4
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LIFT BRIDGE OVER THE OURCQ CANAL.
This bridge, which was inaugurated in 1868, was constructed under the direction of Mr. Mantion, then engineer-in-chief of the Belt Railway. Fig. 1 shows the bridge raised.
The solution adopted in this case was the only feasible one that presented itself, in view of the slight difference between the level of the railway tracks and the maximum plane of the canal water. This circumstance did not even permit of a thought of an ordinary revolving bridge, since this, on a space of 10 inches being reserved between the level of the water and the bottom of the bridge, and on giving the latter a minimum thickness of 33 inches up to the level of the rails, would have required the introduction into the profile of the railroad of approaches of at least one-quarter inch gradient, that would have interfered with operations at the station close by.
[Illustration: FIG. 1.--LIFT BRIDGE OVER THE OURCQ CANAL.]
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
Scientific American Supplement, No. 497 (July 11, 1885), page 3
by Various