An iron cylinder 1½ inches diameter was placed in communication with the pipe connecting the pump and the press, so that the pressure obtained in it was, in proportion to its diameter, the same as that in the large cylinder; and it was fitted with a piston-rod, working in a vertical direction. This piston-rod was connected with a lever, from the end of which a scale-pan was suspended, at a distance from the fulcrum ten times greater than that of the point of attachment of the piston from the same. The weight of the scale-pan and lever were balanced by a large mass of iron at the other end. In the scale-pan a certain weight was placed, proportioned to the proof desired to be obtained; and the action of the pump was continued until the water, rising in the iron cylinder just described, forced up the lever, and with it the weight attached; and thus indicated that the pressure to which it was desired to subject the girder had been reached. The weight to be placed in the scale-pan was thus determined: the diameter of the lever cylinder being 1½ inches, and that of each of those in the proving-frame three inches, the pistons or "rams" in the latter presented together eight times the surface of that in the lever cylinder; which being multiplied by the difference of length of the two parts of the lever, determines the weight for the scale-pan to be one-eightieth of that to which it was desired to prove the girder.
The ordinary gallery girders were tested with a pressure equivalent to a weight of fifteen tons; but it was calculated that, when fixed, the greatest weight they would have to sustain would be seven-and-a-half tons. In one instance, for the sake of experiment, the pressure was continued beyond the proof weight of fifteen tons, to see what amount of strain the girders would bear without fracture, and it was found that a strain of thirty tons produced no injurious effect; but the girder broke with an additional weight of half a ton.
E will now return to describe that portion of the roof which varies in form and arrangement from the rest, namely, the semicircular covering of the transept. This is supported by arched ribs, placed twenty-four feet apart, and constructed of Memel timber, in three thicknesses; the centre-piece four inches thick, with a 2-inch piece on each side of it. They are formed in lengths of about nine feet, placed so as to break joint; that is, the joints of the outer pieces fall upon the centre of the inner one. The thicknesses are fastened together by bolts passing through them about two feet six inches apart, besides being nailed at other points. On the inner circumference of the rib thus constructed there is then placed a piece of timber moulded to correspond with the form of the columns; and on the outer circumference two boards, each one inch thick, are bent round and attached to the rib with strong nails. On both the outer and inner circumference a flat bar of iron is secured by bolts passing through the whole depth of the rib, which, thus finished, measures eighteen inches in depth by eight inches in thickness. The ends of the ribs are fitted into sockets, formed by the upward continuation of the columns, to which they are attached by iron straps.
The ribs, which are supported by the trusses over the main avenue, have their ends bolted down upon a piece of timber secured on the upper portion of the truss; and they are further fixed in their places by oak brackets, forming a spreading foot on each side upon the same piece of timber.
Between these large ribs horizontal timbers, called "purlins," are fixed about nine feet apart, by means of cast-iron shoes, bolted both to them and to the ribs. These serve to support the minor or intermediate ribs, occurring at distances of eight feet apart; which consist of a single square piece of timber, having the two thicknesses of 1-inch board bent round their outer circumference, as on the main ribs. The boards form the gutters or furrows between which rise the ridges, in the same manner as in that portion of the roof which is horizontal.
The ridges, in this case, instead of being cut out of solid pieces, are formed in three thicknesses, bent round to the requisite curve, and so retained by small bolts tying them down to the "purlins." The sash-bars which receive the glass form, as elsewhere, the sloping rafters or supports of the ridge.
Interior View of the Central Avenue towards the West.