856,685. Gyroscopic apparatus. SOC. COMMERCIALE ECA. Dec. 13, 1957, No. 38900/57. Class 97(3) A device for indicating the true vertical on a moving vehicle comprises a platform rotatable about two perpendicular axes gyroscopically maintained parallel to the tangents to two perpendicular great circles and maintained on a fixed true course by rotation about an axis perpendicular to the other two axes, the platform supporting two gyroscopic units for detecting and twice integrating the accelerations of the platform respective along directions parallel to the said tangents, each unit comprising a gyroscope having two degrees of freedom the outer frame of which pivots about a horizontal axis at right angles to the tangent to the corresponding great circle and is unbalanced by any known acceleration-responsive means whilst the inner frame is balanced, vertical and parallel to the said targent and pivots about an axis at right angles to the said horizontal axis with the interpositions between the two frames of a return device, the spinning axis of the rotor being located in a vertical plane passing through the tangent, the angular movement of each outer frame controlling the adjustment of the platform about its two axes, the proportion between each angular displacement of the platform about one of the axes and the angular shifting of the outer frame of the corresponding gyroscope being equal to the inverse of the radius of the earth. If a horizontal platform is mounted in a vehicle which moves through a distance y from Z to Z<1> along a great circle T (Fig. 3), the angle a subtended at O, the centre of earth, is given by the formula a/y=l/R, where R is the radius of the earth. The platform can be kept vertical by turning it through the angle a as the vehicle moves from Z to Z<1>, and this is effected by measuring the acceleration in the y direction and integrating it twice. The double integral appears as a rotation through and angle proportional to y, and by marking the factor of proportionality equal to R, the angle is made equal to a. Further, this value of the factor makes the period of the platform equal to about 84 minutes, so that its movements are dead beat. In the form of double integrator shown in Fig. 2 the rotor 14 is mounted, with its spin axis in the y direction, in a gimbal 12; which is pivoted about a vertical axis in a gimbal 9 which pivots about an axis x which is horizontal and perpendicular to y. A spiral spring 15, or equivalent mechanical or electric device, is connected between the gimbals and surrounds one of the vertical pivots. A weight 11 is fixed to the lower part of gimbal 9. In the event of an acceleration in the direction y, the action of weight 11 is to apply a couple to the gyroscope about axis x, the magnitude being proportional to the acceleration. The resulting rate of precession about the vertical axis is proportional to the acceleration, and the torque set up in spring 13 as the result of the movement of gimbal 12 is proportional to the integral of the precession rate, and therefore to the integral of acceleration. The torque stored in the spring 15 re-acts on the gyroscope to cause precession about axis x. The rate of precession is proportional to the torque in the spring, and the angle through which the gimbal 9 turns is thus proportional to the integral of the torque, and hence to the double integral of the acceleration. If the factor of proportionality is made equal to R, as discussed above, the gimbal 9 turns through an angle a and thus remains vertical during movement along the great circle T. Friction in the bearings must be minimised, and to this end the rotor and gimbal 12 may be enclosed in a casing floated in a liquid, or the spin axis may be vertical and gimbal 9 made horizontal so that the forces in the ball-bearings are radial. A platform 17, Fig. 6, is fixed to a shaft 18 journalled in a vertical frame 20 which is journalled in a horizontal frame 22 pivoted at 23, 23a in a vehicle. On the platform are two double integrators 24, 26 responsive to accelerations in two directions, along great circles at right-angles or, in the example described, along the meridian and the parallel. A gyroscope 28 on the platform maintains the platform in the required direction. Since the axes of the frames 20, 22 are in general inclined to the meridian, electrical signals generated by the double integrators proportional to their angular outputs are split into components a along the axes, recombined, and applied to motors 35, 36 on the frame shafts so that the platform is kept horizontal for all positions of the vehicle. Corrections for errors arising from the rotation of the earth may be made by signals applied to the motors 35, 36.