In consequence of this report the Admiralty ordered the _Waterwitch_ to be built for the trial of the Ruthven propeller. In order to check the vessel by a comparison with one fitted with the ordinary screw, the trials of the armour-plated twin-screw gun-vessel _Viper_ were selected. The _Waterwitch_ was of iron, 778 tons measurement, 162 feet in length by 32 feet beam, and 13 feet 9 inches depth; she was rather broad in proportion to her length, and had a rudder at both ends. Her armour plating was of the usual 4½ inches in thickness at the water-line and on her broadside, and she had athwartship armoured bulkheads across her upper deck with gunports in them, through which it was proposed to fight guns on the line of keel. The main interest of the vessel centred in her machinery. Her first trials on the Thames were so satisfactory that a more extended series at Stokes Bay was determined upon, the two sets of trials lasting about a year. A portion of her bottom was made flat and without external keel. From what may be regarded as a semi-official and certainly expert report of the ship and the Stokes Bay trials, the following description of the vessel may be quoted:-- “In the fore part of this flat surface, in a space about 12 feet square, are one hundred and forty-four perforations 12 inches each in length, and cut laterally through the bottom plates of the vessel, the plate being bent inwards on each side of the cuts to a central depth of about 3 inches. Through these perforations the water on which the vessel is floating finds admission to an oblong iron box, fixed longitudinally and parallel with the vessel’s keelson, closed when the vessel is not under steam by four sluice valves, each having an opening of 2 feet 10½ inches by 1 foot 11½ inches. With the vessel under steam these valves are open, and give further admission to the water to the watertight cast-iron casing, in which is fixed, on a vertical axis, the turbine. This wheel is 14 feet 4 inches in diameter at the bottom plate, and 14 feet at the top. It has twelve blades, and these, with the top and bottom plates, are made of boiler plate about ⅜-inch thick, vertical at the periphery and with the lower edge gradually twisted from near the circumference towards the centre in the direction of its motion. From the cast-iron casing which encloses the wheel, branch off laterally copper pipes, which convey the water from the wheel, or centrifugal pump as it might not inaptly be called, to the discharge pipes and ejection nozzles on the outside of the ship. By this arrangement, therefore, the water which enters through the perforated bottom of the ship passes by way of the sluice valves into the wheel casing, and thence, by the action of the wheel, through the copper conducting pipes and out into the sea again from the nozzles at the end of the discharge pipes on the outside of the ship. The wheel is driven by three cylinders fixed round it at