EVOLUTION OF THE MODERN RAILWAY BRIDGE. 475 



Now, in the case of a masonry arch (Fig. 26), which has the 

 weight all on top, there is no necessity for the tie-rod to hold the 

 ends of the arch together, for the reason that the ends of the arch 

 are always built so as to abut against heavy masonry which will 

 withstand the horizontal thrust, and thus without the interven- 

 tion of any tie members we have a perfect bridge by means of the 



arch, all the weight coming upon the top being passed from one 

 stone to the other in the arch until it reaches the two abutments 

 the same as in a framed truss. 



The suspension bridge (Fig. 28) is nothing more or less than 

 the arch turned upside down. In the arch, as we have seen, the 



SUSPENSION BRIDGE. 

 Fig. 28 



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(A ' / 







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only strain that comes upon it is compression ; in the suspension 

 bridge, on the other hand, the only strain that comes upon the 

 sustaining member or cable which is stretched between the points 

 of support is tension. In the arch bridge all the weight comes 

 above the arch and presses down upon it ; in the suspension bridge 

 all the weight comes below the suspension chains and simply 



