[Illustration: FIG. 11.—Appearance of the earth from a high altitude—10,000 feet or more.] The _air speed indicator_ consists of a combination of Venturi and Pitot tubes, producing a difference of pressure when in motion through the air which is measured on a scale calibrated in air speed. This instrument is important for determining, in combination with wind speed, the _ground speed_ of the plane, on the basis of which is calculated the interval between exposures to secure overlapping photographs. Its accuracy is well above that necessary for the purpose. _Inclinometers_ for showing the lateral and fore and aft angle of the plane with the horizontal, are occasionally used, and have also been incorporated in cameras. The important point to remember about these instruments is that they are controlled not alone by gravity but as well by the acceleration of the plane in any direction. They consequently indicate correctly only when the plane is flying straight. On a bank the lateral indicator continues to indicate “vertical” if the bank is properly calculated for the turn. II THE AIRPLANE CAMERA CHAPTER III THE CAMERA—GENERAL CONSIDERATIONS =Chief Uses of an Airplane Camera.=—The kinds of camera suitable for airplane use and the manner in which they must differ from cameras for use on the ground are determined by consideration of the nature of the work they must do. Four kinds of pictures constitute the ordinary demands upon the aerial photographer. These are single objectives or _pin points_, _mosaic maps_ of strips of territory or large areas, _oblique views_, and _stereoscopic views_. Each of these presents its own peculiar problems influencing camera design. _Pinpoints_ consist of such objects as gun emplacements, railway stations, ammunition dumps, and other objects of which photographs of considerable magnification are desired for study. Here the instrumental requirements are _sufficient focal length of lens_ to secure an image of adequate size; _means for pointing_ the camera accurately; enough _shutter speed_ to counterbalance the speed of the plane; sufficiently _wide lens aperture_ to give adequate exposure with the shutter speed required; _means of supporting_ the camera to protect it from the vibration of the plane. _Mosaic maps_ are built up from a large number of photographs of adjacent areas. In addition to the above requirements, mosaic maps demand _lenses free from distortion_ and covering as large a plate as possible, in order to keep to a minimum the number of pictures needed to cover a given area; means for keeping the camera _accurately vertical_, and means for _changing the plates or films_ and resetting the shutter rapidly enough to avoid gaps between successive pictures. At low altitudes and high ground speeds the interval between exposures becomes a matter of only a few seconds.