144 3Ir. Benjamin Baker [May 20, 



the projecting branches of a couple of trees on opposite sides of the 

 river, and they would lash by grass ropes a central piece to the ends 

 of their cantilevers and so form a bridge. 



The best evidence of approval is imitation, and I am j)leased to 

 be able to tell you that since the first publication of the design for 

 the Forth Bridge, practically every big bridge throughout the world 

 has been built on the j^riuciple of that design and many others are 

 in progress. 



There are three main piers at the Forth known respectively as 

 the Fife pier, the Inch Garvie pier, and the Queensferry pier, and upon 

 each of these there are built huge cantilevers stretching both ways. 

 The Fife pier stands between high and low-water mark, and is 

 separated by a span of 1700 ft. from the Inch Garvie pier, which is 

 partly founded upon a rocky island in mid-stream. Another span of 

 1700 ft. carries the bridge to the Queensferry pier, which is at the 

 edge of the deep channel. The total length of the viaduct is about 

 IJ miles, and this iucludes two sjians of 1700 ft., two of 675 ft., 

 being the shoreward ends of the cantilevers, and fifteen of 168 ft. 

 Including piers, there is thus almost exactly 1 mile covered by the 

 great cantilever spans and another J mile of viaduct approach. The 

 clear headway under the centre of the bridge is 152 ft. at high water, 

 and the highest point of the bridge is 860 ft. above the same datum. 



Each of the main piers includes four columns of masonry founded 

 on the rock or boulder clay. Below low water the j^iers differ some- 

 what in character, according to the local conditions. At Inch Garvie 

 two wrought-iron caissons, which might be likened to large tubs or 

 buckets, 70 ft. in diameter and 50 ft. to 60 ft. high, were built on 

 launching ways on the sloping southern foreshore of the Forth. The 

 bottom of each caisson was set up 7 ft. above the cutting edge, and 

 so constituted a chamber 70 ft. in diameter and 7 ft. high, capable of 

 being filled at the proper time with compressed air to enable men to 

 work as in a diving bell below the water of the Forth. The caisson, 

 weighing about 470 tons, was launched and then taken to a berth 

 alongside the Queensferry jetty, where a certain amount of concrete, 

 brickwork, and staging was added, bringing the weight up to 2640 

 tons. A very strong and costly iron staging had previously been 

 erected, alongside which the caisson was moored in correct position 

 for sinking. Whilst the work described was proceeding, divers and 

 labourers were engaged in making a level bed for the caisson to sit 

 on. The 16 ft. slope in the rock bottom was levelled up by bags 

 filled with sand or concrete. As soon as the weight of caisson and 

 filling reached 3270 tons the caisson rested on the sand bags and 

 floated no more. The high ledge of rock upon which the northern 

 edge of the caisson rested was blasted away, holes being driven, by 

 rock drills and otherwise, under the cutting edge, and about 6 in. 

 beyond for the charges. After the men had gained a little experience 

 in this work no difficulty was found in under-cutting the hard whin- 

 stone rock to allow the edge of the caison to sink, and, of course, 



