ic press so as to clear the roadway
behind, and is then rolled back.

(3) _Draw or Bascule Bridges._--The fortress draw-bridge is the original
type, in which a single leaf, or bascule, turns round a horizontal hinge at
one abutment. The bridge when closed is supported on abutments at each end.
It is raised by chains and counterweights. A more common type is a bridge
with two leaves or bascules, one hinged at each abutment. When closed [v.04
p.0544] the bascules are locked at the centre (see fig. 13). In these
bridges each bascule is prolonged backwards beyond the hinge so as to
balance at the hinge, the prolongation sinking into the piers when the
bridge is opened.

(4) _Swing or Turning Bridges._--The largest movable bridges revolve about
a vertical axis. The bridge is carried on a circular base plate with a
central pivot and a circular track for a live ring and conical rollers. A
circular revolving platform rests on the pivot and rollers. A toothed arc
fixed to the revolving platform or to the live ring serves to give motion
to the bridge. The main girders rest on the revolving platform, and the
ends of the bridge are circular arcs fitting the fixed roadway. Three
arrangements are found: (a) the axis of rotation is on a pier at the centre
of the river and the bridge is equal armed (fig. 33 c), so that two
navigation passages are opened simultaneously. (b) The axis of rotation is
on one abutment, and the bridge is then usually unequal armed (fig. 33 d),
the shorter arm being over the land. (c) In some small bridges the shorter
arm is vertical and the bridge turns on a kind of vertical crane post at
the abutment (fig. 33 e).

(5) _Floating Bridges_, the roadway being carried on pontoons moored in the
stream.

The movable bridge in its closed position must be proportioned like a fixed
bridge, but it has also other conditions to fulfil. If it revolves about a
vertical axis its centre of gravity must always lie in that axis; if it
rolls the centre of gravity must always lie over the abutment. It must have
strength to support safely its own overhanging weight when moving.

At Konigsberg there is a road bridge of two fixed spans of 39 ft., and a
central span of 60 ft. between bearings, or 41 ft. clear, with balanced
bascules over the centre span. Each bascule consists of two main girders
with cross girders and stringers. The main girders are hung at each side on
a horizontal shaft 8-5/8 in. in diameter, and are 6 ft. dee