In the Gray stabilizer several metal balls are slowly rotated in a tray above the center of gravity of the gyroscope. Specially shaped grooves or compartments limit the freedom of motion of these balls so that when the gyro is inclined the balls travel at different distances from the center on the ascending and descending sides. By this scheme a couple is produced about the axis through the center and the low point of the disc, which tilts the apparatus to the gravitational vertical. In an alternative form the balls are carried past the low point by their momentum and are prevented from returning by the walls of the containing compartment, which have meanwhile been advanced by the rotation of the erector as a whole. The net result is to shift the center of gravity of the system of balls in the proper direction to erect the gyro. The rectifying action is purposely made quite slow so that the displacements of the balls due to pseudo-gravity will be averaged out. In a design due to Lucian, small pendulums work through electric contacts to actuate solenoids which in turn move small weights in the appropriate directions to give the desired tilt. Response is made fairly quick and delicate, and pseudo-gravity, due to turns and rolls, is rendered inoperative by the contacts breaking whenever the pendulums swing more than three or four degrees. This can only happen if they move too quickly for the erecting forces to act, reliance being here placed on the characteristic differences of action in respect to time of real and pseudo-gravitational forces. Besides the neutral gyroscope as just considered there is the pendular or top type, in which the center of gravity is not in the plane of the supports. In general this type depends on a couple resulting from the gravitational pull and the inevitable friction of the supports to slowly tilt the axis to the gravitational vertical. This type is slower to respond than the designs in which a definite couple in the proper direction is provided and it reaches the true vertical only through a circuitous path. [Illustration: FIG. 80.—Diagram of camera linked to gyroscopic stabilizer.] Three methods of controlling a camera by a gyroscope are suggested. One is to fasten the gyroscope rigidly to the camera and mount the whole system on gimbals. A second is to mount both camera and gyro side by side on gimbals, linking the two so that the camera is moved parallel to the gyro (Fig. 80). A third method is to utilize the gyro to make electric contacts to operate motors which in turn move the camera.