Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/515

Rh AKCHES.] BUILDING 465 pressure, are themselves opposed to the back of the same joints, so that its power of resistance is made equal to that of the bricks themselves, except at the ends ; which, in such works as we have supposed, are remote, and may be protected by the use of cement in their joints, whilst mortar is used in the rest. Rough arches are those lough in which the bricks are roughly cut with an axe to a wedge rches. form, and are used over openings, such as doors and windows, when the work is to be plastered on the outside, or in plain back fronts, outhouses, garden-walls, &c., when, however, they are neatly pointed with what is called a tuck or tucked joint. .Semicircular and elliptical arches are generally made plain, or without cutting the bricks ; but arches composed of a smaller segment of a circle (vulgarly and technically called scheme arches), if not gauged, are cut or axed. Very flat arches are distinguished by the term camber, from the French cambrer, to round like an arch. It is arches of this kind which are generally employed over windows and doors in external work, and they too are either cut or gauged. Gauged arches are iauged composed of bricks which are cut and rubbed to gauges rches. au( j m0 ulds, so as to form perfectly fitting parts, as in masonry. Gauging is equally applicable to arches and to walling, as it means no more than the bringing every brick exactly to a certain form, by cutting and rubbing, or grinding it to a certain gauge or measure, so that it will exactly fit into its place, as in the finer works of masonry. Gauged brickwork is set in a putty instead of common mortar, but it is seldom used except for arches in the fronts of houses, &c., which are to be neatly finished. These are for the most part straight, and are generally from 11 to 12 inches in depth, or the height of four courses of brickwork. Their value as arches will be best understood by reference to the diagram, fig. 7, Plate XX., by which it appears that all the material between the soffit of the straight arch or head of the opening b c, and the dotted line b f c, is useless, the intrados or soffit of the really efficient part of the arch being at that dotted line itself. This is the arc of an angle of 60, its chord, the width of the opening, being the base of an equilateral triangle constructed on it, and the joints are the radii of a circle whose centre is at a. b d and c e, the continuations of the sides of the triangle or radii a b and a c, are technically termed the skew-back of the arch. Sometimes the arc is made under a more acute angle, in which case the skew-back is less, that is, the external angles c b d and bee are less obtuse ; a smaller unavailable portion of the arch is thus left between the arc and its chord, but that portion is less securely retained under the flatter segment, because the joints or radii diverge less, or are more nearly parallel. These gauged arches being, as they for the most part are, but a half brick in thickness, and not being tied by a bond to anything behind them for, indeed, almost the whole, if not the whole, of their height is occupied behind by the reveal and the wooden lintel require to be executed with great care and nicety. It is a common fault with workmen to rub the bricks thinner behind than before, to insure a very fine joint in front. This tends to make the work bow outwards ; it should rather be inverted, if it be done at all, though the best work is that in which the bricks are gauged to exactly the same thickness throughout. The same fault occurs when a gauged arch is inserted in an old wall, on account of the difficulty of filling up with cement the space behind the bricks. Fig. 8, Plate XX. is a transverse section of fig. 7, and the gauged arch, lintel, &amp;lt;fcc., in it show the total discon nection of the gauged arch with any surrounding brickwork to which it might be bonded. The absurdity of construct ing arches circular on plan, especially in a thin unbonded shell of bricks, is so clear as hardly to require notice. It is generally held that nothing but its own components Plates ai should be admitted into a brick wall, except what is wood- absolutely necessary for its connection with the other parts ljricks - of a building, such as wall-plates and wood-bricks (and that these should be avoided as much as possible), tem plets, lintels, &c. Wall-plates are applied to receive the ends of the joists, and distribute the weight of the floor to which they belong equably along the walls. If the joists tailed singly on the naked bricks, their thin edges would crush those immediately under them, and the rest of the brickwork would escape immediate pressure altogether. Wall-plates may be avoided by the use of framed floors, which are carried by a few large beams, under whose ends stout pieces of timber or stone, 2 or 3 feet in length, are placed. These supports are intended, like a wall-plate, to distribute the weight over a considerable part of the wall, and prevent the necessity of placing the beam on the naked friable bricks, and are called templates. As bond timbers and wood-plates are now interdicted by the Build ings Act in London, the joists have to be tenoned into trimming joists carried by brick, stone, or iron corbels. Lintels are used over square-headed windows and doors, Lintels. instead of arches in brickwork. They are useful to pre serve the square form and receive the joiner s fittings, but they should always have discharging arches over them, and Discharg- should not tail into the wall at either end more than a few in s arche inches, that the discharging arch be not wider than is absolutely necessary. Fig. 9, Plate XX., indicates the ele vation of the inside of part of an external wall with a window in it, and shows the lintel over the opening with a discharging arch over it, and wood-bricks under its ends, on the jambs of the opening. Discharging arches should be turned over the ends of beams, and templets also, as in fig. 10. They may generally be quadrants of a circle or even flatter, and should be turned in two or more half bricks over doors and windows, and other wide openings, but over the ends of beams they need not be in more than one Wood- half brick. Wood-bricks are used to prevent the necessity Bricks. of driving wedges into the joints of brickwork to nail the joiner s work to. They are pieces of timber generally cut to the size and shape of a brick, or portion of one, and worked in as bricks in the inner face of a wall, where it is known the joiners have occasion for something of the kind. This is principally in the jambs of the windows and doors for their fittings, and along the walls, at proper heights, for the skirtings or wainscotting, as the case may be. The use of bond timbers in brick walls is objectionable because Bond of their liability to shrink and swell, to decay, and to tim be set on fire ; and in England the use of timber in walls has, since the extension of the manufacture of iron in these countries, been in a great degree superseded by that metal in the form known as hoop iron. Thin and Hoop iro narrow strips of this metal are laid in the bed joints of mortar, at intervals more or less frequent according to the nature and character of the work, with the best effect in respect of compactness and consequent strength. An improvement on the straight band has been introduced by Mr Tyerman, whereby a notch is made and the tongue bent down, which coming at each hollow of the bricks tends to afford a better hold on the mortar. It will be generally found that a brick wall built with Brick am mortar and faced with ashlar has settled inward to a a f hl r less or greater extent, as the work has been more or less carefully performed. Indeed, in the nature of things it cannot be otherwise, unless the brick backing be worked in some cement which sets and hardens at once ; for the outer face is composed of a layer of unyielding material, with few and very thin joints, which p erhups do not occupy a fiftieth part of its height, while the back is built up of an infinity of small parts, with fully one-eighth its height IV. - 59