any great extent, upon the railways of this country. This bridge is called the Post bridge (Fig. 24), taking its name from its inventor. The characteristic features are that the compression members are inclined at what is claimed to be the most economical angle—that is, the most economical in regard to the amount of strength obtained for the amount of material used. They are
so inclined as to cross one panel of the bridge, while the tie-rods, running at an angle with the braces, cross two panels. This is the only advantage that can be claimed for this form of truss, and much of this so-called advantage is more than counterbalanced by some of the difficulties encountered in the actual construction; and whether the bridge really in itself is a more economical bridge than the Pratt, yet remains to be practically proved.
We stated in the beginning that bridges consist of arches and suspension bridges as well as framed trusses. The relation between
the framed truss and the arch will be readily seen by an examination of Fig. 25. Take a truss of the Pratt pattern; then, in place of having the top chord parallel with the lower chord, let the compression members be increased in length, as shown in the drawing, and the top chord take the form of an arch, and we have the bow-string girder. The ends of the arch on each side are simply held together by means of the lower chord, which acts as a tie-rod upon them.
