CENTRE of a Sphere, a point in the middle, from which all lines drawn to the surface are equal.
To an unskilful carpenter this bridge does not seem essentially different from the centring of M. Hupeau for the bridge of Orleans; and indeed, in many cases, it requires reflection, and sometimes very minute reflection, to distinguish between a construction which is only an addition of frame to frame till the width be covered, from a construction where one frame works on the adjoining one transversely, pushing it in one part, and drawing it in another. The ready way for an unlettered artist to form a just notion of this point, is to examine whether he may distinguish saw through the connecting piece from one end to the other, and make them two separate frames. Whenever this cannot be done without that part opening, it is a construction by composition. Some of the beams are on the stretch; and iron straps, extending along both pieces, are necessary for securing the joint. The bridge is no longer a piece of masonry, but a performance of pure carpentry, depending on principles peculiar to that art. Equilibrium is necessary in the first construction; but, in the second, any inequality of loading is made ineffectual for hurting the edifice, by means of the stretch that is made to operate on some other piece. We are of opinion that this most simple employment of the distinguishing principle of carpentry, by which the beams are made to act as ties, will give the most perfect construction of a wide bridge. One polygon alone should contain the whole of the abutments, and one other polygon should consist entirely of ties; and the beams which form the radii, connecting the angles of the two polygons, complete the whole. By confining the attention to these two simple objects, the abutments of the outer polygon, and the joints of the inner one, may be formed in the most simple and efficient manner, without any collateral connections and dependencies, which divide the attention, increase the complication, and commonly produce unexpected and hurtful strains. It is for this reason that we have so frequently recommended the centring of the bridge of Orleans. Its office will be completely performed by a truss of the form of fig. 23, where the polygon ABCDEF, consisting of two layers of beams, if one is not sufficient, contains the whole abutments, and the other is nothing but an iron rod. In this construction, the obtuseness of the angles of the lower polygon is rather an advantage. The braces , which are wanted for trussing the middle of the outer beams, will effectually secure the angles of the exterior polygon against all risk of change. The reader must perceive that we have now terminated in the best construction of the Norman roof. We indeed think it the best general form, when some moderate declivity is not an insuperable objection. When this is the case, we recommend the general plan of the centring of the bridge of Orleans. We would make the bridge (we speak of a great bridge) consist of four trusses, two to serve as the outsides of the bridge, and two inner trusses, separating the carriage-way from the footpaths. The road should follow the course of the lower polygon, and the main truss should form the rails. It might look strange, but we are here speaking of strength; and evident, but not unwieldy strength, once it becomes familiar, is the surest source of beauty in all works of this kind. (B. B. B.)