ting the base DB of the angular rib in g, i, 4, and n. Draw gh, ik, lm, and 10, each perpendicular to DB, cutting the diagonal rib at h, k, m, and o. Then making the distances GH, IK, IM, and NO equal to the corresponding distances gh, ik, Im, and no, through the points H, K, M, O draw a curve which will be the under edge of that for the bottom of the ribs QG, RI, SL, TN, and UC, shown complete on each side of the square plan. If each of the circular segments on each side of the square plan be turned up at right angles to the plan ABCD, the ribs will then stand in their true position.

BRIDGES.

Fig. 735.

2095. We shall in this work confine ourselves to the simplest forms of timber bridges, which, as well as those of stone, will be found fully treated of in the Encyclopædia of Engineering, by Mr. Cresy, which forms one of the series. As they mostly depend on the principle of the truss, where the span is large, and this combination of timbers we have already explained; so in stone bridges the principle of construction of the arch is the chief matter for consideration, and to that a large portion of this work has been devoted; hence, on the part of the architect, we do not resign his pretension to employment in such works, for which, indeed, as respects design, his general education fits him better than that of the engineer.

2096. The bridge over the Brenta, near Bassano, by Palladio, is an example of a wooden bridge (fig. 736.), which is not only elegant as a composition, but one which is economical

Fig. 736.

and might be employed with advantage where it is desirable that the piers should occupy a small space, and the river is not subject to great floods. The same great architect, in his celebrated Treatise on Architecture, has given several designs for timber bridges, the principles of whose construction have only been carried out further in many modern instances He was the earliest to adopt a species

of construction by which numerous piers were rendered unnecessary, and thus to avoid the consequences of the shock of heavy bodies against the piers in the time of floods. Of this sort was the bridge he threw over the rapid torrent of the Cismone (fig. 737.) whose span was 108 feet.

2097. Palladio has given a design for a timber bridge (fig. 738.) which is remarkable as having been the earliest that has come to our knowledge, wherein the arrangement is in what may be called framed voussoirs, like the arch stones of a bridge, a principle in later days carried out to a great extent, and with success, in iron as well as timber bridges.

Fig. 737.

Fig. 738.

2098. We shall conclude our section on practical carpentry with a method of constructing timber bridges proposed by Price in his Treatise on Carpentry, and one not dissimilar in principle to the method of Philibert de Lorme, before mentioned. The bridge (fig. 759.) is sup

posed to consist of two principal ribs ik. The width of the place is spanned at once by an arch rising one sixth part of its extent. Its curve is divided into five parts, "which," says Price, "I purpose to be of good seasoned English oak plank, of 3 inches thick and 12 broad. Their joint or meeting tends to the centre of the arch. Within this rib is another, cut out of plank as before, of 3 inches thick and 9 broad, in such sort as to break the joints of the other. In each of these ribs are made four mortices, of 4 inches broad and 3 high, and in the middle of the said 9-inch plank. These

Fig. 759.

mortices are best set out with a templet, on which the said mortices have been truly divided and adjusted. Lastly, put each principal rib up in its place, driving loose keys into some of the mortices to hold the said two thicknesses together; while other help is ready to drive in the joists, which should have a shoulder inward, and a mortice in them outward; through which keys being drove keep the whole together. On these joists lay your planks, gravel, &c. ; so is your bridge compleat, and suitable to a river, &c. of 36 feet wide."

2099. "In case the river, &c. be 40 or 50 feet wide, the stuff should be larger and more particularly framed, as is shown in part of the plan enlarged, as I. These planks ought to be 4 inches thick and 16 wide; and the inner ones, that break the joints, 4 inches thick and 12 broad; in each of these are six mortices, four of which are 4 inches wide and 2 high; through these are drove keys which keep the ribs the better together; the other two mortices are 6 inches wide and 4 high; into these are framed the joists of 6 inches by 12; the tenons of these joists are morticed to receive the posts, which serve as keys, as shown in the section K, and the small keys as in L; all which inspection will explain. That of M is a method whereby to make a good butment in case the ground be not solid, and is by driving two piles perpendicularly and two sloping, the heads of both being cut off so as to be embraced by the sill or resting plate, which will appear by the pricked lines drawn from the plan I and the letters of reference." Price concludes: "All that I conceive necessary to be said further is, that the whole being performed without iron, it is therefore capable of being painted on every part, by which means the timber may be preserved; for though in some respects iron is indispensably necessary, yet, if in such cases where things are or may be often moved, the iron will rust and scale, so as that the parts will become loose in process of time, which, as I said before, if made of sound timber, will always keep tight and firm together. It may not be amiss to observe, that whereas some may imagine this arch of timber is liable to give way, when a weight comes on any particular part, and rise where there is no weight, such objectors may be satisfied that no part can yield or give way till the said six keys are broke short off at once, which no weight can possibly do."

SECT. V.

JOINERY.

2100. Joinery is that part of the science of architecture which consists in framing or joining together wood for the external and internal finishings of houses, such as the linings of walls and rough timbers, the putting together of doors, windows, stairs, and the like.

It requires, therefore, more accurate and nicer workmanship than carpentry, being of a decorative nature and near the eye. Hence the surfaces must be smooth and nicely wrought, and the joints must be made with great precision. The smoothing of the wood is called planing, and the wood used is called stuff, which consists of rectangular prisms roughly brought into shape by the saw, such prisms being called battens, boards, and planks, according to their breadth and thickness.

2101. We shall give but a succinct account of the joiner's tools; an acquaintance with their forms and uses being sooner learnt by mere inspection over a joiner's bench than by the most elaborate description.

TOOLS.

2102. The first is the bench, whose medium height is about 2 feet 8 inches, its length about 10 or 12 feet, and its width about 2 feet 6 inches. One side is provided with a vertical board, called the side board, pierced with holes ranged at different heights in diagonal directions, which admit of pins for holding up the object to be planed, which is supported at the other end of it by a screw and screw check, together called the bench screw, acting like a vice. The planes used by the joiner are the jack plane, which is used for taking off the roughest and most prominent parts of the stuff, and reducing it nearly to its intended form. Its stock, that is, the wooden part, is about 17 inches long, 3 inches high, and 3 inches broad. The trying plane, whose use is nearly the same as that last described, but used after it, the operation being performed with it by taking the shaving the whole length of the stuff, which is called trying up, whereas with the jack plane the workman stops at every arm's length. The long plane, which is used when a piece of stuff is to be tried up very straight. It is longer and broader than the trying plane, its length being 26 inches, its breadth 3 inches, and depth 3 inches. The jointer, which is still longer, being 2 feet 6 inches long, and is principally used for obtaining very straight edges, an operation commonly called shooting. With this the shaving is taken the whole length in finishing the joint or edge. The smoothing plane, which, as its name imports, is the last employed for giving the utmost degree of smoothness to the surface of the wood, and is chiefly used for cleaning off finished work. It is only 7 inches long, 3 inches broad, and 2 inches in depth. The foregoing are technically called bench planes.

2103. The compass plane which in size and shape is similar to the smoothing plane, except that its under surface or sole is convex, its use being to form a concave cylindrical surface. Compass planes are therefore of various sizes as occasion may require. The forkstaff plane resembles the smoothing plane in size and shape, except that the sole is part of a concave cylindric surface, whose axis is parallel to the length of the plane. The form is obviously connected with its application, and, like the last named, it is of course of various sizes. The straight block is employed for shooting short joints and mitres, instead of the jointer, which would be unwieldy: its length is 12 inches, its breadth 3 inches, and depth 23 inches.

2104. There is a species of planes called rebate planes, the first whereof is simply called the rebate plane, being, as its name imports, chiefly used for making rebates, which are receding planes formed for the reception of some other board or body, so that its edge may coincide with that side of the rebate next to the edge of the rebated piece. The length of the rebate plane is about 9 inches, its depth about 3 inches, and its thickness varies according to the width of the rebate to be made, say from 13 to inch. Rebate planes vary from bench planes in having no tote or handle rising out of the stock, and from their having no orifice for the discharge of the shavings, which are discharged on one side or other according to the use of the plane. Of the sinking rebating planes there are two sorts, the moving fillister and the sash fillister, whereof, referring the reader to the tool itself, a sight of which he can have no difficulty in procuring, the first is for sinking the edge of the stuff next to the workman, and the other for sinking the opposite edge, whence it is manifest that these planes have their cutting edges on the under side. Without enumerating many other sorts which are in use, we shall mention merely the plough, a plane used for sinking a cavity in a surface not close to the edge of it, so as to leave an excavation or hollow, consisting of three straight surfaces forming two internal right angles with each other, and the two vertical sides two external right angles with the upper surface of the stuff. The channel thus cut is called a groove, and the operation is called grooving or plowing. This species will vary according to the width from the edge; but it is generally about 73 inches long, 33 inches deep.

2105. Moulding planes are for forming mouldings, which, of course, will vary according to the designs of the architect. They are generally about 9 inches long, and 3 inches deep. When mouldings are very complex, they are generally wrought by hand; but when a plane is formed for them they are said to be stuck, and the operation is called sticking.

2106. The bead plane is used very frequently in joinery, its use being for sticking mouldings whose section is semicircular; when the bead is stuck on the edge of a piece of stuff to form a semi-cylindric surface to the whole thickness, the edge is said to be

beaded or rounded. When a bead is stuck so that it does not on the section merely fall in with its square returns, but leaves a space thus, between the junctions at the

sides, it is said to be quirked. The beads or planes vary from very small sizes up to the inch and bead. They may however be larger, and are sometimes stuck double and triple. The snipebill plane is one for forming the quirk, whereof we have spoken; but we do not think a detailed description of it necessary, more than we do of those which are made for striking hollows and rounds.

2107. The stock and bit is the next tool to be mentioned. Its use is for boring wood, and the iron, which varies as the size of the bore required, is made in a curve on its edge of contrary flexure so as to discharge the wood taken out. It fits into what is called the stock, which has a double curved arm working on spindles, the end opposite to the bit being pressed by the body, whose weight against the whole instrument is the power whereby the operation is performed. The bit is also called a pin, or gouge bit. It is an important tool, and much used. (See AUGER in Glossary.)

2108. Countersinks are bits for widening the upper part of a hole in wood or iron for the head of a screw or pin, and are formed with a conical head. Rimers are bits for widening holes, and are of pyramidal form whose vertical angle is about 3 degrees. The hole is first pierced by means of a drill or punch, and the rimer then cuts or scrapes off the interior surface of the hole, as it sinks downwards, by pressing on the head of the stock. According to the metal on which they are to be used they are differently formed.

2109. The taper shell bit is conical both within and without. Its horizontal section is a crescent, the cutting edge being the meeting of the interior and exterior conic surfaces. Its use is for widening holes in wood. Besides the above bits, there are some which are provided with a screw-driver for sinking small screws into wood with more rapidity than the unassisted hand will accomplish.

2110. The brad awl, the smallest boring tool, the gimlet, and the screw driver, are so well known, that it would be waste of space to do more than mention them, the commonest of instruments in the science of construction.

2111. The variety of chisels is great. They are well known to be edge tools for cutting wood by pressure on it, or by percussion with a mallet on its handle. The firmer chisel is a tool used by the carpenter as well as the joiner for cutting away superfluous wood by thin chips. Those are best which are made of cast steel. If much superfluous wood is to be cut away, a strong chisel, with an iron back and steel face, is first used with the aid of the mallet, and then a slighter one with a very fine edge. The first is the firmer first mentioned, and the last is called a paring chisel, in the use whereof the force employed is from the shoulder or hand.

2112. The mortice chisel, whose use is for cutting out rectangular prismatic cavities in stuff is made of considerable strength. The cavity it so cuts out is called a mortice, and the piece which fits into it a tenon, whence the name of the tool. This chisel is one acted on only by the percussion of the mallet.

2113. The gouge is used for cutting concave forms in stuff. It is, in fact, a chisel whose iron is convex.

2114. The drawing knife is an oblique-ended chisel, or old knife, for drawing in the ends of tenons by making a deep incision with the sharp edge, guided by that of the tongue of a square, for which purpose a small part is cut out in the form of a triangular prisın. The use of this excavation is to enter the saw and keep it close to the shoulder, and thus make the end of the rail quite smooth, for by this means the saw will not get out of its

course.

2115. There are many species of the saw, which is a thin plate of steel, whose edge is indented with teeth for cutting by reciprocally changing the direction of its motion. The varieties are the ripping saw, which is used for dividing or splitting wood in the direction of the fibres; its teeth are large, the measure being usually to the number of eight in 3 inches, such teeth standing perpendicularly to the line which ranges with the points: the length of the plate or blade of this saw is about 28 inches. The half ripper is used also for dividing wood in the direction of the fibres: the plate of this saw is as long as of that last described, but it has only three teeth in an inch. The hand saw, whose plate is 26 inches long, contains fifteen teeth in 4 inches; it is used for cross cutting, as in the direction of the fibres; for which purposes the teeth recline more than in the two former saws. The panel saw has about six teeth in an inch, the length of its plate being the same as the last; but in this and the hand saw thinner than in the ripping saw it is used for cutting very thin wood, either with or across the fibres. The tenon saw is most used for cutting wood transverse to the fibres, as the shoulders of tenons. The plate of a tenon saw is from 14 to 19 inches long, having eight to ten teeth in an inch. This saw not being intended to cut through the whole breadth of the wood, and the plate being too thin to make a straight kerf, or to keep it from buckling, it has a thick piece of iron fixed on the edge opposite to the teeth, called the back. From the opening for the fingers through the

handle of this and the foregoing saws being enclosed all round, it is called a double handle. The sash saw is used for forming the tenons of sashes; its plate is 11 inches in length, having about thirteen teeth to the inch. It is sometimes backed with iron, but more frequently with brass. The dovetail saw is used for cutting the dovetails of drawers and the like; its plate is backed with brass, it contains fifteen teeth in about one inch, and is about 9 inches long. The handles of this and the last saw are only single. The compass saw, for cutting wood into curved surfaces, is narrow, thicker on the cutting edge as the teeth have no set, and is without a back; the plate, near the handle, is about an inch broad, and about a quarter of an inch at the other extremity, having about five teeth to the inch; the handle is single. The keyhole, or turning saw, in its plate resembles the compass saw, but the handle is long, and perforated from end to end for inserting the plate at any distance within the handle; there is a pal in the lower part of the handle, through which is inserted a screw for fastening the plate therein. As its name implies, it is used for turning out quick curves, as keyholes, and is therefore frequently called a keyhole saw.

2116. The teeth of all saws, except turning and keyhole saws, are bent alternately on the contrary sides of the plate, so that all the teeth on the same side are alike bent throughout the length of the plate, for the purposes of clearing the sides of the cut made in the wood by it. The saw is a tool of great importance in every case where wood is to be divided, for by its means it can be divided into slips or scantlings with no more waste than a small slice of the wood, whose breadth is equal to the depth of the piece to be cut through, and the thickness of it equal to no more than the distance of the teeth between their extreme points on the alternate sides of the saw measured on a line perpendicular to them; whereas, by any other means, such as the axe for instance, large pieces of timber could only be reduced in size by cutting away the superfluous stuff, which would be no less a waste of labour than of the material used; and even then it would have to be reduced to a plane surface.

2117. Joiners use the hatchet, which is a small axe, for cutting away the superfluous wood from the edge of a piece of stuff when the part to be cut away is too small to be sawed.

2118. The square consists of two rectangular prismatic pieces of wood, or one of wood, and the other, which is the thinnest, of metal, fixed together, each at one of their extremities, so as to form a right angle both internally and externally; the interior right angle is therefore called the inner square, and the exterior one the outer square. Squares are, for different applications, made of different dimensions. Some are employed in trying up wood, and some for setting out work; the former is called a trying square, and the latter a setting out square. To prove a square it is only necessary to reverse the blade after having drawn a line on the surface to which it is applied: if the line of the blade on reversal do not coincide with that first drawn, the square is incorrect.

2119. The bevel consists, like the square, of a blade and handle; but the tongue is moveable on a joint, so that it may be set to any angle. When it is required to try up many pieces of stuff to a particular angle, an immoveable bevel ought to be made for the purpose; for unless very great care be taken in laying down the moveable bevel, it will be likely to shift.

21 20. The gauge is an instrument used for drawing or marking a line on a piece of stuff to a width parallel to the edge. It consists generally of a square piece with a mortice in it, through which runs a sliding bar at right angles, called the stem, furnished with a sharp point or tooth at one extremity, projecting a little from the surface; so that when the side of the gauge next to the end which has the point is applied upon the vertical surface of the wood, with the toothed side of the stem upon the horizontal surface, and pushed and drawn alternately by the workman from and towards him, the tooth makes an incision from the surface into the wood at a parallel distance from the upper edge of the vertical side on the right hand. This line marks precisely the intersection of the plane which divides the superfluous stuff from that which is to be used. When it is required to cut a mortice in a piece of wood, the gauge has two teeth in it, and is called a mortice gauge, one tooth being stationary at the end of the stem, and the other moveable in a mortice between the fixed tooth and the head; so that the distances of the teeth from each other, and of each from the head, may be set at pleasure, as the thickness of the tenon may require.

2121. The side hook is a rectangular prismatic piece of wood, with a projecting knob at the ends of its opposite sides. The use of the side hook is to hold a board fast, its fibres being in the direction of the length of the bench, while the workman is cutting across the fibres with a saw or grooving plane, or in traversing the wood, which is planing it in a direction perpendicular to the fibres.

2122. The mitre be consists of three boards, two, called the sides, being fixed at right angles to a third, called the bottom. The bottom and top of the sides are all parallel; the sides of equal height, and cut with a saw in two directions of straight surfaces at right angles to each other and to the bottom. forming an angle of 45 degrees with the sides. The mitre box is used for cutting a piece of tried up stuff to an angle of 45 degrees with two