EVOLUTION OF THE MODERN RAILWAY BRIDGE. 479 



different parts of the bridge while it is being put together. This 

 is a great expense ; and in some cases, where the bridge is far 

 above the water, the current very rapid, or an existing necessity 

 of not obstructing the water-way, becomes impossible. In the 

 erection of a cantilever, each cantilever span is balanced on its 

 own pier and built out each side from the tower in such a manner 

 as to preserve this balance until the shore ends are anchored 

 firmly, after which the river ends can be extended as far as de- 

 sired. The suspended span, which is never of extraordinary 

 length, can usually be built directly from the ends of the can- 

 tilever spans, and the necessity of false work entirely done away 

 with. 



In all bridges of long span that the weight of the bridge itself 

 is by far the greatest load that the bridge has to bear. In bridges 

 of short spans, the weight of the locomotive and train coming 

 upon them suddenly constitutes the greatest load, as the weight 

 of the train is greater than the weight of the bridge itself ; but as 

 the size of the bridge increases, its weight increases very rapid- 

 ly, and the weight of the locomotive and train becomes almost 

 nothing as compared with the weight of the bridge itself ; that is, 

 if any of these long bridges are built strong enough to hold them- 

 selves up, with a very slight margin of safety above that, there is 

 scarcely any danger of their ever falling from any weight that 

 could come upon them from an outside load. For this reason, in 

 building short-span bridges, the amount of economy that can be 

 exercised in the use of material is very small, as the bridge must 

 be built stiff and rigid, even if this necessitates the using of much 

 more material than the absolute weight of the tender and locomo- 

 tive that come upon it would demand. For this reason plate gird- 

 ers or flanged girders have many advantages connected with their 

 use for short-span bridges, because the whole amount of material 

 used is comparatively of little value, and extreme stiffness and 

 rigidity are the result ; while in the case of long-span bridges, such 

 as cantilever, or any remarkably long bridge, every calculation 

 must be made with the greatest care in order to reduce the amount 

 of necessary material as much as possible, because by reducing the 

 amount of material used the weight is reduced, and that again 

 reduces the amount of material. The factor of safety to be used 

 can be only two or three in long-span bridges, while in short spans 

 it should run up even to ten. 



Although cantilever bridges are of rather recent use in this 

 country and in Europe, and much has been written claiming them 

 as an invention of modern times, still, the same can be said of them 

 that was said of suspension bridges that there at present exists 

 in Japan, built by the order of the Mikado two hundred and fifty 

 years ago, as perfect and scientific a cantilever bridge as any that 



