14 ANNUAL OF SCIENTIFIC DISCOVERY. 



of wire composing the 1 2 cables is 4,950 tons, so that they will in an 

 ordinary state of the bridge be capable of supporting five times the 

 strain which they are actually called upon to bear ; and when the 

 platform is rilled with loaded teams they will have the power of resist- 

 ing three times the strain produced by the bridge itself, and three times 

 the additional strain produced by the teams. The anchorage of the 

 bridge is formed on the Wheeling side by very heavy anchoring-irons, 

 which are imbedded in the earth, and surrounded on all sides by a 

 ponderous pile of massive masonry. On the island side, continuous 

 links of wrought iron are imbedded in the massive wing walls, so that 

 there need be no apprehension of a failure in this portion of the struc- 

 ture. This bridge was built by a joint stock company, who have 

 a charter from the State of Virginia, and the engineer is Charles 

 Ellet, Jr. 



IRON-ARCHED BRIDGE ON THE PENNSYLVANIA CENTRAL 



RAILROAD. 



THE chief peculiarity of this bridge consists in its iron-arch, which 

 is extended to a very considerable span, and furnishes a highly impor- 

 tant test of the powers of resistance, both of the material itself and of 

 the particular form in which it is used. At the same time it is per- 

 fectly safe, for if the arch fails, the truss without is sufficient to sus- 

 tain any weight that can come upon the bridge. The general arrange- 

 ment of the truss is that of the well-known Howe Bridge. The arch 

 is constructed of a centre rib of cast iron, 7 inches deep, with upper 

 and lower horizontal Ranches, 5 inches wide ; two rolled iron plates are 

 placed on the top and two on the bottom of the cast rib, breaking- 

 joint with the rib and with each other, artd secured by clamps at prop- 

 er intervals. Below the chords are solid cast-iron skew-backs, and 

 castings of suitable form to connect with the skew-back and to receive 

 the ends of the arch are placed on the top of the lower chord. As it 

 was believed that the failure of cast-iron bridges generally results from 

 the inequality of pressure upon the joints, they were separated to the 

 distance of one fourth of an inch, and spelter was poured into them in 

 a melted state. The castings were made with inch holes near the 

 ends, through which rods were passed to assist in raising them. The 

 most important advantage to be derived from the peculiar arrangement 

 exhibited in this structure was the practical test of the power of re- 

 sistance of a counter-braced iron arch on a large scale. The counter- 

 braces being placed above the arch, and resting against it by adjusting 

 or set screws, and there being at short distances vertical posts of oak, 

 also terminating in a set screw resting on the arch, it will be readily 

 perceived, that, by loosening the counter-brace screws, and tightening 

 those on the posts, the bridge will be raised upon the arch, so that the 

 latter will bear the whole weight of the truss and its load. This ex- 

 periment has thus far proved entirely successful, and shows that the 

 counter-braced arch, which is the lightest and cheapest system possi- 

 ble, is also perfectly reliable for spans of any magnitude. In the 

 Franklin Institute Journal for September, from which we abridge this 



