HAVING considered at sufficient length the strains of different kinds which arise from the form of the parts of a frame of carpentry, and the direction of the external forces which act on it, whether considered as impelling or as supporting its different , we must now proceed to consider the means parts, we this form is to be secured, and the connec tions by which those strains are excited and com municated.
The joinings practised in carpentry are almost in finitely various, and each has advantages which make it preferable in some circumstances. Many varieties are employed merely to please the eye. We do not concern ourselves with these : Nor shall we consider those which are only employed in connecting small works, and can never appear on a great scale ; yet even in some of these, the skill of the carpenter may be discovered by his choice ; for in all cases, it is wise to make every, even the smallest, part of his work as strong as the materials will admit. He will be particularly attentive to the changes which will necessarily happen by the shrinking of timber as it dries, and will consider what dimensions of his fram ings will be affected by this, and what will not ; and will then dispose the pieces which are less essential to the strength of the whole, in such a manner that their tendency to shrink shall be in the same direc tion with the shrinking of the whole framing. If he do otherwise, the seams will and pans be split asunder. He will dispose his hoardings in such a manner as to contribute to the stiffness of the whole, avoiding at the same time the giving them positions which will produce lateral strains on truss beams which bear great pressures ; recollecting, that although a single board has little force, yet many united have a great deal, and may frequently per form the office of very powerful struts.
Our limits confine us to the joinings which are most essential for connecting the parts of a single piece of a frame when. it cannot be formed of, one beam, either for want of the necessary thickness or _length ; and the joints for connecting the different sides of a trussed frame.
Much ingenuity and contrivance has been be stowed on the manner of building up a great beam of many thicknesses, and many stuke methods are practised as great nostrums by dient artists: but when we consider the manner in which the cohesion of the fibres performs its office we will clearly see that the simplest are equally effected with the most refined, and that they are less apt to lead us into false notions of the strength of the assemblage.
Thus, were it required to build up a beam for a great lever or a girder, so that it may act nearly as a beam of the same size of one log—it may either be done by plain joggling, as in Plate L. fig. 17. A, or by scarfing, as in fig: 17. B or C. If it is to act as a lever,
having the gudgeon on the lower side at C, we be. lieve that most artists will prefer the form B and C at least this has been the case with nine-tenths of those to whom we have propOsed the question. The best informed only hesitated ; but the ordinary artists were all confident in its superiority ; and we found their views of the matter very coincident. They con sidered the upper piece as grasping the lower in its hooks ; and several imagined that, by driving the one very tight on the other, the beam would be stronger than an entire log ; but if we attend care fully to the internal procedure in the loaded lever, we shall find the upper ome clearly the strongest. If they are formed of equal logs, the upper one is thicker than the other by the depth of the joggling or scarfing, which we suppose to be the same in both ; consequently, if the cohesion of the fibres is the intervals is able to bring the uppermost filaments into full action, the form A is stronger than B, in the proportion of the greater distance of the upper filaments from the axis of the fracture : this may be greater than the difference of the thickness, if the wood is very compressible. If the gudgeon be in the middle, the effect, both of the joggles and the scarfings, is considerably diminished ; and if it is on the upper side, the scarfings act in a very different way. In this situation, if the loads on the arms are also applied to the upper side, the joggled beam is still more superior to the scarfed one. This will be best understood by resolving it in imagination into a trussed frame. But when a gudgeon is thus put os that side of the lever which grows convex by the strain, it is usual to connect it with the rest by a powerful strap, which embraces the beam, and causes the opposite point to become the resisting point. This greatly changes the internal actions of the filaments, and, in some measure, brings it• into the same state as the first, with the gudgeon below. Were it pos sible to have the gudgeon on the upper side, and to bring the whole into action without a strap, it would be the strongest of all ; because, in general, the re sistance to compression is greater than to extension. In every situation the joggled beam has the advan tage; and it is the easiest executed. (See Note GG.) We may frequently gain a considerable accession of strength by this building up of a beam; especially if the part which is stretched by the strain be of oak, and the other part be fir. Fir being so much supe rior to oak as a pillar (if Muschenbroek's expert• meats may be confided in), and oak so much prefer able as a tie, this construction seems to unite both advantages. But we shall see much better methods ' of making powerful levers, girders, &c. by trussing.