All these forms admit of pooping well ; and by mak ing the interior part somewhat full, we may compress the soft heart of the tree so far as to render the as semblage much stronger than an entire tree of equal size.
Timber is also pieced, in order to increase its length ; and there are a variety of methods of effecting this, some of them very ingenious. The strongest, and at the same time the simplest, method of lengthening a beam, whether it be a post, a tic, or a rafter, is to com pose it of several pieces, which are so placed that they may break joint, to use the mason's phrase. The neces sary connection among them can be given in any of the ways we have just mentioned, Fig. 14. We arc thus enabled in ties, to make certain of the cohesion of all the pieces but one ; and it is obvious, that the strength in creases with the number of pieces. Besides, the connec tion among them becomes the more certain, in conse quence of the increase of the surfaces of attachment.
Fishing a beam is something of this kind. It consists 4 clapping a piece of solid wood over a joint, and attach ing it firmly to both parts, either by bolts or tabling. (Fig. 15.) A strap is frequently placed over such a joint, connecting the !reads of neighbouring bolts. This is mcncly a fish of iron.
An eminent writer has said, that no tie of the same thickness of timber can be so strong as two pieces laid over each other for a certain length, and bolted toge ther. (Fig. 16.) Now this is certainly wrong, for that very assemblage may be made stronger by dowelling across the joint with a thin key. Neither is the form 17. No. 1. so strong as the other mode, where tne pieces have a square abutment, and is every way stronger than No. 2. But as a tic, the case is very dif ferent.
The bolts arc the only means of connection, and as great dependence seems frequently had upon that sort of fastening, it may be worth while to examine it a little more closely. It is clear, that whenever we pass the joint 11U, we can have only the tenacity of one half of the beam ASTC. Fig. 17. No. 3. Now, how do we take hold of the other part RUDI1 ? It is by means of a bolt, and we hold this by means of the bit beyond it. This piece is only kept in its place by the connecting matter, which attaches it to the fibres. Our only hold, therefore, is a thin plate of the connecting matter on each side of the bolt, and parallel to it. The second bat gives us a hold of more of the connecting matter, but that still acts by means of the same fibres ; and the third bolt does also the same thing, which spews, by the way, the impro priety of placing all the bolts in one line along the fibres, as is frequently done. But besides this, when the part comes to be stretched, the bolts will necessarily yield a little, and then bear only on one side of the holes ; the fibres near the junction may be expected to yield ; their number resisting an iron bolt is small, and the position of the bolt enables it to act as a lever in crip pling them. Accordingly, such bolts quickly work loose, and, in reciprocating strains especially, the as semblage is very apt to give way at once. Placing a strap over the joint to connect the various bolt heads, may prevent the leverage above mentioned, and will be an unquestionable benefit against a transverse strain.
But, even with every precaution, the firmness which ap pears to be produced by bolting, when carefully con sidered, is by no means so great as is generally imagined. The key placed across the joint of Figs. 18, 19, brings as many fibres into action as a bolt, and being so much shaken, is not exposed to any leverage, and consequently cannot cripple the fibre in that event. This key need be no thicker than just to take hold of the wood, with out tearing it up ; the greater the distance between these mortices, the deeper they must be cut, since the greater will be the lateral adhesion. As to the breadth of the key itself, this must depend on its hardness ; and, in order to have it as small as possible, it is common to make it of some much harder substance than the beam, as of oak, or of iron. The latter is said to be apt to cripple the fibre ; but perhaps this is more to be ascribed to injudicious driving, for these keys are usually made with a small taper, and, in order to obtain firmness and closeness, the workman is apt to proceed too far. Every species of connection in carpentry is liable to this defect, and the necessity of guarding against it must be carefully and constantly impressed upon the workman. The joints should be close, and all motion prevented, and the pieces made firmly to adhere, that no corner or single fibre may be unequally loaded ; but any thing beyond that is useless and dangerous.
Since, therefore, the apparent firmness of adhesion, produced by bolts and screws, does not communicate any real additional strength, and that an equal adhesion may be produced even by a wooden connection, the ingenuity of the carpenter is left more at liberty ; and it may not be improper to inquire, whether stronger modes of adhesion are not to be found than those either of Fig. 17, or 18. It is unquestionable, that if the two surfaces of No. 2. Fig. 17. were tabled together, the joint would be greatly stronger, and this can be done with very little, if any loss of wood ; nay, the adhering parts near the ends of the joint being so near the edge, and of course so much farther removed from the neutral fibre, would render the assemblage stronger against a transverse strain than even Plate CX111. Fig. 17. No. 1. The tabling will be more perfect by lengthening the joint, that is, by making it more oblique. But there is a limit to that, independent of the waste of timber; for when the distance between the alternate projections is so great, as to tear the fibre ere the connecting matter part, there can nothing be gained by increasing the stir face of attachment. Fig. 20. exhibits another method of piecing a beam, which will enable it to resist a con siderable transverse strain ; and, as a tic, it is at least as strong as any such form as Fig. 17. The lower piece is supposed to be oak, and by tightening the key above, its transverse strength is greatly increased. Duhamel has made many experiments on an assemblage somewhat similar to this, and has shown us, that this built beam of three pieces may, in some cases, he much stronger than an entire one. The ingenious carpenter will see, that the principle is capable of being applied in many other situations. As a tic, its chief value consists in tak