Page:Cassier's Magazine Volume XV.djvu/22

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also, by means of stamping presses, for manipulating it into forms which had not previously been thought possible. All the rivet-holes in the steelwork of the bridge were drilled, and consequently the drilling plant was a very important part of the whole. Mr. Arrol's system of first assembling the parts together and then drilling through the various thicknesses, while placed in the same position as they would occupy when ultimately fixed in the structure itself, was carried out to a very large extent, with the result that the rivet-holes were found, practically speaking, mathematically correct. The traveling drilling machines, which passed over the various parts of the work, were used not only for drilling the main portions of the girders, but also for the many miles of steel tubes required in the structure.

Some idea of the extent of the plant employed may be gained from the fact that it took over a year to have it designed and made ready to begin operations, but even long afterwards large additions were continually being made and applied. It may be of interest to add that the cost of the temporary plant was about £500,000. This included several small steamships, 1,000,000 cubic feet of timber, 1200 tons of service bolts, 60 miles of wire ropes, jacks and rams almost innumerable, and other manifold kinds of machinery and plant in proportion.

During the time the workshops were being got into condition the material for the caissons for the foundations was being prepared by outside contractors. They were then put together on the shores of the Forth. These caissons were all 70 feet in diameter, and after being built and launched sideways, after the fashion of launching a ship, were carried into the waters of the Forth and thereafter floated to the position where they were to be finally sunk into the bed of the river. The sinking of these caissons was one of the novel and most interesting operations in connection with the great structure. After the caissons had been floated over the site they were to occupy, concrete was gradually filled in till there was not only sufficient to sink them to the bed of the river, but also enough to force them into the ground as the material was being excavated in the caisson itself. The caisson proper may be looked upon as a huge diving bell into which the men descended through air locks and shafts, and as the work of removing the material from the inside proceeds, the caisson lowers itself into the bed of the river. This process was continued till the caisson arrived at its final depth, after which