=Gyroscopic Mountings.=—The ideal support for the aerial camera will undoubtedly be one embodying gyroscopic control of the camera's direction. By proper utilization of the principles of the gyroscope it is to be expected that not only can the camera be maintained vertical, but it may be supported anti-vibrationally as well. At the present time the problem of gyroscopic control is in the experimental stage, so that only the elements of the problem and the possible modes of solution can be laid out. The gyroscope consists essentially of a heavy ring or disc rotating at a high speed on an axis free to point in any direction (Fig. 79). If mounted so that the axes of the supporting gimbals pass through the center of gravity of the rotating disc, the result is a _neutral_ gyroscope. Its characteristic is that its axis maintains its _direction fixed_, but this fixity is with respect to space and not with respect to the gravitational vertical. Consequently, as the earth revolves the inclination of the gyroscopic axis changes with respect to the earth. In latitude 45° this change is approximately a degree in five minutes. Furthermore, the action of friction in the supports, which can never be entirely eliminated, also acts to slowly alter the direction of the gyroscopic axis. Therefore, unless some _erector_ is applied even the gyroscope will not perform the task required of it. [Illustration: FIG. 79.—Diagram of simple gyroscope.] Before discussing possible forms of erectors it may be noted in general, first, that these must depend upon gravity; second, that such being the case, they must respond to the resultant of gravity and any acceleration, that is, to the _apparent_ or _pseudo-gravity_. As already seen, this pseudo-gravity, during a turn, is exactly what limits the usefulness of the pendular support, and necessitates recourse to the gyroscope. The problem thus becomes one of making an erector-gyroscope combination which will respond to true gravity and not to pseudo-gravity.