Improvement in the Construction of Bridges, §c. 281 
stantly tightening of wedges and other parts; which, however, 
cannot possibly raise a bridge which has once settled, or become 
weak, from such looseness of its parts, and the consequent vibra- 
tions thereby occasioned. 
Nor is cast iron, whatever expense for it may be incurred, any 
more than a very partial remedy ; for still, the wood will both 
shrink and be, by the great pressure of the parts, compressed ; so 
that in a span of 100 feet, if made in a shape of parallelogram 
trusses, with a tie-string piece at the bottom, to prevent a horizon- 
tal thrust or pressure against abutments and piers; and, as ustal 
in bridges depending on exact and expensive execution of car- 
pentry, with king-posts at every eight or nine feet distance, and 
filled up between them, with any kind of double braces, whether 
with iron footings, or even without them; still the shrinkage 
and compression, both, of each post, must take place, and, con- 
sequently, the accumulation of shrinkage and compression of all 
the posts. Some eleven or twelve, in the hundred feet span, will 
Operate to one end, viz. to give the bridge motion, by use, and a 
_ depression to a line below its first position. By wedges, care- 
fully driven, and with the most prompt attention at all times to 
them, a part of the evil may be prevented, but by no means can 
it be fully prevented, even in sosmall aspace as 100 feet. When 
spans, however, of 150, 175, 200, and from 200 to 300 feet, are 
required, and, of course, the strength requisite for the support of 
such a span, so very much greater, while, at the same time, the 
accumulation of shrinkage and compression of timber becomes 
twice or thrice as great as in the span of 100 feet, from the fact 
that so many more such posts are necessary to its construction, it 
must be, beyond all doubt, perceived, that such constructions, 
from these disadvantages in their execution, by which they are— 
inevitably exposed to such disadvantageous frailty in the material, 
for which there is no remedy, and which the mode of using or 
combining does not provide against, or remedy in the execution, 
must be defective and of short duration. 
Hence it appears, to a certainty equal to mathematical, that, at 
No reasonable expense can there be bridges of the other modes of 
construction of wide spans, constructed with an arrangement of 
the materials, such as to admit so much pressure or strain, on 
comparatively a few points; and, at nearly all of them, the strain, 
pending so much on a pressure against the side grain of a ma- 
Vol. xxxvim, No, 2.—Jan.~March, 1840. 36 
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