The simplest and most obvious scarfing (after the one.now mentioned•is that represented in fig. 24.,No. 1. and 2. If considered merely as two pieces of wood joined, it is plain that, as a tie, it has but half the strength of an entire piece, supposing that the bolts (which are the only connections) are fast in their holes. No. 2. requires a bolt in the middle of the scarf to give it that strength ; and, in every other part, is weaker on one side or the other. (See Note IIH.) But the bolts are very apt to bend by the vio lent strain, and require to be strengthened by uniting their ends by iron plates ; in which case it is no longer a wooden tie. The form of No. 1. is better adapted to the office of a pillar than No. 2.; especi ally if its ends be formed in the manner shown in the elevation No. S. By the sally given to the ends, the scarf resists an effort to bend it in that direction. Besides, the form of No. 2. is unsuitable for a post; because the pieces, by sliding on each other by the pressure, are apt • to splinter off the tongue which confines extremity.
sections of the and CD.
Pig. 25. and 26. exhibit the most approved forni of a. scarf, whether for a tie or for a post. The key represented in the middle is not essentially neces sary; the two pieces might simply meet square there. This form, without a key, needs no bolts (al though they strengthen it greatly); but, if worked very true and close, and with square abutments, will hold together, and will resist bending in any direc tion. But the key is an ingenious and a very great improvement, and will force the parts together with perfect tightness. The same precaution must be observed that we mentioned on another occasion, not to produce a constant internal strain on the parts by overdriving the key. The form of fig. 25. is by far the best ; because the triangle of 26 is much easier splintered off by the strain, or by the key, than the square wood of 25. It is far prefer able for a post, for the reason given when speaking of fig. 24., No. 1. and No. 2. Both may be formed with a sally at the ends equal to the breadth of the key. In this shape fig. 25. is vastly well suited for joining the pirts of the long corner posts of spires and other wooden towers. Fig..25., No. 2., differs from No. 1. only by having three keys. The princi and the longitudinal strength are the same. The pal scarf of No. 2., tightened by the three keys, enables it to resist a bending much better.
' None of these scarfed tie-beams can have more than one-third of the strength of an entire piece, unless with the assistance of iron plates ; for if the key be made *inner than one-third, it has less than One-third of the fibres to pull by.
We are confident, therefore, that when the heads of the bolts are connected by plates, the simple form Of fig. 24.. No. 1. is stronger than those more inge nious scarfings. It may be strengthened against la teral bending by a little tongue, or by a sally ; but cannot have both.
The strongest of all methods of piecing a tie-beam would be to set the parts'end to end, and grasp them between other pieces on each ride, as in fig. 27. Plate LI. This is what the ship-carpenter calls fishing a beam; and is a frequent practice for occasional repairs. Mr Perronet used it for the tie-beams or stretchers, by which he connected the opposite feet of a centre, which was yielding to its load, and had pushed aside one' of the piers above four inches. Six of these not
only withstood a strain of 1800 tons, but, by wedg ing' behind them, he brought the feet of the truss 21 inches nearer. The stretchers were 14 inches by 11' of sound oak, and could have withstood three times that strain. Mr Perronet, fearing that the great length of the bolts employed to connect the beams of these stretchers would expose them to the risk of bending, scarfed the two side pieces into the middle piece. The scarfing was of the triangular kind (Trait de Jupiter), and only an inch deep, each face being two fbet long, and the bolt passed through close to the angle.
In piecing the pump rods, and other wooden stretchers of at engines, no dependence is had on scarfing; and the engineer connects every thing by iron straps. We doubt the propriety of this, at least in 'cases where the bulk of the wooden con nexion is not inconvenient. These observations must suffice for the methods employed for connect mg the parts of a beam ; and we now proceed to con sider what are more usually called the joints of a piece of carpentry.
Where the beams stand square with each other, and the strains are also square with the beams, and in the plane of the frame, the common mortise and tenon is the most perfect junction. A pin is gene rally put through both, in order to keep the pieces united, in opposition to any force which tends to part them. Every carpenter knows how to bore the lmle for this pin, so that it shall draw the tenon tight into the mortise, and cause the shoulder to butt close, and make neat work : and he knows the risk of tear ing out the bit of the tenon beyond the pin, if he draw it too much. We may just observe, that square holes and pins are much preferable to round ones for this purpose, bringing more of the wood into action, with less tendency to split it. The ship-carpenters have an ingenious method of making long wooden bolts, which do not pass completely through, take a very fast hold, though not nicely fitted to their holes, which they must not be, lest they should be crippled in driving. They call it foxtail wedging. They stick into the point of the bolt a very thin wedge of hard wood, so as to project a proper distance ; when this reaches the bottom of the hole by driving the bolt, it splits the end of it, and squeezes it hard to the side. This may be practised with advantage in car pentry. If the ends of the mortise are widened in wards, and a thin wedge be put into the end of the tenon, it will have the same effect, and make the joint equal to a dovetail. But this risks the splitting the piece beyond the shoulder of the tenon, which would be unsightly. This may be avoided as follows: Let the tenon T, fig. 28. have two very thin wedges a and c struck in near its angles, projecting equally at a very small distance within these, put in two shorter ones b, d, and more within these if neces sary. In driving this tenon, thewedges a and c will take first, and split off a thin slice, which will easily bend without breaking. The wedges b, d, will act next, and have a similar effect, and die others in suc cession. The thickness of all the wedges taken to gether must be equal to the enlargement of the mor tise toward the bottom.