=Devices for Heating the Camera.=—Parts of the camera mechanism which depend on the uniformity of action of springs or upon adequate lubrication are susceptible to change with variation of temperature. At high altitudes low temperatures are met which may freeze ordinary machine oils or may cause springs to seriously alter their tension, even to break. To meet this difficulty, and probably also to dispel the occasional condensation of moisture on the optical parts, the German cameras are equipped with an electrical heating coil placed just below the shutter, and arranged to connect with the general heating and lighting current of the plane. Two contacts are ordinarily provided, for offsetting the effects of temperatures of -15 and -30 degrees centigrade. An additional function of this heating coil is perhaps to maintain the sensitiveness of the plates or film. III THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS CHAPTER XIV THEORY AND EXPERIMENTAL STUDY OF METHODS OF CAMERA SUSPENSION =General Theory.=—In addition to the limitation of exposure set by the ground speed of the plane another limitation is set by the _vibration_ of the camera. This may be caused either by the motor, or by the elastic reactions of the plane members to the strains of flight. Unlike the movement of the image due to the simple motion of the plane, movements due to vibration may be eliminated by proper anti-vibrational mounting of the camera. The effect of vibration may show as an indistinctness of the whole image—this is its only effect with a between-the-lens shutter—or as a band or bands of indistinctness parallel to the curtain opening (Fig. 76). These are due to shocks or short period vibrations during the passage of the focal-plane shutter. The obvious remedy for vibration troubles is to mount the camera on some elastic, heavily damping support, like sponge rubber or metal springs. Such a mounting should, however, be designed on sound principles derived from a proper analysis of the nature and effect of the possible motions of the camera. Otherwise, the vibrational disturbances may be increased rather than diminished by the camera mount. Such an analysis, based merely on general mechanical principles, shows that all motions of the camera are resolvable into _six_. These are three _translational_ motions, namely, two at right angles in one plane such as the horizontal, and one in the plane at right angles to this (vertical); and three _rotational_ motions, one about each of the above directions of translational motion as an axis (Fig. 77). [Illustration: FIG. 76.—Captured German photograph, showing zones of poor definition due to vibration during passage of focal plane shutter aperture.]