Page:Forth Bridge (1890).djvu/17

Rh of erection, three timber booms, each boom consisting of three heavy Oregon pine logs, were moored to the west of Inchgarvie. The logs were octagon section, from 2 ft. to 2 ft. 6 in. across, and about 100 ft. long, and were strongly bound together by three heavy iron belts to each boom. Mooring blocks weighing nearly 40 tons each were laid down in the bed of the Firth, some 120 yards west to the booms. On the side of the island heavy iron stakes were fixed in the rock, and to these or else to some of the iron columns, $1 1/4$ in. cable chains were attached. The other ends of these chains were shackled to the pointed ends of large floating buoys, and the same shackles received similar chains coming from the large mooring blocks. On the tops of the buoys were large rings, and to these were attached the chain bridles from the ends of the floating booms, or rather from the iron belts near the ends. Sufficient slack was allowed in all chains for rise and fall of tide. These timber booms have been the means of saving many small boats and sailing schooners from certain shipwreck.

With the calm weather in winter and early spring the sea fogs or eastern haars occur with tolerable frequency, and have caused much anxiety on account of the necessity of having to carry large numbers of workmen—many hundreds every morning and night—from either shore to their destinations. These fogs come up the Firth like a solid wall of dazzlingly white cloud, sometimes leaving the tops of the towers standing out clearly in the sunshine, at other times hanging some 50 ft. to 100 ft. up in the air, and leaving the lower portions quite clear. The effect is well shown in the illustration from a photograph given on Plate XIII.

The wind pressure to be provided for in the calculations for bridges in exposed positions is 56 lb. per square foot, according to the Board of Trade regulations, and this twice over the whole area of the girder surface exposed, the resistance to such pressure to be by deadweight in the structure alone.

The most violent gales which have occurred during the construction of the Forth Bridge are given with the pressures recorded on the wind gauges in the annexed Table, No. I.

* These data are unreliable, owing to faulty registration by the indicator needle, as will be explained. They were altered after this date. The barometer fell to 27.5 in. on that occasion—over ¾ in. within an hour.

It is worthy of observation that only one gale from easterly direction is recorded—January 5, 1888—but there have been a number of gales from that quarter registering between 15 lb. and 16 lb. and up to 20 lb. per square foot, and, of course, the same from other directions.

The pressure gauges which were put up in the summer of 1882 on the top of the old castle on Inchgarvie, and from which daily records have been taken throughout, were of very simple construction. As the object was to ascertain only the maximum pressures which the structure would eventually have to resist, the maxima only were taken. The most unfavourable direction from which the wind pressure can strike the bridge is at right angles to the longitudinal axis, or nearly due east and west, and two out of the three gauges were fixed to face these directions, while a third was so arranged as to register for any direction of wind. There was no provision made for registering intermediate pressures, nor particulars as to direction of wind or times of occurrence, except in so far as the records were taken generally at 9 a.m. every day.

The principal gauge (Fig. 17) is a large board—20 ft. long by 15 ft. high, or 300 square feet area—set vertically with its faces east and west. The weight of this board is carried by two rods suspended from a framework surrounding the board, and so arranged as to offer as little resistance as possible to the passage of the wind, in order not to create eddies near the edge of the board. In the horizontal central axis of the board there are fixed two pins, which fit into the lower eyes of the suspension-rods, the object being to balance the board as nearly as possible. Each of the four corners of the board is held between two spiral springs, all carefully and evenly adjusted so that any pressure exerted on either face will push it evenly in the opposite direction; but on such pressure being removed, the compressed springs will force the board back to its normal position. To the four corners four wires are attached, uniting in pyramidal formation in one point, whence a single wire passes over a pulley to the registering apparatus below. This in the original arrangement consisted of two levers at right angles to one another, the the shorter one—about one-third of the other—being acted on by the wire from the windboard, the longer one with an index pointer registering the amount upon prepared paper slips. The indication by the pointer was thus about three times the amount of travel of the wire up and down. In order to ascertain, to some extent, how far great gusts of wind are quite local in their action, and exert great pressure only upon a very limited area, two circular spaces—one in the exact centre, and one in the right-hand top corner about—18 in. in diameter, were cut out of the board, and circular plates inserted which could register independently the force of the wind upon them.

By the side of this large square board, at a distance of about 8 ft., another gauge—a circular plate of $1 1/2$ square feet area, facing east and west—was fixed up with separate registration. This was intended as a check upon the records given by the large board.

Another gauge of the same dimensions as the last, but with the disc attached to the short arm of a double vane, so that it should face the wind from whatever direction it might come, was set up (see Fig. 18.) This also had a separate registering apparatus arranged in the same manner as the first two described.

Fig. 17 gives two views of the large wind-board board with the two independent gauges at A and B. Fig. 18 shows the revolving wind gauge, consisting of a circular disc hung in four bent springs, and rods which centre in the point M. Here a horizontal crossbar connects the two opposite springs, and to this bar, immediately opposite the centre of the hollow vertical spindle P, a small chain is attached, which passes through a slot in P over the pulley N, and right down the centre to the registering apparatus below. The double vane is fixed upon P, which revolves in the socket column R. The small fixed gauge is of the same construction and area,