A caution must be noted with regard to center of gravity mountings. Any change in the camera, in particular the substitution of a short for a long lens cone, must be made so as to cause no alteration of the relative positions of the center of support and the center of gravity. Either the short cone must be weighted, or additional supporting pivots must be provided in the plane of the new center of gravity. =The Italian and G. E. M. Mountings.=—These mounts (Figs. 49 and 59) are similar in that the protection from vibration is furnished by an elastic support at the bottom of the camera. Tests show that these two cameras give very similar results, of the unsatisfactory sort to be expected from this kind of mounting in view of the lessons of the last chapter on the proper point of support. Fig. 94, _a_, shows a trace given by the Italian mount. The permissible exposure, on the criterion adopted, is very short with either mount, about 1/200 second. =The Brock Suspension.=—This consists of a pair of frames into which the camera is fitted by ball bearing pivots, so that it is free to move in any direction (Fig. 60). In order to permit gravity to control the direction of the camera, the point of support is made considerably (ten inches) above the center of gravity. Air dash pots are provided for damping the swings. As already explained, the pendular method of support is in basic contradiction to the requirements for vibration elimination. Tests of the Brock suspension, shown in Fig. 94, _b_, indicate it to be of low efficiency in damping out the short period vibrations which are responsible for poor definition. CHAPTER XVI THE INSTALLATION OF CAMERAS AND MOUNTINGS IN PLANES