each one of the pair.

Span. Rise. Stringer. Post. Rod. Rods.
Feet. In Feet. (single.) (double.)

15 1-7/8 12 x 12 6 x 8 2-1/8 diam. or 1-1/2 diam.
20 2-1/2 12 x 14 7 x 8 2-1/2 " 1-3/4 "
25 3-1/8 12 x 16 8 x 8 2-3/4 " 2 "
30 3-3/4 13 x 18 9 x 9 3 " 2-1/8 "

It is as well to tenon the post into the beam, and also strap it
firmly with iron plates--and the end should be shod with iron to form
a saddle for the rods to bear upon.

Now if we should make a bridge, on the plan of Fig. 3, Pl. I., 75 or
100 feet, or perhaps more, in length, the braces A F and F C, would
not only be very long but very large and heavy, and one chief
requisite in a good bridge is, to have all the beams so proportioned
that they will resist all the strains acting upon them, without being
unnecessarily large. It now becomes necessary to have a different
arrangement of the parts of the truss in order to obtain increased
length of span.

Suppose we have a span, of 40 feet, as represented in Fig 2, Pl. I.
Now instead of running the braces from A C until they meet in a point,
as before we stop them at a, and c, and place the straining beam, a c,
between them to prevent those points from approaching, suspend the
points B and D from them, and start the braces B b and D b--and, if
the truss were longer, would continue on in the same manner as far as
needful. To prevent the. truss from altering its form, as shown by the