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 and delivering it on the marsh at the back of the bank of the river Thames at Crossness, Kent. A stage constructed of wooden piles commanded a series of barge beds, and the unloading dredger running from end to end of the stage, lifted and delivered the materials on the marsh behind the river wall at the cost of 1 d. per cub. yd.