138 LECTURE 



about a mile and a half below the falls, by Charles Ellet,* in 1848, 

 and in 1852, it having been decided to carry the railroad across the 

 river, the large bridge was commenced at the same locality, by Mr. 

 Roebling. The fact that the English engineers had decided that sus- 

 pension bridges \vere impracticable for railroad purposes, made the 

 proposition to construct one at this place a bold enterprise, and 

 great praise is due not only to the genius and skill of the engineer, 

 but also to the president and directors of the road to whose liberal 

 vieAvs the engineering profession is indebted for this grand experi- 

 ment. 



Suspension bridges for ordinary traffic were in common use in Eu- 

 rope and in this country, and the well-known one at Fribourg, with a 

 span of 807 feet and an elevation of 167 feet above the water, built 

 in 1830, had remained in good condition after more than twenty years 

 of use; but the doubt existed whether such a structure could be made 

 stiff enough to resist the undulations caused by a moving weight as 

 heavy as that of a locomotive and a loaded train of cars. 



The cables of the Niagara bridge are four in number, each ten 

 inches in diameter, and made up of 3,640 iron wires of No. 9 gauge, 

 anchored firmly into the solid rock about 230 feet from the edge of 

 the cliff, and passing over massive stone towers 76 feet in height, and 

 from these the roadway is suspended by wire ropes. Iron drawn into 

 the form of wire is known to have its strength greatly increased in 

 proportion to its weight, and, therefore, it can be much more econo- 

 mically employed than in the form of rods or chains. 



Were the roadway merely made strong enough to support the load 

 to be brought upon it, the bridge would be depressed, owing to the 

 flexibilit)^ of the cables at the point occupied by the load, and Avould 

 rise at other points, inducing an oscillation that would soon be destruc- 

 tive to the bridge, and the roadway is therefore trussed or framed 

 after the manner of a timber bridge, so as to resist any local depres- 

 sion and to distribute the weight over a great length of the cable, 

 and thus prevent such oscillation. 



In addition to that, a number of wire stays run from the top of the 

 tower to points in the truss, on the principle shown in figure 19, which 

 fix those points and thus add materially to the stiffness. Again, the 

 track itself rests upon longitudinal girders of great strength, which 

 serve to distribute the immediate load to more distant parts of the 

 truss. These are briefly the points upon which the stiffness of the 

 bridge depends. The railroad is placed on the upper floor of the 

 truss, while the lower is used for carriages and foot passengers. 



It was held by English engineers that a -truss which would give 

 sufficient stiffness to the cables would be strong enough to sustain it- 

 self and the load, or that the cables would be useless, but this is 

 plainly disproved both by calculation and by the practical result of 

 the Niagara bridge, in which the truss is exceedingly light, and when 

 it serves only the legitimate purpose of stiffening, while the weight 



* While these pages are going througb the press our country is called upon to moiira 

 the untimely death of this eminent engineer, who, with geneious patriotism, devoted his 

 talents and his life to the defence of the Union. 



