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THE POPULAR EDUCATOR. 



To return to our classification of iron railway bridges, we 

 have first to mention the suspension bridge. Structures of this 

 kind are usually built in the following manner. A massive 

 tower of masonry is erected on each side of the stream, these 

 towers being termed the abutments. Over the tops of the 

 towers are passed chains formed of bars of iron, the ends of 

 which chains are sunk to a great depth in the ground, and 

 firmly embedded therein. From these chains iron rods are 

 hung, to support the roadway which passes from tower to 

 tower. Among the good examples of ordinary suspension 

 bridges may be mentioned that at Hammersmith, on the 

 Thames ; Hungerford Bridge, afterwards removed to Clifton ; 

 and the bridge of this kind across the Menai Straits. 



In the earlier days of railway construction, this form of 

 bridge was not considered suitable for railway traffic. The 

 passage of a train over such a bridge would depress the chain 

 and roadway at either end, raising it at the centre, and so 



This is a form peculiar to railway architecture, the great engi- 

 neer who designed the first having before him no previous struc- 

 ture similar in kind although applied to other purposes, as in 

 the case of the suspension bridges to suggest the idea and guide 

 him in carrying it out. The first tubular bridge was that 

 thrown by Mr. Robert Stephenson across the Menai Strait, not 

 far from the suspension bridge to which we have before alluded. 

 It was a necessary condition, imposed by the Admiralty, that the 

 central arches of a bridge across this strait should be of more 

 than 400 feet span, and 100 feet above the surface of the water 

 at the highest tide, to preserve the freedom of the navigation. 

 In studying the problem of how to accomplish this end with 

 the utmost security, Mr. Stephenson determined, after repeated 

 experiments, to adopt a tubular form for his girders, and to 

 make his bridge, so to speak, a tunnel suspended in the air. A 

 further series of experiments convinced him that a rectangular 

 tube was the most suitable for the purpose, possessing far greater 



KAILWAY BRIDGE AT CHEPSTOW. PROCESS OF CONSTRUCTION. 



endanger the security of the entire fabric. But this tendency 

 was counteracted by various devices to stiffen the roadway ; 

 and a great engineering triumph was achieved when, in 

 1848, a railway suspension bridge, spanning a chasm of 800 

 feet, was built over the Niagara river below the falls. , In 

 this bridge wire cables are made to support a rectangular tube, 

 which carries both an upper and a lower roadway, the former for 

 the railway traffic, and the lower for that of vehicles and foot 

 passengers. A representation of this very remarkable structure 

 is given in one of our engravings. 



Another kind of suspension bridge is occasionally employed 

 for railway purposes. In this form the roadway is supported 

 by chains and rods which hang from wrought-iron tubes, these 

 tubes passing from tower to tower or from pier to pier, instead 

 of the chains before mentioned. The great railway bridges at 

 Chepstow and Saltash are formed in this manner. The method 

 of their construction will be understood by a reference to our 

 illustration of the first-named example, on the South Wales 

 Railway. 



We come now to the second classification of iron railway 

 bridges namely, those constructed on the tubular principle. 



strength than tubes either cylindrical or elliptical in form. His 

 idea, as finally resolved on and carried out, was, therefore, that 

 of a long tube, in section an oblong square, made of plates of 

 cast-iron closely riveted together, the chief weight and support- 

 ing power of the material being massed in a cellular form at 

 the top and bottom of the structure. Two million rivets were 

 employed throughout the tube. These rivets, when brought to 

 the works, were formed with only one end flattened, holes being 

 punched in the plates to receive them. Each rivet, before- being 

 fixed in position, was made red-hot in a furnace, then taken up 

 by pincers and inserted in its place, when the unflattened end 

 of the hot metal was hammered on the inside until another 

 head was formed, and the plates were thus securely bound 

 together. 



The Britannia Tubular Bridge, when completed, consisted of 

 four spans over the Strait, the two central spans 460 feet each 

 in length, and the two at the sides half those dimensions. A 

 representation of this structure is given in our illustrations. An 

 immense bridge of the same character was afterwards erected 

 across the St. Lawrence at Montreal. It consists of a tube 

 more than 6,500 feet long, divided into twenty-five spans, the 



