1887.] on Bridging the Firth of Forth. 143 



figures iu their drawings to give a scale to tlie buildings, do we 

 require something at Queensferry to enable visitors to appreciate the 

 size of the Forth Bridge. If we could transport one of the tubes of 

 the great Britannia Bridge from the Menai Straits to the Forth, we 

 should find it would span little more than one-fourth of the space to 

 be spanned by each of the great Forth Bridge girders. To get an 

 idea of the magnitude of the latter, stand in Piccadilly and look 

 towards Buckingham Palace, and then consider that we have to span 

 the entire distance across the Green Park, with a complicated steel 

 structure weighing 15,000 tons, and to erect the same without the 

 possibility of any intermediate pier or support. Consider also that 

 our rail level will be as high above the sea as the top of the dome of 

 the Albert Hall is above street level, and that the structure of our 

 bridge will soar 200 feet yet above that level, or as high as the toj^ of 

 St. Paul's. 



It is not on account of size only, that the Forth Bridge has 

 excited so much general interest, but also because it is of a j)reviously 

 little known type. I will not say novel, for there is nothing new 

 under the sun. It is a cantilever bridge. One of the first questions 

 asked by the generality of visitors at the Forth is — Why do you call 

 it a cantilever bridge ? I admit that it is not a satisfactory name, 

 and that it only expresses half the truth, but it is not easy to find a 

 short and satisfactory name for the type. A cantilever is simply 

 another name for a bracket, but the 1700 feet openings of the Forth 

 are spanned by a compound structure 'consisting of two brackets or 

 cantilevers and one central girder. Owing to the arched form of the 

 underside of the bridge many persons hold the mistaken notion that 

 the principle of construction is analogous to that of an arch. In 

 preparing for this lecture the other day, I had to consider how best 

 to make a general audience appreciate the true nature and direction 

 of the stresses on the Forth Bridge, and after consultation with some 

 of our engineers on the spot a living model of the structure was 

 arranged as follows : — Two men sitting on chairs extended their arras 

 and supported the same by grasping sticks butting against the chairs. 

 This represented the two double cantilevers. The central girder was 

 represented by a short stick slung from one arm of each man, and the 

 anchorages by ropes extending from the other arms to a couple of 

 piles of brick. When stresses are brought on this system by a load 

 on the central girder, the men's arms and the anchorage ropes come 

 into tension, and the sticks and chair legs into compression. In the 

 Forth Bridge you have to imagine the chairs placed a third of a mile 

 apart and the men's heads to be 360 feet above the ground. Their 

 arms are represented by huge steel lattice members, and the sticks or 

 props by steel tubes 12 feet in diameter and 1^ inches thick. 



I have evidence that even savages when bridging in primitive 

 style a stream of more than ordinary width, have been driven to the 

 adoption of the cantilever and central girder system as we were 

 driven to it at the Forth. They would find the two cantilevers in 



