The tension at the centre of the Lower Chord is found by _dividing the_
_product of the weight of the whole bridge and load by the span_, by
sight times the height--or letting T=tension in lbs., W=weight of
bridge and load in lbs., S=span in feet, and h=rise or height--we have

W x S
T = ----- --.
8 h

[TeX: $T = \frac{W \times S}{8 h}$]

In this case we have taken the rise at 1/8 of the span, which is
evidently the best ratio between those dimensions, as it equalizes
the vertical and horizontal forces. As to the proportions of the
_bays_ or _panels_, (or that portion of the truss bounded by two
adjacent verticals, as struts or ties, and the chords,) the ratio of
the rise (or the vertical distance between the centre lines of the two
chords,) and the length on the chord should be such, that the diagonal
truss members may make an angle of about 50° with the chords; as the
size of the timbers is increased by decreasing the angle, and, if the
angle is increased, there are more timbers required.

Mr. G.L. Vose, in his admirable work on R.R. Construction, observes
very truly that "The braces, at the end of a long span, may be nearer
the vertical than those near the centre, as they have more work to do.
If the end panel be made twice as high as long, and the centre panel
square, the intermediates varying as their distance from the end, a
good architectural effect is produced."