JOINT. In constructive operations the word joint is used to express the means adopted to ensure a greater length in any part of the work than can be obtained by the use of the natural materials, or to form large masses of work ; or to enable one part of the work to turn upon another, in the manner of a door, casement, &e. The different kinds of joints of the first description assume names dependent upon their forms, positions, and the materials in which they are executed ; and the joints of the second description assume names dependent upon the manner and extent of opening they are intended to secure.
Thus, in masonry and brick-work. the aides, or vertical faces of the materials within the face of the walls, are called joints, in contradistinc tion to the horizontal faces or beds ; and they are called either heading or stretching-joints, as they may be laid with their shorter or longer dimensions in the same direction as the face. They may be perpen dicular to the face or dovetailed, as in the courses of the Eddystone Lighthouse ; and in arches they are made normally to the curve of the intrados ; in ornamental work they are sometimes rusticated, or made either with a species of sunk rebate or with splays on the edges, thus : and they are either (in ashlar masonry) cramped on the top, dowelled, or made with a joggle thus : this form of joint is used for landings of staircases. When straight architraves are carried over wide openings, it is customary to form in them what are called concealed arch-joints ; a very vicious style of construction, it may be added.
The various descriptions of joints used in timber-work differ from those of masonry and brick-work, in this respect : that they are expressly made for the purpose of lengthening the several pieces of wood, rather than of enabling them to form parts of a structure, whose stability should consist in the reaction of the laws of gravity, on the materials employed, upon those materials themselves, as in the case of stone and brick-structures. The joints of wood work then are : No. 1, the lop-joint, in which one piece of timber is cut out so as to allow the other to fit over it and to be fastened simply by nailing.
No. 2, the dore-tailed lap-joint, in which the faces of the Laps are cut with an inclination from one another, so as to offer a resistance to the lateral displacement of the timber; and this object is more effectually attained by the scarf-joint, No.3, especially when it is (an in the sketch) keyed and wedged. In certain positions, as in tie-beams of long span, the scarf-joints, instead of being made horizontally, are made vertically ; but, In all such works it is customary to stiffen them by means of bolts and of plates of wrought-iron, on either side of the scarf. In joiner's work, lap-joints, grooved and tongued-joints, tenon-joints and splayed-headings arc used, for a description of which see JOINERY.
In metal works, joints are sometimes lapped, dovetailed, and scarfed, as in carpentry ; but they are also either welded, brazed, or soldered, according to the nature of the metal and of the purposes intended to be effected. Welding consists simply in bringing the ends of the metal to such a beat, as to allow the percussion exercised upon them, when in that state, to effect a complete homogeneous junction of the respective ends—to make the lengthened bar, in fact, precisely analo gous in Its powers of resistance to longitudinal strains to those of the original bars. Brazed joints are those made when edges of iron, copper, brass, &c., are run together by means of an alloy consisting of brass and zinc, with occasionally small proportions of tin and silver. SOLDERED JOINTS are very nearly the same as brazed joints, or they are those made when surfaces of metal 'are united by means of a more easily fusible material, Which is run between the said surfaces, and binds them together by chemical and cohesive force ; and thus according as the metal surfaces are similar or different, it becomes necessary to vary the nature of the solder to be used. The most Important observation to be made with respect to the execution of either of these fused joints' is, that it is essential to keep the faces of contact perfectly bright, and free from any kind of dirt or extraneous bodies. In metal work, rivet joints are also used.