Page:Popular Science Monthly Volume 36.djvu/485

Rh 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 combining 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.



Every bridge is composed of two or more trusses. The ordinary 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 together across the top, when the train runs through upon the bottom chord, and therefore they can only be used for very short