central vertical rod. When these eight
spades are drawn up by means of chains, they form one flat table or
tray at right angles to the central rod. In operation the spades hang
vertically, and are dropped into the material to be excavated; the
chains are then drawn up, and the table thus formed holds the material
on the top, which is lifted and discharged by releasing the spade.
This apparatus has been extensively used both in Great Britain and in
India for excavating in bridge cylinders.

The clam shell dredger consists of two hinged buckets, which when
closed form one semi-cylindrical bucket. The buckets are held open by
chains attached to the top of a cross-head, and the machine is dropped
on to the top of the material to be dredged. The chains holding the
bucket open are then released, while the spears are held firmly in
position, the buckets being closed by another chain. Bull's dredger,
Gatmell's excavator, and Fouracre's dredger are modifications with
improvements of the clam shell dredger, and have all been used
successfully upon various works.

Bruce & Batho's dredger, when closed, is of hemispherical form, the
bucket being composed of three or four blades. It can be worked by
either a single chain or by means of a spear, the latter being
generally used for stiff material. The advantage of this form of
dredger bucket is that the steel points of the blades are well adapted
for penetrating hard material. Messrs Bruce & Batho also designed a
dredger consisting of one of these buckets, but worked entirely by
hydraulic power. This was made for working on the Tyne. The excavator
or dredger is fixed to the end of a beam which is actuated by two
hydraulic cylinders, one being used for raising the bucket and the
other for lowering it; the hydraulic power is supplied by the pumps in
the engine-room. The novelty in the design is the ingenious way in
which the lever in ascending draws the shoot under the bucket to
receive its contents, and draws away again as the bucket descends. The
hydraulic cylinder at the end of the beam is carried on gimbals to
allow for irregularities on the surface being dredged. The hydraulic
pressure is 700 lb. per sq. in., and the pumps are used in connexion
with a steam accumulator.

An unloading apparatus was designed by Mr A. Manning for the East &
West India Dock Co. for unloading the dredged materials out of barges