—Figure 4 exhibits what is called "dovetailed mortis ing, " for timbers that are to remain flush and that meet at an angle, gen erally a right angle. The dovetail is so made that it can be lifted out, but not pulled apart, while the breaking off of the dovetail is prevented by an additional notch let in at the top. Figure 5 is a shouldered tenon appli cable for the same purposes as Figure 4, but with the difference that the tenon is prevented from leaving its place by the introduction of a wooden pin. Figure 6 exhibits a dovetailed corner-joint, the shape of the notch portion (somewhat hook-like) preventing, the solid part from being displaced in either direction.
7 shows the various forms of cogging used for tim bers that cross one another without being let in flush, but are merely notched; the cogs at a, b, and care called respectively side, middle, and cross cogs. The overlying timbers—not given in the Fig-ure—must be tnade to correspond in notches, so as to bring the floors of both in contact, and to prevent displacements in either direction. The joints in Figures 8 and 9 are end cog,gings. Figure shows the dovetailed form, and Figure 9 the form of the side cog.
Mortise and TC71011.—Figure ro shows the common mortise and tenon used where vertical and horizontal timbers meet. A pin serves to prevent displacement. The breadth is about one-third that of the timber. Figure is a so-called " slit tenon," chiefly used on the upper ends of rafters. Figure 12 shows the oblique tusk tenon used where struts are set into hori, zontal or vertical timbers. The cut for the mortise extends over the whole breadth, and is intended to prevent the splitting of the tenon, taking up, as it does, chiefly, the thrust of the strut.
Besides the joints which have been shown, various other forms arc met with in practice, but they may be regarded merely as modifications of those above described.
Petioses ofjoints.—The principal purposes served by joints are length ening ties, struts, and beams, supporting- beams on beams, plates, and posts, connecting struts with ties, and using them as joints for ties and braces. Much depends upon the skill displayed in arrangement and in selecting particular forms for the accomplishment of various purposes. An English writer forcibly observes that simple joints are much more likely to be securely made than more elaborate ones. Scarfing is one of the most
common methods, but for important purposes it is considered inferior in reliability.
Principles of Construction strongly recommended include the following: (1) To cut the joints and arrange the fastenings so as to weaken the pieces of timber as little as possible; (2) To endeavor to place each abutting sur face in a joint perpendicular to the pressure it has to transmit; (3) To pro portion the area of each part of the joint and fastenings to the maximum stress it has to resist; (4) To form the joints with reference to the greatest attainable a7.'oidance of the shrinking and expanding of the wood; (5) To form and fit the different parts of each joint so as to spread the stresses with approximate uniformity over the sections of the timber and fastenings com posing it; (6) To make special efforts to devise safeguards against careless workmanship.
.11c1bods of illaking the methods or tools employed, it is stated that in scarfing, cogging, and notching the shoulders are cut in with a saw, but the cheek is generally struck out with a mallet and chisel or an adze. Tenons should be made entirely with the saw, which is also used in cutting weclg-es. AIortises are frequently bored at the ends with an auger whose diameter equals their thickness, the intervening part being taken out with a wide chisel and the ends squared down with a chisel as broad as the thickness of the mortise. In some cases iron is employed to strengthen wooden joints, and a variety of devices is used in facilitating such operations. They are considered specially advantageous in connection with fished joints, in which tbe ends of timbers abutting square against each other may be connected by means of fish-plates made either of wood or of iron. When iron struts are used, care should be taken to insure sufficient strength for the purposes they are intended to sei've. Uncertainties in regard to this matter induce some skilful carpenters to restrict their nse as much as possible, but the growing scarcity of timber and increasing cheap ness and abundance of iron and steel, together with the increase of facili ties for testing all classes of metallic building- material, will presumably extend the use of iron straps, stirrup irons, and iron tie-rods.