Improvement in the Construction of Bridges, §c. 285 
circle, so flat as to be wholly incapable of bearing so great a 
weight as that of the bridge itself, and the travel over it, which 
it would be required to sustain also. 
It is a well established fact, ascertained by practical experience, 
that a flat segment, or, which is the same, a small portion of the 
circumference of a circle or other curve, when applied to the 
arch of.a bridge, executed in wood, becomes so much exposed to 
the compression of its wood, by a thrust-strain, as to. be wholly 
inadequate to the purpose. The reason of which is founded in 
the plain mathematical principle, that as any curved arch of a 
given span, loaded with a given weight, approaches, by its low 
altitude, to a horizontal line, its exposure to the compression of 
its materials, in a thrust manner of strain, increases to an almost 
incredible degree ; so much so, that the wood, which does not 
increase in its density, or power to resist compression, but remains 
Stationary in this respect, becomes too weak and entirely insuffi- 
cient, until at last, on a near approach tothe horizontal line, even 
one tenth of its own weight could not be sustained. 
In wide spans, to raise the arch so as to give it its adequate 
power to support a bridge, would present so large a volume to 
the wind, and that too with such great leverage, as might, in- 
deed, create reasonable fears for the safety of such a construction. 
The opinions and descriptions of several eminent engineers in 
England, in their late publications on bridges and railroads, are 
here introduced. 
David Stevenson, in his sketch of the Civil Engineering of 
North America; London, John Weale, Architectural Library, 59 
High Holborn, 1838; has the following account of this mode. 
He however, did not see but a small number of those that are 
well constructed. 
“Plate 9th is a drawing of ‘'Town’s Patent Lattice Bridge,’ 
which is much employed on the American railways. This con- 
Struction is sometimes used for bridges of so large a span as 220 
feet, and it exerts no lateral thrust, tending to overturn the piers 
on which it rests. A small quantity of materials, of very small 
Scantling, arranged in the manner shown in the plates, possesses 
@ great degree of strength and rigidity. 
“For this drawing, I am indebted to Mr. Moncure Robinson, 
of Philadelphia, who is constructing many large bridges on this 
Principle, on the Philadelphia and Reading Railway, several of 
which I examined, both in their finished and unfinished state. 
