through a ball-and-socket joint at the lower end of the spindle and escapes through apertures arranged tangentially at the periphery and through an axial hole at the upper end of the spindle; the former acts by recoil, after the manner of the well-known "Hero" engine, to maintain the velocity of rotation, and the latter acts in a very ingenious manner to operate one or another of a series of hydraulic cylinders by which the " platform " (on which may be mounted a searchlight or a light gun) is held in a horizontal plane. It is well known that water, escaping by an orifice, is capable of generating a pressure little short of that by which it is impelled. Thus in close proximity with the axial nozzle four apertures are arranged, each communicating with one of four hydraulic cylinders arranged to act on the platform. The latter is mounted on trunnions to swivel universally and carries the gyroscope mounting, so that if by the rolling or pitching of the vessel the platform begins to move rotationally in any direction, the jet of water issuing from the axial nozzle passes into one or other of the orifices, and there generates pressure in whichever of the hydraulic cylinders is in a position to correct the motion of the platform, and so maintain its constancy of direction. The platform is thus made to copy at all times the position of the gyroscope, and so partakes of its steadiness. The above form of relay is a singularly beautiful contrivance, inasmuch as it acts without any contact, frictional or otherwise, so that the gyroscope is unconstrained, and is not influenced by the mechanism on which it operates.
The weight of the gyroscope wheel is borne in most part by the water pressure at the socket joint, which serves as a "foot-step" bearing. Any remaining friction is utilised to retard the precession and so to bring the wheel into its position of static stable equilibrium; the centre of gravity of the rotating mass is arranged slightly below the point of support.
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