EVOLUTION OF THE MODERN RAILWAY BRIDGE. 469 



abutments (Figs. 15 and 16). There the weight comes directly- 

 down from the top of the bracing, and the lower end of the braces 

 are held in place by the masonry abutments. 



From some combination of these trusses can be constructed 

 any form of bridge, with the exception of the suspension bridge 

 and the arch. By increasing the number of panels or by com- 

 bining a number of king-post trusses (Figs. 17 and 18), we have 

 what is known as the pony truss, and for short spans one that is 

 used to a great extent on all railroads. 





PONY TRUSS. 



rif/. 17 



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PONY TRUSS. 



Fig. 18 



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FINK TRUSS. 



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BOLMAN TRUSS, Fig.20 



Every bridge is composed of two or more trusses. The ordi- 

 nary railroad bridge is composed of simply two trusses, one upon 

 each side. These trusses are connected at the top and bottom, 

 and the train can either run over the top of the bridge, or through 

 the bridge on the bottom chord. 



In the pony truss, the only distinctive feature is that the 

 trusses are not deep enough to allow of their being fastened to- 

 gether across the top, when the train runs through upon the bot- 

 tom chord, and therefore they can only be used for very short 



