There are several methods of lengthening timber, either by joining whole pieces of the same transverse sections, and which are to come in contact in One or several plinies, or by forming the connection by means of a third piece, or by building the piece to be lengthened in several thicknesses, making the joints abutting upon each other on the solid of the piece with which they conic in con tact on the parallel joint. It is evident, that two bodies united, and intended to act as one in a state of tension, can never be so strong as either piece taken separately.
Tabling is a mode of indenting the ends of the pieces which !Orin the scarf, so as to resist a longitudinal strain ; the pieces, therethre, require to be held together, or otherwise the notches and the tables which fit into them would require to lie dovetailed. In this construction, the tables between the notches would be a very feeble support, as they are apt to split away. It must also be observed, that one single table, or one abutting part of resistance, is stronger, and much more cosy to execute, than two or four ; and that the resisting part should have as little projection as possible, because such projection diminishes the cohesive force, by a quantity of the timber equal in section to the abutting puts. Two pieces of timber may be very firmly fixed, by making the ends of the tables, instead of abutting. to form a tapering mortise, so that when the two pieces are brought close at the Connecting surfaces, a wedge driven into the cavity will bring all the parts of the joint into contact.
Every two pieces of timber require to be held together by sonic three compressing them equally on each side, particularly when the pieces are light ; for which purpose iron bolts are very convenient, they acting as a tie, and having the same effect as two equal and opposite forces would have in coin pressing the beam on each side of the scarf; and as iron is of great strength, the bore made to receive the bolt will not be so huge as to diminish the section, and consequently the firmness of the timber at the scarf, in any considerable degree; whereas, when wooden pins are used, they require a large bore, which weakens the timber, and the two pieces thus connected are not so firmly compressed, or, indeed, compressed at all, but are held together almost solely by friction.
No limited distance can he specified fur the length of the scarf; though it may be observed, generally, that a long seat f has no of in diminishing the cohesive strength of a compound piece of timber. On the contrary, a long scarf
gives :01 opportunity of increasing the number of bolts, which are the only ties when no taldings are used, as is the case where the abutting parts unite only by compression. it must here be understood, that all such abutting parts diminish the cohesive force in the proportion of their abutting surface to that of the whole section at any one of the abutments ; so that should a sears' consist of a series of steps, formed by parallel and perpendicular to two of the opposite sides, the transverse sides of these steps to those of the piece should be all equal ; and the greater the number or steps, the less will the strength be impaired ; but if they lie unequal, the timber will lie weakest at the greatest section or compressed abutment, and it' few, the section will be large, and the piece consequently deprived of a proportional degree of strength. We may also add, if' the two pieces be strapped longitudinally across their abutting parts, the cohesive force will be considerably assisted thereby.
There is nil part of carpentry which requires greater cor rectness in workmanship than scarfing ; as all the indents should bear equally, otherwise the greater part of the strength will he lost Hence we see how very unfit some of the complicated forms shown in the old works on carpentry were the purpose. It is certainly the height of absurdity to render the parts difficult to be fitted, when the whole of the strength depends on their fitting well. '' Bit many.," says Professor Robison, seem to aim at making the beam stronger than if it were of one piece ; and this inconsiderate project has given rise to many wlihnsical modes of tabling and scarfing." Having already shown many varieties of sea•fings under the general head of CARPENTRY, we shall here only point out the most approved forms for practical purposes, by way of illustrating the preceding observations.
Figure I. Two pieces of timber connected by a single step on each piece. Here more than half the power is lost ; neither is the scarf so capable. of resisting the three of tension as a single piece of timber would be, were it sawed half through its thickness from the opposite side, at a distance equal to the length of the scarf: however, if assisted by straps, it may perhaps be capable of resisting a much greater force, particularly if each opposite surface be bolted on the sides of the transverse joints through the straps.