rods, each will sustain a strain of 67200 lbs.--and, at 15,000 lbs.
per square inch, will have an area of 4.48 sq. inches, and, by Vose's
Tables, must have a diameter of 2-1/2 inches. The sizes of the rods in
each set will decrease towards the centre of the bridge as the weight
becomes less.

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


=Counterbraces.= Now, as to the necessity of Counterbracing, there are
various opinions. The object of it is to stiffen the truss and check
vibrations. If a load be placed over any panel point, it causes that
portion of the truss to sink, and produces an elevation of the
corresponding panel point at the other end of the truss--thus
producing a distortion, which change of form is resisted by proper
counterbraces. The strain to which this timber is subjected is caused
by the moving load on one panel only--and requires only scantling of
the size of the middle braces. These counterbraces should not be
pinned or bolted to the braces where the cross--as their action is
thereby entirely altered--but it is well to so confine them as to
prevent vertical or lateral motion.


=Shoes.= Formerly it was the custom to foot the braces and counters on
hard wood blocks on one side of the chord, the vertical rods passing
through and screwing against a block on the other side--thus the whole
strain tended to crush the chord across its fibres. This is now
remedied by the use of cast iron blocks, bearing on one side of the
chord, but having tubes extending through to the other side, where the
washer plate for the bolts fits firmly on their ends, forming a