898 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



erection of a proposed lattice bridge over the Frith of Forth, 

 between Blackness and Charleston, about 14 miles from Edin- 

 burgh. This bridge is to be in four spans of 500 feet each, and 

 64 feet deep, and 125 feet above high water. Although I hare 

 not had the good fortune to meet with any more than a meagre 

 description of this bridge, I apprehend that the lattice system is 

 not alone relied on, but that chains or tension l)ars are introduced 

 to take the load, the lattice system bein<]^ merely a counter-brace 

 to the chains and a support to the roadwa}^ This of course is 

 merely a surmise on my part, for without some such arrangement, 

 I do not see how the tremendous eifort to buckle the lattice bars 

 towards the ends of the truss can be counteracted, without im- 

 mense weight being added in the form of gussets, angle irons and 

 streets. I think I can give you in a few words an idea of the 

 maximum strains on a trussed beam, when the weights are carried 

 from panel to panel. The strength of any beam is proportional 

 to the square of its depth, and there is one stage in varying the 

 depth of a truss, in which the maximum compressive strains in the 

 top chord, and the tensile strains in the liottom chord, are exactly 

 equal to the greatest w^eight of the truss when loaded. And that 

 takes place when the depth is equal to one-eighth the span. Let 

 us take a pair of girders of 650 feet span, one-eighth of which would 

 be 81| feet. Such girders would weigh, together with their 

 greatest varia])le load (allowing 20 feet for roadway), about 3,000 

 tons, which measures the compression exerted in the upper chord, 

 or tension in the loAver; half of Avhich, or 1,500 tons, must be 

 sustained by the end diagonals and verticals. Without unAvieldly 

 proportion of parts, the resistance to flexure of verticals 81^ feet 

 long would be very small, when an enormous duty, as you see, 

 devolves upon. them. 



Suspension Bridges. 

 "VVe come now to consider the elements and principles of the 

 suspension system; a system every one naturally turns to when the 

 problem that called forth this paper is presented for solution because 

 of its adaptation to long spans, its ease of construction, doing away 

 entirely with false works, and the grace and beauty of its appeaiJ- 

 ance. Now, in itself, a suspension bridge is only immensely strong 

 ■when simply regarded as supporting a dead weight — it has no 

 element of stiifness whatever. The slightest unequal loading wall 

 throw it out of equilibrium and produce a tremor in all its parts, 



