Observe, that the efficacy of both methods de entirely on the difficulty of causing the piece between the cross. joints to slide along the timber to which it adheres. Therefbre, if this be moderate, it is wrong to make the notches deep ; for as soon as they are so deep that their ends have a force suf ficient to push the slice along the line of junction, nothing is gained by making them deeper ; and this sequires a greater expenditure of timber.
Scarfings are frequently made oblique, as is fig. 18.; but we imagine that this is a bad practice. It begins to yield at a poidt, where the wood is crippled, and splintered off, or at least bruised out a little : as the pressure increases, this part, by squeezing broader, causes the solid parts to rise a little upwards, and gives them some tendency, not only to push their antagonists along the base, but even to tear them up a little. For similar reasons, we disapprove of the favourite practice of many art ists, to make the angles of their scarfings acute, as in fig. 19. Thiseften causes the two pieces to tear each other up.. The abutments should always be perpendicular to the directions of the pressures. Lest it should be forgotten in its proper place, we may extend this injunction also to the abutments of different pieces of a frame, and recommend it to the artist even to attend to the shrinking of the timbers by drying. When two timbers abut obliquely, the joint be most full at the obtuse angle of the end ; because, by drying, that angle grows more ob tuse, and the beam would then be in danger of splin tering off at the acute angle.
It is evident, that the nicest work is indispensably necessary in building up a beam. The parts must abut on each other completely, and the smallest play or void takes away the whole efficacy. It is usual to give the butting joints a small taper to one side of the beam, so that they may require moderate blows of a maul to force them in, and the joints may be per fectly close when the external surfaces are even on each side of the beam. But we must not exceed in the least degree for a very taper wedge has great force ; and if we have driven the pieces together by very heavy blows, we leave the whole in a state of violent strain, and the abutments are perhaps ready to splinter off' by a small addition of pressure. This is like too severe a proof for artillery : which, though not sufficient to burst the pieces, has weakened them to such a degree, that the strain of ordinary service is sufficient to complete the fracture. The workman is tempted to exceed in this, because it smooths off and conceals all uneven seams ; but he must be watched. It is not unusual to leave some abut ments open enough to admit a thin wedge reaching through the beam. Nor is this a bad practice, if the wedge is of materials which is not compressed by the driving or the strain of service. Iron would be preferable for this purpose, and for the joggles, were it not that by its too great hardness it cripples the "fibres of timber to some distance. In consequence
of this, it often happens that, in beams which are subjected to desultory and sudden strains (as in the levers of reciprocating engines), the joggles or wedges Widen the holes, and work themselves foam: There fore skilful engineers never admit them ; and indeed as few bolts as possible, for the same reason : but when.resisting a steady or dead pull, they are not so improper, and are frequently used.
Beams are built up, not only to. increase their dimensions in the direction of the strain (which we have hitherto called their depth), but also to increase their breadth, or the dimensions perpendicular to the strain. We sometimes double the breadth of a gir cler which is thought too weak for its load, and where we must not increase the thickness of the flooring.
The mast of a great ship of war must be madel bigger athwartship, as well as fore and aft. This is one of the nicest problems of the art ; and profes sional men are by no means agreed in their Alin:lions about it. We do not presume to decide ; and shall content ourselves with exhibiting the different me thods.
The most obvious and natural method is that shown in fig. 20. It is plain that (independent of the connection of cross bolts, which are used in them all when the beams are square) the piece C cannot bend in the direction of the plane of the figure without bending the piece D along with it. This method is much used in the French navy. : but it is undoubtedly I imperfect. Hardly any two great trees are of equal quality, and swell or shrink alike. If C shrinks more than D, the feather of C becomes loose in the groove wrought in D to receive it; and when the beam bends, the parts can slide on each other like the plates of a coach spring ; and if the bending is in the direction ef; there is nothing to hinder sliding but the bolts, which soon work themselves loose in the bolt-holes. , Fig. 21. exhibits another method. The two halves of the beam are tabled into each other in the sane manner as in fig. 17. It is plain that this will not be affected by the unequal swelling or shrinking, because this is insensible in the direction of the fi bres ; but when bent in the direction a b, the beam is weaker than fig. 20. bent in the direction ef. Each half of fig 20. has, in every part of its length, a thick ness greater than half the thickness of the beam. It is the contrary in the alternate portions of the halves of fig. 21. When one of them is bent in the direc.. tion AB, it is plain that it drags the other with it by means of the cross butments of its tables, and there can be no longitudinal sliding. But unless the work is accurately executed, and each hollow com pletely filled up by the table of the other piece, there will be a lateral slide along the cross joints sufficient to compensate for the curvature ; and this will hin der the one from compressing or stretching the other in conformity to this curvature.