of their lengths. The best method to obviate this sagging, as it is