of ideas, gleaned from the various works of
standard authorities, and modified by the author's practice, embodied
in book form. To give a correct list of all the books consulted would
be simply impossible;--but it is well to state that the Hand-book of
Railroad Construction, by Prof. G.L. Vose, under whom the author
served as an Engineer, has been used as authority in many cases where
there has been a difference of opinions among other authors. Some
parts have been quoted entirely; but due credit has been given, it is
believed, wherever such is the case.

It is not claimed that this little work covers the whole ground, but
it is intended to describe, and explain thoroughly, three or four of
the more prominent styles of Truss, leaving the other forms of Wooden
Bridges to a subsequent volume.

Abutments and Piers, as well as Box and Arch Culverts, belonging more
properly to masonry, will be treated of hereafter under that head.

Iron Bridges form a distinct class, and may be mentioned separately at
some future period.

If this small volume should lead the student of Engineering to examine
carefully the best Bridges of modern practice, and study the larger
scientific works on this art, the author will feel satisfied that his
efforts have not been entirely in vain.

_Cambridge, February 23, 1874._

TOWER'S

Modern American Bridge Building.

BRIDGE BUILDING

The simplest bridge that can be built, is a single beam, or stick of
timber, spanning the opening between the abutments--but this is only
of very limited application--(only for spans of 20 feet and less)
owing to the rapid increase in sectional dimensions which is required
as the span becomes greater.

Next comes the single beam supported by an inclined piece from each
abutment meeting each other at the middle point of the under side of
the beam--or, another arrangement, of two braces footing securely on
the beam and meeting at a point above the middle point of the beam,
which is suspended from the apex of the triangle formed by them, by
means of an iron rod--These arrangements may be used up to 50 feet.
For any span beyond 50 feet, modifications of this arrangement are
used which will be described hereafter. Now let us investigate shortly
the different strains that the various parts of a bridge have to
bear--and the strength of the materials used. The theory of strains in
bridge trusses is merely that of the Composition and Resolution of
Forc