Besides, in the case of a revolving bridge, since the bottom of the
latter would be but ten inches above the water level, and the rollers
would have to be of larger diameter than that, it would have been
necessary to suppose the roller channel placed beneath the level of the
water, and it would consequently have been necessary to isolate this
channel from the canal by a tight wall. The least fissure in the latter
would have inundated the channel.

As the Ourcq Canal had no regular period of closing, it was necessary
to construct the bridge without hinderance to navigation. The idea of
altering the canal's course could not be thought of, for the proximity
of the fortifications and of the bridge over the military road was
opposed to it. Moreover, the canal administration insisted upon a free
width of 26 feet, which is that of the sluices of the St. Denis Canal,
and which would have led to the projection of a revolving bridge of 28
feet actual opening in order to permit of building foundations with
caissons in such a way as to leave a passageway of 26 feet during
operations.

For these reasons it was decided to construct a metallic bridge that
should be lifted by means of counterpoises and balanced after the manner
of gasometers.

The free width secured to navigation is 28 feet. The bridge is usually
kept raised to a height of 16 feet above the level of the water in order
to allow boats to pass (Fig. 2). In this position it is balanced by four
counterpoises suspended from the extremities of chains that pass over
pulleys. These counterpoises are of cast iron, and weigh, altogether,
44,000 pounds--the weight of the bridge to be balanced, say 11,000
pounds per counterpoise. Moreover, each of the four chains is prolonged