When the, side of a roof is a plane sur face, except at the eaves, at which place it is concave, the roof is said to have a bell cast at that place.
The general names of the timbers are, straining pieces, tie pieces, and bearers ; under straining pieces are included, prin cipal rafters, camber beams, hip and val ley rafters, collar beams, or straining beams, straining sills, struts, auxiliary raf ters, or principal braces and studs.
Under the pieces are included, tie beams, diagonal ties, and truss posts; and under bearers are included. plates, pur lins, common rafters, small rafters, ridge pieces, boarding and dragon beams.
The sloping sides of roofs are of two kinds, single and double, or plain and carcase: single roofs are those which have one row of rafters upon the same side ; double or carcase roofs are those which have two ties of rafters; the lower tie sup porting the upper by the intervention of transverse pieces called purlins.
Stone Bridges. A stone bridge is a thick wall built across a hollow, with one, two, three, or a series of apertures, formed in to arcades, which either serve to lighten the masonry, or to give passage to a stream of water, or both.
When a stone bridge is resolved upon, the first consideration is its place : in this several particulars should be taken into consideration, and the advantages com pared to the disadvantages. As the height of the bridge depends on the banks of the river, the expense will be increased according to their height: therefore a con venient situation should be chosen, where the banks will be adequate in height to that necessary for the bridge, though the expense will be increased by the length of the bridge. In most cases, where the river runs in a valley, a wide part of the stream must be preferred to a narrow part, as the water at this narrower part has not only a greater degree of velocity of itself, but the velocity would also be increased by the piers of the bridge ; in times of heavy floods it would be liable to be thrown down, and in a navigable river the navigation would be impeded. As the expense depends on the bed of the river, it must also be taken into the account.
These being settled, the form and height of the arches come next tinder consideration ; the height of the arches, which determine that of the bridge, de pends on the rise of the water in time of floods ; and whether there is to be a na vigation, and what kind of vessels there are to pass.
Stone bridges ought to be constructed with as few arches as possible, which will not only give greater beauty, but will re quire fewer foundations, piers, and cen terings, and also easier passage for craft. The piers ought to be so proportioned as to enable them to withstand the thrust of the adjacent arches, though the rest were thrown down. The number of arches ought to be odd, in order that one may stand in the middle, where the stream has its greatest velocity.
When the passage-way along the top of the bridge is a convex curve, the arch es should diminish from the middle to wards each extreme, so as to be similar to the middle one ; this will allow a more free passage to the water, the velocity being greatest in the middle. With re spect to the choice of arches, the ellipti cal, cycloidal, and equilibrated arches, are not only convenient, in allowing more room for the passage of ships at the hanches, but they require fewer materials than most other curves of the same di mensions.
When the extrados is convex, and the height of the arch small in proportion to the span, a segment of a circle may be used with success : in this case the arch should not exceed BO degrees.
These particulars being fixed, the prac tice is as follows :—When the foundation of a stone bridge is to be laid in a river which is not very deep, a single or dou ble inclosure of wood is formed, and the intervening space is rammed well with clay or chalk, to prevent the water from coming in. These inclosures are either made with piles driven closely together, and dovetailed at their jointings, or by piles driven at certain distances from one another, and grooved on the sides oppo site each other, and the intervals re shut with boards let in between the grooves. This kind of fence against the water is called a batterdeaux, or coffer-dam. The batterdeaux, or coffer-dam, requires a good foundation of solid earth or clay. If the bed of the river be of a loose consist ence, the water will ooze through it in too great abundance. The sides of the inclosure must be made very strong, and well braced within, to prevent the am bient water from forcing its way into the batterdeaux.