ON BRIDGES. 137 



abandoned and the simple beam decided upon. The bridge is com- 

 posed of four spans, the two centre ones being 460 feet and those at 

 the ends 230 feet. The land beams, or tubes as they are called, were 

 built in their places on scaffolds raised from the ground; but the tubes 

 occupying the centre spans were built on platforms on the shore, 

 afterwards floated into position, and raised by hydraulic presses to 

 their places, one hundred and four feet above the level of the water. 



It is difficult, without a detailed description, to give an idea of the 

 magnitude and interest of this operation; but when we know that 

 each beam weighed 1,589 tons, was 470 feet in length, and had to be 

 floated during a single tide, from its original place on the shore, sev- 

 eral hundred yards to the foot of the piers, brought nicely into a 

 niche in the inasonry while still on the floating pontoons which sup- 

 ported it, and then hoisted steadily through a vertical height of 104 

 feet, we can readily imagine that the work required the greatest care 

 and the most perfect arrangements. There being two large spans to 

 the bridge and two beams to each span, for the double track, there 

 were four to be thus moved, and the successive operations were 

 watched with great interest by the large crowds that assembled to 

 witness them. 



These operations, as well as the magnitude of the w^ork, gave this 

 bridge a place in the public mind, and a w^orld-wide importance that 

 perhaps it hardly merits in an engineering point of view. 



The two large spans are 460 feet in the clear — the small ones 230. 

 The total length of the bridge is 1,511 feet, and the single line weighs 

 3.1 tons per lineal foot. "The two tubes, in their entire length and 

 their complete state, contain 9,360 tons of wrought iron, 1,015 tons of 

 cast iron, and 165 tons of permanent way. They are composed of 

 about 186,000 separate pieces of iron, pierced by seven millions of 

 holes, and united by upwards of two millions of rivets. They con- 

 tain 435,700 feet or eighty-three miles of angle-iron, and their total 

 weight is 10,540 tons." The total cosi of the bridge was about three 

 millions of dollars. 



Elaborate preliminary experiments, costing over twenty-five thou- 

 sand dollars, were made to determine the proportions of the beams, 

 by Mr. Fairbairn, to whom much of the credit is due. 



The history of this bridge and its construction, in two beautiful 

 volumes, with elaborate plates, by Edwin Clark, will always be 

 considered a classical work both by the practical engineer and the 

 student of science. 



The other structure to which I referred is the celebrated bridge 

 built over the Niagara river for the railroad passing through New 

 York and Canada, by John A. Roebling, C. E. The difficulties here 

 to be overcome were the crossing of a chasm 800 feet wide and 245 

 feet deep, at the bottom of which was a deep river with a furious 

 current, in which no boat could live or no scaffolding could be fixed; 

 of course no central piers could be used, and a single clear span was 

 required. 



A carriage-bridge had been constructed at this point, which is 



