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THE POPULAR SCIENCE MONTHLY. 



hangs from thera. 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 swim- 

 ming 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 eco- 

 nomical 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, obstruct- 

 ing navigation to a great extent, and otherwise prove an incon- 

 venience. 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 



