Page:Popular Science Monthly Volume 36.djvu/492

 hangs from them. In the suspension bridge the cables, chains, or any other flexible devices are stretched between the two points of support, the ends carried over the tops of the towers, and firmly anchored in the ground beyond. Then the roadway of the bridge is simply hung by tie-rods from this suspension cable.

The suspension bridge is undoubtedly one of the oldest forms in existence. At the time when our ancestors were either swimming across creeks, or cutting trees, making them fall across in order that they might walk over on them—that is, one thousand years ago—the Japanese were building suspension bridges which are in use to-day, using iron chains for suspension cables, and in every way building them in as scientific a manner as the East River Bridge in New York is built to-day. Of course, there was a certain crudeness as to the methods which were used, but this in no way affected the scientific principles on which the bridges were built. It is not our purpose, however, in this paper to take up the question of suspension bridges.

We pass now to the last form, and in this country at least the latest form, of the framed truss—that is, the cantilever bridge. The object of the cantilever bridge is to make possible the economical construction of long, clear spans of a rigid truss, and thus do away to a great extent with the necessity of suspension bridges, as there are many disadvantages besides the mere one of expense that are connected with the use of suspension bridges. The other advantages of the cantilever will be taken up later.

To show the development of the cantilever bridge, we will take two king-post trusses (Fig. 29); putting them together, we



form a bridge of two spans, which has an abutment at each end and a pier in the center. In case this was for the passage of a river, the center pier would come directly in the center, obstructing navigation to a great extent, and otherwise prove an inconvenience. We use the king-post truss merely as the most simple form of truss that is built. In any other form that could be built the result would be the same; that is, for a bridge of two spans there would be a pier in the center of the river, and for any span that could be built of any of the types of bridges which we have noticed thus far the amount of open space that would be left in