254
DOCK ENGINEERING.
stone should be of a very hard and durable quality. Granite is almost
invariably used, but greenheart timber has also been employed with, it is
stated, most satisfactory results. The wear of the heelpost is said to be
less, and the cost of dressing the surface of the quoin much reduced.*
At the base of the hollow quoin is situated the foundation stone to
receive the gate-pivot casting.
In cases where chains are used for manæuvring the gates, it will be
advisable to attach a check chain from the top of the mitre post to a volute
or other spring fixed in the neighbourhood of the square quoin, to avoid
violent impact against the sill.
Recesses for sliding and rolling caissons (fig. 179) åre usually rectangular
chambers constructed normally to the axis of the passage. They require to
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Fig. 179. —Caisson Recess at Greenock.
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be slightly longer than the width of waterway, and to be slightly wider than
the caisson itself. In some cases sufficient room is left between the caisson
and the side wall of the chamber to allow of men conveniently effecting
repairs to the caisson should such be necessary, the chamber being rendered
watertight, temporarily, by timber dams. A strong covering is expedient
on account of the traffic overhead.
4. The side walls of a lock are preferably constructed without any batter
on the face. With the water at widely varying levels there would be a
danger of two vessels, locking outwards side by side, nipping each other
unless the walls were plumb. Where a ship caisson is used for closing
the entrance, and, for that purpose, is floated into grooves in the walls, a
slight batter is inevitable, but the method is unusual for locks and the
contingency remote.
* Moncrieff on “ Dock Gates of Iron and Steel,” Min, Proc. Inst. C.E., vol. oxvii.