O proceed to the gallery itself, supported by the girders just described. The timbers supporting the floor are so arranged that the weight of each bay of twenty-four feet square is distributed equally to the four girders inclosing it, and in such a manner as to bear upon them at the points immediately over the vertical standards.
In the transverse direction of the building two pairs of joists, eight feet apart in each bay, are formed into trusses by tie-rods, 1⅜ inches diameter, passed through a cast-iron shoe at each end, and pressing up two "struts," which are made to bear against the under-side of binding-timbers running longitudinally, or crossing the joists, and immediately under them. The cast-iron shoes for the trusses are bolted down to the girders, and serve at the same time to receive the standard supports of the gallery railing. The ends of the binding-timbers are secured by bolts and oak suspension-pieces to the other two girders inclosing the square. Joists about two feet six inches apart bear from girder to girder parallel to the trusses, and resting on the binding-timbers. On these is laid the floor, 1¼ inches thick, grooved and iron-tongued. A light cast-iron railing, forming a kind of trellis-work, is fixed between the columns, and is capped with a round mahogany hand-rail. From the view at page 60 the arrangement of the galleries will be readily understood.
ROM the very important office which the girders perform throughout the building, but more particularly those supporting the galleries, it was of the utmost importance that, previously to their being fixed in their places, the soundness of the casting should be proved; for it could hardly be expected that so large a number of girders could be produced without some of them being defective. The ordinary means of testing girders, by loading them with weights, would have occupied far too much time; and therefore an ingenious apparatus was devised by Mr. C. H. Wild for this purpose, by the use of which the testing of a girder occupied but a few minutes.
FRAME AND HYDRAULIC PRESS FOR TESTING THE GIRDERS.
It consisted of a very strong cast-iron frame rather longer than the girder, the bottom of which was formed by two fixed beams placed eight inches apart, and supported a few inches above the ground. At each end of these a cast-iron standard was firmly bolted between them and rose to a height rather greater than the depth of the girder to be tested; on the inner faces of these standards two "shoulders" were formed, which received the projections cast on the ends of the girder, as before mentioned. Between the fixed beams below, at two points dividing the whole length into three equal parts, were placed strong cylinders, with rising pistons connected with a forcing-pump, together with which they formed a Bramah's hydraulic press. A girder being placed in this frame, in an inverted position, the force applied by means of the pistons rising from the cylinders acted upon it precisely at those points, and in the same manner, as the load from the gallery or the roof would do when afterwards fixed in its place.
The essential parts of the Bramah's press may be thus briefly described. It consists of two cylinders, the diameter of one being considerably larger than that of the other. The smaller cylinder is fitted with a solid plunger or piston, by means of which water may be forced from it into the larger; this being also fitted with a rising piston, the force is communicated by it to the weight which it is desired to raise. The power obtained by means of this apparatus arises from the distributive power of fluids and the practical incompressibility of water, and it is proportioned to the difference of the diameters of the two cylinders; so that if a pressure of one pound per square inch be applied on the surface of the piston in the smaller cylinder, and the piston in the larger cylinder present a surface ten times greater, the power is multiplied by that number; whilst, in addition, the lever power used in applying the pressure to the smaller piston is obtained. The cylinders are fitted with valves, so arranged as to prevent the return of the water from the larger to the smaller, while the apparatus is in action, and thus the power is accumulated in the former.
In the instance before us, the two 3-inch cylinders already alluded to in the proving-frame took the place of the larger cylinder of the ordinary apparatus; and they were connected with the forcing-pump by a strong metal tube. When a girder had been fixed in the frame for proving, the force-pump was worked till the pistons underneath the girder carried it off its lower bearings and pressed it upwards against the "shoulders," by which it was firmly held, and the pressure was then continued until the amount previously fixed upon as necessary for proof had been obtained. This was ascertained by means of a self-adjusting apparatus attached to the hydraulic press.