So 



NA TURE 



[May 26, 1887 



And yet the Forth which '' bridled the wild High- 

 lander," and especially that part of it where the bridge 

 crosses, should be well enough known to every reader of 

 fiction, for it has been made the scene of many adven- 

 tures. Mr. Louis Stevenson's thrilling story, " Kid- 

 napped," will have been read by most of you ; the hero of 

 that story was kidnapped at the very spot where the 

 bridge crosses, so I can describe the point of crossing in 

 David Balfour's own words : — 



" The Firth of Forth (as is very well known) narrows at 

 this point, which makes a convenient ferry going north, 

 and turns the upper reach into a land-locked haven for 

 all manner of ships. Right in the midst of the narrows 

 lies an island with some ruins ; on the south shore they 

 have built a pier for the service of the ferry, and at the 

 end of the pier, on the other side of the road, and backed 

 against a pretty garden of holly-trees and hawthorns, I 

 could see the building which they call the Hawes Inn." 



Such was the appearance of the spot 1 50 years ago. The 

 middle pier of our bridge now rests on the island referred 

 to, and the Hawes Inn flourishes too well, for being in 

 the middle of our works its attractions prove irresistible 

 to a large proportion of our 3500 workmen. The accident 

 ward adjoins the pretty garden with hawthorns, and many 

 dead and injured men have been carried there, who would 

 have escaped had it not been for the whisky of the Hawes 

 Inn. 



I would wish if possible now to convey to my hearers 

 some clear impressions of the exceptional size of the 

 Forth Bridge, for even those who have visited the works 

 and noted the enormous gaps to be spanned on each side 

 of Inch Garvie, may yet have gone away without realiz- 

 ing the magnitude of the Forth Bridge as compared with 

 the largest railway bridges hitherto built. For the same 

 reason that architects introduce human figures in their 

 drawings to give a scale to the 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 P'orth, 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. And yet it was of this 

 Britannia Bridge that Stephenson, its engineer, thirty 

 years ago said : — ■" Often at night I would lie tossing 

 about, seeking sleep in vain. The tubes filled my head. 

 I went to bed with them, and got up with them. In the 

 gray of the morning, when I looked across Gloucester 

 Square, it seemed an immense distance across to the 

 houses on the opposite side. It was nearly the same 

 length as the span of my tubular bridge ! " 



Our spans, as I have said, are each nearly four times 

 as great as Stephenson's. To get an idea of their magni- 

 tude, 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 possibihty of any intermediate pier or sup- 

 port. 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 

 top of St. Paul's. The bridge would be a startling object 

 indeed in a London landscape. 



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 previously 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 satis- 

 factory name for the type. A cantilever is simply 

 another name for a bracket. The 1700-feet openings of 



the Forth are spanned by a compound structure consist- 

 ing of two brackets or cantilevers and one central girder. 

 Owing to the arched form of the under-side of the bridge, 

 many persons hold the mistaken notion that the principle 

 of construction is analogous to that of an arch. In pre- 

 paring 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 arms 

 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 1 2 feet in 

 diameter and ij inch thick. 



I have remarked that the principle of the Forth Bridge 

 is not novel. When Lord Napier of Magdala accom- 

 panied me over the works one day he said : " I suppose 

 you touch your hat to the Chinese?" and I replied 

 " Certainly," as I knew that a number of bridges on the 

 same principle had existed in China for ages past. 

 Indeed, 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 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. 

 This is no imagination, as 1 have actual sketches of such 

 bridges taken by exploring parties of engineers on the 

 Canadian Pacific and other railways, and in an old book 

 in the British Museum I found an engraving of a most 

 interesting bridge in Tibet upwards of 100 feet in span, 

 built between two and three centuries ago, and in every 

 respect identical in principle with the Forth Bridge. 

 When I published my first article on the proposed Forth 

 Bridge some four years ago I protested against its being 

 stigmatized as a new and untried type of construction, 

 and claimed that it probably had a longer and more 

 respectable ancestry even than the arch. 



The best evidence of approval is imitation, and I am 

 pleased to be able to tell you that since the first publica- 

 tion of the design for the Forth Bridge, practically every 

 big bridge throughout the world has been built on the 

 principle of that design and many others are in progress. 



Piers. — Having referred thus briefly to the general 

 principle of the Forth Bridge, I will now describe more 

 particularly the details of the structure, commencing with 

 the piers. 



There are three main piers, 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 



feet from the Inch Garvie pier, which is partly founrled 



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



1700 feet carries the bridge to the Queensferry pier, which 



is at the edge of the deep channel. The total length of 



the viaduct is about i^ mile, and this includes two spans 



of 1700 feet, two of 675 feet, being the shoreward 



' ends of the cantilevers, and fifteen of i6Sfeet. Including 



I piers, there is thus almost exactly one mile covered by 



I the great cantilever-spans, and another half-mile of via- 



