is used for both temporary and permanent works. Plate IV, Figs. 1, 2,
3 and 4, give some of the best varieties in use. Figs. 1 and 2, may be
used up to 15 feet in height; Fig. 4, up to 20 feet; and Fig. 3, to 30
ft. The distance apart of the various bents should not exceed 10 or 12
ft., unless bracing is introduced between them, and the bents should
always be raised above the ground a few feet on a solid masonry
foundation. Want of space forbids any mention of abutments and piers,
which really come more properly under the head of masonry.

Iron Bridging is gradually working its way into favor, and Will
probably eventually supersede wooden trusses;--but in many cases wood
is the only material at hand--and therefore some knowledge of Wooden
Bridging is desirable. It is intended to follow this pamphlet with a
portfolio of sheets containing working drawings of several kinds of
Wooden Bridges, taken from actual measurements of some of the best
specimens of the different styles of Truss in use.

* * * * *

=PRACTICAL NOTES.=

When putting a truss together in its proper position, on the
abutments, 'false works' must first be erected to support the parts
until they are so joined together as to form a complete
self-sustaining truss. The bottom chords are first laid as level as
possible on the false works, then the top chords are raised on
temporary supports, sustained by those of the lower chord, and are
placed a few inches higher at first than their proper position, in
order that the web members may be slipped into place. When this is
done the top chords are gradually lowered into place. The screws are
then gradually tightened, (beginning at the centre and working towards
both ends,) to bring the surfaces of the joints into proper contact,
and by this method, the camber forms itself, and lifts the lower
chords clear of the false works, leaving the truss resting only upon
its proper supports. The subjoined Table will be found useful in
estimating the strains on a truss when proportioning a bridge for any
moving load.

Table of weights per running foot of a bridge, (either of wood or
iron,) including weights of floor, lateral bracing, &c., complete, for
a single track.

Clear Weight of
Span. Bridge.
Tons. lbs.

25 .266 596
30 .281 629
40 .313 701
50 .343 768
60 .374 838
70 .404 905