The circle of the mechanical arts; containing practical treatises on the various manual arts, trades, and manufactures / By Thomas Martin.

  • Martin, Thomas (Civil engineer)
Date:
1813
Catalogue details

Licence: Public Domain Mark

Credit: The circle of the mechanical arts; containing practical treatises on the various manual arts, trades, and manufactures / By Thomas Martin. Source: Wellcome Collection.

    55/712 (page 41)
    pressed down, they may he connected together before the superstructure is proceeded with * and ki some cases it may be expedient to make these masses no larger than piles of wood, to which, however, they might be preferable, as being of eheaper and more durable materials. EXPLANATION of FIGURES ill PLATE 1 of BRIDGES. “ Figure 1, ( Plate 1) A, section. B, plan of a foun- dation mass of brick-work, for a wharf wall in deep water, b b b, parts added gradually to the bottom. Figure 2, A, section of a mass of brick-work to form part of an embankment, and at the same time to serve other purposes of an ordinary building; part of which shews the superstructure, and the parts gradually added to the bottoms to strengthen them as the superstructure advances, and as by that means a sufficient buoyancy is obtained. B, plan of the same. Figure 3, A, section. B, plan of a cylindrical mass of masonry or brick-work, applicable for the foundation of a break-water, or for other purposes. The lines a a distinguish the superstructure and the parts added. Figure 4, A, longitudinal section. B, transverse section. C, plan of one of the constituent parts to be prepared ashore for the bottom of a mass of any extent, which may be formed by a number of such constituent parts put together afloat. ” Wooden Btidges now demand our attention. There are various methods of constructing wooden bridges, so as to answer valuable purposes, even to last a very considerable time. The simplest case of these edifices is that, in which the road-way is laid over beams placed horizontally, and supported at each end by piers or posts. This method, how- ever, is deficient in strength, and width of opening: it is therefore necessary in all works of magnitude, to apply the principles of trussing, as used in roofs ,and centers. Wooden bridges of this kind are there- fore stiff frames of carpentry, in which, by a proper disposition, beams are put, so as to stand in place of solid bodies, as large as the spaces which the beams enclose; and thus two or more of these are set in a hutment w ith each other, like vast arch stones. One of the most important points to be considered in wooden bridge building, is the seasoning and pre- paring of the wood, so as to make it lasting. In seasoning of wood for this purpose, the follow ing particulars should be attended to : it is well know n that the decay of fir timber is generally ow ing td the sappy nature of its exterior surface, which is by no means capable of being removed by any immediate application of paint, previous to its being seasoned : on the contrary, it Isas been proved, that such an application is actually injurious, since it hinders the free admission of air and heat, which would have the property of extracting that sappy quality which so much contributes to decay and rottenness. In con- sequence of this practice, the sap strikes inwardly, and makes its way to the heavt-of the wood, the sub- stance of which is presently destroyed. As a means of preventing this evil, some burn and scorch the timber over a flaming fire, turning it about till every side acquire a sort of crusty surface; and in doing this, it necessarily follows, that the external moisture is dissipated. After this process, a mixture of pitch and tar, sprinkled with sand and powdered shells, may be advantageously applied to tlie parts undefc water. Those more in sight, after being well scorched, and while the wood is hot, should be rub- bed over with linseed oil, mixed with a little tar, This w ill strike deeply into the grain of the wood, and will soon harden so as to receive as many coats of paint as may appear necessary. It has been found, that fir timber, this prepared, is nearly equal to oak for durability. Palladio has given several excellent designs of wooden bridges, Figure 1 Plate 2, of Bridges, one of w hich wras erected on the river Cismone, which separates Italy from Germany,and is 100 feet wide; this width is divided into six equal parts, and at the end of each part, excepting at the banks, which are, strengthened with pilasters of stone, the beams .are placed, that form the breadth of the bridge, upon which, a little space being left at their ends, were placed other beams lengthways, which form the sides. Over these, directly upon the first, the king ports are disposed on each side ; these king posts are connected to the beams, which form the breadth of the bridge, by means of irons passing through the projecting ends of the beams, and bolted and pinned through both. The invention of this bridge Palladio considers as very worthy of attention, as it may serve on all occasions where posts cannot with convenience be introduced in the river. He describes also other methods of constructing wooden bridges, without posts in the water, like the fore-mentioned. The bridges after the first method are to be made in this manner : Figure 2, the banks being strengthened by pilasters when necessity requires, one of the beams forming the breadth of the bridge is to be placed with one end upon the pier, and the other end abbut- ting against the first queen post, which is to be con- nected with the beams by irons. Then the second beam being placed at a distance equal to the space between the first beam and the pier, which is to be supported in like manner with a queen, and strut and queen posts, and thus proceeding, as far as re- quired, observing to have a king post in the middle of the length, in w hich the struts meet both ways, and collar beams between all the posts, which stiffen and support the whole construction. It should be recollected that when bridges are constructed after this manner that they are to be wider at the extre- mities, and contract tow ards the middle. The bridge represented in Fig. 3, is also from Palladio. Its upper part, which is the support of the whole, is inscribed in a fiat segment of a circle $ M the