The propelling power of the hydraulic wheel is obtained from the force and volume of the column of water ejected by the wheel from the discharge pipes, on a principle that a gun recoils on being discharged, but with this difference, that the recoil from the water-wheel is continuous. If the column of water were discharged towards the stern the vessel moved forward, and if towards the stem it moved in the other direction; if discharged in both directions the vessel remained stationary, and if discharged forward on one side and towards the stern on the other, the vessel turned either on her centre as on a pivot, or if the pressure were greater in one direction than in the other, in a circle the size of which depended on the pressure of the discharge from either set of nozzles. No reversing of the engines or of the hydraulic wheel was required under any circumstances, the direction and force of the discharge being regulated by a series of valves. The hydraulic wheel was fixed immediately over the sluice valves and water-box, and revolved in a cast-iron circular case 19 feet in diameter. The wheel was itself 14 feet 6 inches in diameter and weighed eight tons, and was fitted with eleven vertical or radial arms and blades. The engines were of 160 nominal horse-power, and steam was supplied by two ordinary tubular boilers. At her trial the _Waterwitch_ covered the measured mile in Long Reach in 6 minutes 20 seconds. At other trials later in the day she averaged 9 knots. The shape of the vessel and the fact that she could be steered in either direction with equal facility were of undoubted advantage from the point of view of manœuvring, but the trials can hardly be called successful so much as experimental, as it was ascertained that she would probably have done better had her nozzles been differently placed and provision made for altering the size of the nozzles according to the speed at which the vessel was required to travel. The machinery itself, however, worked beautifully. The Government ordered a number of comparative tests to be made in which the efficacy of the _Waterwitch_ method could be judged against that of the double-screw system installed in the gunboats _Viper_ and _Vixen_, all three vessels being of the same size. The two gunboats were not the best of their kind as they had double sternposts with a cavernous recess between them and flat overhanging sterns. Mr. M. W. Ruthven, son of the inventor of the system, it being under his father’s patent that the _Waterwitch_ machine was built, in addressing the Institute of Marine Engineers a few years ago, said: