A form of bridge that has been used to some extent on the Baltimore
and Ohio Railroad, by Mr. Latrobe, is the Arch Brace Truss. In this
form of Truss the braces lead directly from the abutments to the head
of each vertical; thus the load is transferred at once to the
abutments, without passing through a series of web members. The
counterbracing is effected by means of a light lattice,--and is
applied to both sides of the chords, and the intersections of the
diagonals are fastened while the bridge is strained by a load--thus
preventing recoil--so that the effect of a moving load is to lighten
the strain on the lattice--without otherwise affecting the Truss.

[Illustration: Pl. III. with Fig. 1., Fig. 2., Fig. 3., Fig. 4., Fig. 5.]

There are two models of this style of bridge, to my knowledge; one
built by Prof. G.L. Vose, on a scale of 1/2 an inch to the foot,
and representing a span of 150 feet, which supported 2,500 lbs. at
the centre, and a movable load of 150 lbs., proving itself to be
strong and rigid enough for any thing. The other, on a scale of 1
inch to the foot, and representing a span of 76 feet, was built by
the Class of '73, of the Thayer Engineering School, under the
writer's direction, and though bearing very heavy weights, has never
been thoroughly tested--it has, however, been subjected to the
sudden shock of 1040 lbs. falling 20 inches, without injury, several
times. Subjoined are the dimensions of the models mentioned.

DIMENSIONS OF A MODEL OF AN ARCH BRACE TRUSS.

G.L. VOSE.