Engineering Wonders of the World
Volume I

324 ENGINEERING WONDERS OF THE WORLD. THE SOUTH APPROACH SPANS, LOOKING NORTH. (By kind permission of Messrs. Baker and Hurtzig.) In the middle distance is seen one of the South Queensferry Caissons afloat. silt, and sand. The last, therefore, were of the greatest depth, one of them extending downwards 89 feet below high-water level. Work on the bridge was commenced at the end of 1882. The first things to be done were to fix the exact positions of the piers for the towers, and to pre- Work pare workshops, wharves, jet- commenced. ties, etc., for handling the immense quantities of metal and stone required for the enterprise. The centre points of the piers were established by a series of triangulations based on lines of known length fixed on the shores, and verified by means of a steel wire carefully measured in the following manner. Two posts carrying knife edges exactly 1,700 feet apart were set up beside the North British Railway. The wire was then passed over the knife edges, and tightened till it had a certain amount of droop at the centre. The droop having been measured by level, and the temperature noted, the wire Measuring was marked by copper tags Operations, soldered to it at the points where it had rested on the knife edges. By straining the wire subsequently over posts which had been set up on the Fife shore, on Inchgarvie, and at the edge of the south channel, in accordance with the trigono- metrical observations, and by giving the wire the original droop at the original temperature, the calculations were checked and found to be accurate within an inch or two. On the Queensferry shore the sloping ground was terraced over an area of sixty acres to accommodate the workmen and their families, drill-roads, storehouses, timber