uld not be less
than 9 ft. for wagons and cavalry; for highway bridges not less than
14 ft.

Ramps at the ends of a bridge, if intended for artillery, should not
be steeper than 1 on 7. For animals, slopes steeper than 1 on 10 are
inconvenient.

If the bridges are high, hand rails should be provided. A single rope
may suffice, or it may have brush placed upon it to form a screen.

A guard rail should be provided along each side of the roadway, near
the ends of the flooring planks. In hasty bridges it may be secured by
a lashing or lashings through the planking to the stringer underneath.
Otherwise it may be fastened with spikes or bolts.

=1112. Spar bridges.=--This name is applied to bridges built of round
timbers lashed together. Intermediate points of support are provided
by inclined frames acting as struts to transmit weight from the middle
of the bridge to the banks. The single-lock and double-lock bridges
with two and three spans of 15 ft., respectively, are the ones of most
utility.

The first step in constructing a spar bridge is to measure the gap to
be bridged and select the position of the footings on either bank.
Determine the distance from each footing to the middle point of the
roadway if a single-lock, or the two corresponding points of a
double-lock bridge. Next determine and mark on each spar except the
diagonals the places where other spars cross it. The marking may be
done with chalk, or with an ax. If possible a convenient notation
should be adopted. As, for example, in marking with chalk, a ring
around the spar where the edge of the crossing spar will come, and a
diagonal cross on the part which will be hidden by the crossing spar.

A simple way to determine the length of spars is the following: Take
two small lines somewhat longer than the width of the gap, double each
and lash the bights together. Stretch them tightly across the gap so
that the lashing comes at the middle as at _A_, Fig. 8. Release one
end of each and stretch it to the footing on the same side as
indicated by the dotted lines. Mark each line at the footing _C_ or
_C'_, and at the position chosen for the abutment sill, _B_ or _B'_.
Cut the lashing and take each piece of rope to its own side. The
distances _AB_ and _AB'_ are the lengths between the transoms, and
with 2 ft. added give the length of road bearers required. The
distances _AC_ and _AC'_ are the lengths of struts from butt to top of
transom, and with 3 ft added, g