282 Improvement in the Construction of Bridges, 6^C. 



terial so frail, and so certain to be operated upon by two such for- 

 midable evils as shrinkage and compression, and that, too, in the 

 accumulated quantity of from twenty to thirty posts, and twice 

 or thrice the number of braces, all of which also admit of the 

 same evil, to a very considerable extent. Time has shown, and 

 will in future show, the truth of these observations, to such an 

 extent as will fully remedy the evil. 



The original mode of using the arch, by Burr, Wernwag, Field, 

 and many others, it must be admitted, had the very important 

 advantage of sustaining the most important portions of the strain, 

 in the direction of the length of the materials, as in the arch- 

 pieces, which, indeed, were the main support of the structure. 

 In these constructions, in which the arch is so conspicuous for 

 the strength and beauty of the superstructure, (for beautiful, it 

 must be admitted the arch is, when applied with good taste,) there 

 seem to be evils too great to be overcome, by the most profound 

 science, or the most refined practical experience in execution. 

 Some of which are — 



1st. The great expense of construction, too great, by far, to be 

 incurred, except at a iew points, where the great importance of 

 the work, and a command of great wealth can be united, 



2d. The great horizontal thrust against abutments and piers, 

 requires great expense in its construction ; and even then, when 

 an accident destroys one arch, the others, by their own gravity, 

 destroy not only themselves, but their piers also, to any length 

 to which the bridge may extend. In bridges of many arches, 

 therefore, it would be fearfully imprudent to construct them in 

 this manner, even if means might be had for the purpose. Two 

 bridges of this kind were erected over the Schuylkill, at Phila- 

 del^.hia, many years ago; one of three arches, the other of one 

 arch ; and although so short, each one cost a very large sum to 

 its proprietors. A third, for the Western Raih'oad, was erected 

 six or seven years ago, a short distance above the other two, on 

 fine stotje piers of solid masonry, laid in coffer-dams. It has five 

 or six arches, but in their construction the more modern mode of 

 attempting to add what is tended a tie, to the arches forming a 

 level road-way, and at the same time, relieving the arches from 

 the horizontal thrust or pressure against abutments and piers. 

 This mode has recently been much practised, but it is very ques- 

 tionable whether, in many instances, this kind of tie for the 



