282 Improvement in the Construction of Bridges, §e. 
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 Iength 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 profoun 
science, or the most refined practical experience in execution. 
Some of which are— 
Ist. The great expense of construction, too great, by far, to be 
incurred, except at a few points, where the great importance of 
the work, and a command of great wealth can be united. 
> 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. / W0 
bridges of this kind were erected over the Schuylkill, at Phila 
delphia, many years ago; one of three arches, the other of one 
arch; and although so short, each one costa very large stm 0 
its proprietors. A third, for the Western Railroad, was erect 
six or seven years ago, a short distance above the other two, 0P 
fine stone 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 termed a tie, to the arches forming 4 
vel road-way, and at the same time, relieving the arches foo 
the horizontal thrust or pressure against abutments aud piers 
This mode has recently been much practised, but it is very ques 
tionable whether, in many instances, this kind of tie for 
