PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 897 



quickly after passing 150 feet, more rapidly, however, in the 

 latter than in the former, as the numerous long rods cause great 

 vibration. Some Fink bridges of 200 feet span, have been built 

 on the line of the Baltimore and Ohio railroad, Avhich, I believe, 

 give satisfaction. The Bollman bridge is not a stift' bridge at all, 

 nor can it be made so, for even take a span of fifty feet, the ten- 

 sion rods would be over fifty feet long and at a very flat angle, 

 rendering it impossible to make them taut, without bringing au 

 undue strain upon them. Both these bridges have considerable 

 merit, and although they cannot be looked upon, as you readily 

 perceive, for the great spans required over the East river, this 

 paper would be incomplete without noticing them. This same, 

 remark applies to the triangular truss so much in vogue in Eng- 

 land, and represented in this diagram (fig. 4). 



It is the simplest of all trusses, and exceedingly easy in its 

 computations. The Newark Dyke bridge (England) is the longest 

 span ever erected on this principle, and is 240 feet, on a skew of 

 ninety-seven feet. The proposed International bridge over the 

 Niagara river, at Buffalo, upon which I was engaged while engi- 

 neer of bridges for the A. & G. W. Railway, was to be in spans 

 of 250 feet, and to be built entirely of wrought iron (boiler plate). 



The streets of the I 1 section made from plates and anglcirons 



melted together, the ties of plates merely, and the chords were 



to be box shaped, thus: 



r Tr . 



This system of truss is open to the 



same objections to great spans that was mentioned in connection 

 with the Whipple bridge, viz. : the transmission of weights, 

 always increasing, by a series of triangles to the points of sup- 

 port. The Lattice bridge, shown below (for which America must 

 get credit, as the system was used in timber bridges long l>efore 

 Sir John MacNeil thought of introducing it on the other side, in 

 wrought iron), comes properly under the head of trussed girders, 

 the diagonals being multiplied, but acting in the same way as 

 before described (fig. 5). 



In addition to the difficulties in adapting this description of 

 truss to long spans, the lattice arrangement is very deficient in 

 vertical strength, which becomes very markedly so towards the 

 abutments, requiring various stiffening arrangements to keep the 

 chords in their proper position. There have been some bold 

 examples on this principle, which bid fair to be outstript by the 



[Am. Ixst.] EEE 



