MISCKI.I.ANY 



777 



The same general principles apply to the con- 

 struction of iron as of stone bridges, but the 

 greater cohesion and adaptability of the material 

 give more liberty to the architect, and much 

 greater width of span is possible. At first, iron 

 bridges were erected in the form of arches, and 

 the material employed was cast iron; but the 

 arch has now been generally superseded by the 

 beam or girder, with its numerous modifications; 

 and wrought iron or steel is likewise found to be 

 much better adapted for resisting a great tensile 

 strain than cast-metal. Numerous modification- 

 exist of the beam or girder, as the lattice-girder, 

 bow-string-girder, etc. ; but of these none is more 

 interesting than the tubular or hollow girder, 

 first rendered famous from its employment by 

 Robert Stephenson in the construction of the rail- 

 way bridge across the Menai Strait, and connect- 

 ing Anglesey with the mainland of North Wales. 

 This is known as the Britannia Tubular Bridge. 

 The tubes are of a rectangular form, and con- 

 structed of riveted plates of wrought iron, with 

 rows of rectangular tubes or cells for the floor and 

 roof respectively. The bridge consists of two 

 of these enormous tubes or nollow beams laid 

 side by side, one for the up and the other for the 

 down traffic of the railway, and extending each 

 to about a quarter of a mile in length Other 

 tubular bridges of importance are tne Conway 

 Bridge, over the River Conway, an erection iden- 

 tical in principle with the Britannia Bridge, but 

 on a smaller scale; the Brotherton Bridge over 

 the river Aire; the tubular railway bridge across 

 the Damietta branch of the Nile, which has this 

 peculiarity, that the roadway is carried abore 

 in-tead of through the tubes; and the Victoria 

 Bridge over the St. Lawrence, Canada. In 

 many respects this structure is even more re- 

 markable than the Britannia Bridge, being sup- 

 ported by twenty-four piers, and nearly two 

 miles in length, or about five and a half times 

 that of the bridge across the Menai Strait. A 

 girder railway bridge across the Firth of Tay at 

 Dundee was opened in 1887, being the second 

 built at the same place, after the first had given 

 way in a great storm. It is two miles seventy- 

 three yards long, has eighty-five spans, is seven- 

 tv-seven feet high and carries two lines of rails. 

 The bridge over the Firth of Forth, at Queens- 

 ferry, a notalile -t picture, has two chief spans 

 of 1,710 feet, two others of 680 feet, fifteen of 

 168 feet, and seven small arches, and will give 

 a clear headway for navigation purposes of 150 

 feet above high-water of -prim: tides. Tin- 

 great spans consist of a cantilever at either 

 680 feet long, and a central girder i 

 Both the above bridges are built to carry the 

 lines of the North HritMi Railway. The ('"ruin- 

 lin Railway Viaduct. South Wales.' having l.-n 

 girders supported on open- work more 



>r height than length, l-mif 

 feet high bridges, l>ein<- 



independent of central supports, do not interfere 

 with the river, and may be erected where it is 

 impracticable to build bridges of any other kind 

 The entire weight of a suspension* bridge rests 

 upon the piers at either end from \\lnch 



suspended, all the weight ln-ing In-low the p 



ipport Such I. ridges always swing a little. 

 giving a vibratory movement which imparts a 



peculiar sensation to the passenger. The modes 

 of constructing these bridges are various. The 

 roadway is suspended either from chains or 

 from wire-ropes, the ends of which require 

 to be anchored, that is attached to the solid 

 rock or masses of masonry or iron. One of 

 the earlier of the great suspension bridges 

 is that constructed by Telford over the 

 Strait near the Britannia Tubular Bridge. 

 finished in 1825; the opening between the 

 points of suspension is 580 feet. The Hammer- 

 smith Chain-bridge, the Union Suspension 

 bridge near Berwick, and the suspension bridge 

 over the Avon at Clifton are other British 

 examples. On the European Continent, the 

 Fribourg Suspension bridge in Switzerland, 

 span 870 feet, erected, 1834, is a celebrated 

 work ; as is that over the Danube connect- 

 ing Huda with Pesfh. In America the lower 

 suspension bridge over the Niagara, seven 

 miles below the falls, supported by win' 

 cables, is 822 feet long; it nas two floors or 

 roadways connected together but fifteen feet 

 apart, the lower serving for ordinary traffic, 

 the upper carrying three lines of rails, 245 

 feet above the river. Another bridge, close 

 to the falls, has a span of 1,250 feet. The 

 Cincinnati bridge over the Ohio has a span of 

 1.057 feet. A suspension bridge of great mag- 

 nitude, connecting the city of New York with 

 Brooklyn, was opened in 1883. The central 

 or main span is 1,595$ feet from tower to 

 tower, and the land spans between the towers 

 and the anchorages 930 feet each; the ap- 

 proach on the New York side is 2,492 feet, 

 long, and that on the Brooklyn side, 1,901 

 feet, making the total length 5,989 feet. The 

 height of the platform at the center is 135 

 feet above high water, and at the ends 119 

 feet. The roadway is eighty-five feet broad, 

 and is divided into five sections, the two out- 

 side for vehicles, the two inner for trolley- 

 cars, the middle one, twelve feet above tin- 

 rest, for foot-passengers. Cost over $15,000,- 

 000. Though the oldest bridges on re, 

 \\ere- built of wood, like the Suhlician Bridge 

 at Rome, or that thrown by Ctrsar across the 

 Rhine, it is only in certain places and for 

 certain purposes that wood is much used at 

 P resent. In modern time- < iermany has been 

 the school for wooden bridges. Perhaps the 

 most ceh-1. rated of all wooden bridges was 

 that which spanned the Rhine at Schaffhnusrn 

 in Swit/.erlaml. This was 364 feet in length 

 and eighteen feet broad. It was designed 

 Mid executed by Ulric Crul.enman. a village 

 carpenter, in 1758, and was destroyed by the 

 :,-h. in 1 :'.. In the Tniied States, where 

 timber is still in common u-e. we have some fine 

 :,ples. the Tret,- the Delaware, 



ed in 1X04; the bridge over the Susque- 

 hanna. etc. 



Some of the most notable development- in 

 the art of bridge construction are to be found 

 in North America, when- an enormous railway 



rsing a country of pi- md 



nes, has given an exceptional stimuli 

 the 



I, rid >nd boldness of de-iirn. 



the reduction of the number of members to a 



