432 EEPORT — 1882. 



adding successive portions to the ends of the cantilevers until the same 

 are complete. The central girders will also probably be erected on the 

 overhanging system, temporary connections being formed between the ends 

 of the cantilevers and the central girders. The closing lengths or key-pieces 

 at the centre of each 1,700-feet span will be put in on a cloudy day, when 

 there is little variation in temperature, and the details will be so arranged 

 that the key-piece can be completed and the temporary connections cut 

 away in a few hours, so as to avoid any temporary inconvenience from 

 expansion and contraction. 



No special difficulty will arise with respect to the foundations, though 

 the works will be, of course, on an unusually large scale. The island of 

 Inchgarvie is of trap rock, and the central pier at that spot will consist 

 of four cylindrical masses of concrete and rubble- work faced with granite, 

 and having a diameter of 45 feet at the top and 70 feet at the bottom. 

 The height above high water will be 18 feet, and the depth below the 

 same will vary from 24 feet to 70 feet. After the sloping face of the rock 

 foundation has been cut into steps, wrought-iron caissons will be floated 

 out, lowered into place, and filled with concrete lowered through the water 

 iu hopper-bottomed skips. Queensferry Pier will be founded on boulder 

 clay. Open-topped cylindrical caissons, 70 feet diameter, with an external 

 and internal skin 7 ft. G in. apart, will be floated out and lowered into 

 jolace. The space between the skins will be filled with concrete, to give 

 strength and weight to overcome frictional resistance in sinking. Grab 

 excavators will be carried on a turntable top to the caissons to remove the 

 earth from the interior, and pneumatic apparatus will be supplied to 

 enable men to clear boulders from the cutting edge of the caissons. From 

 a depth of 6 feet below low water upwards, the masoniy will be built in 

 the dry, inside a movable caisson connected by an indiarubber joint with 

 the permanent caisson below it. The piers will be carried down at least 

 10 feet into the boulder clay, which will give depths ranging from 68 feet 

 to 88 feet below high water and 18 feet less at low water in the respective 

 cylinders. In round numbers the weight of one of the cylindrical piers 

 at Queensferry may be taken at 16,000 tons, and the combined vertical 

 pressure on the top of the pier from the dead weight of superstructure, 

 rolling load, and wind pressure at 8,000 tons ; so the load on the clay 

 would average about 6 tons per square foot over the area of the founda- 

 tion. This is an insignificant amount on such bard clay as that at the 

 Forth, and the margin is ample, therefore, to allow of the unequal distri- 

 bution of the pressure due to the action of partial blasts of wind and of a 

 certain amount of expansion in the tubes connecting the piers. 



The total length of the great continuous girder is 5,330 feet, or say a 

 mile, and of the viaduct approaches 2,754 feet, or rather over half a mile. 

 There is nothing calling for special remark in the viaduct. The piers 

 will be of rubble masoniy, faced with granite, and the superstructure of 

 iron lattice girders with buckled plate floor and trough rail bearers, as in 

 the instance of the main spans. The main girders, spaced 16 feet apart, 

 will be placed under the railway, and there will be a strong parapet and 

 wind screen to pi-otect the trains. 



To engineers little need be said respecting the stiffness of a girder 

 bridge having the proportions adopted in the present case. The central 

 girder has a depth of one-seventh of the span, whilst the cantilevers may 

 be looked upon as halves of a girder having a depth of one-fourth of the 

 span. Exceptional depths like these confer exceptional stiffness, and there 



