FIG. 131.—Aerial bombardment of Trieste. Note falling bombs in center of picture; and exploding anti-aircraft shells over the water. Italian official photograph.] [Illustration: FIG. 132.—Example of spotting requiring exposure at exact instant. Explosion following burst of bomb in ammunition dump. British official photograph.] Sights for the observer have been discussed already. Here again the negative lens is to be preferred, but while the pilot's lens needs only directing lines in the axis of the plane (unless he takes the picture), the observer's lens needs both an accurate center mark and an additional upper or lower sighting point. Accurate alignment of these marks with the camera axis must be arranged for in precise spotting. Accurate spotting work requiring the delineation of fine detail calls for cameras of considerable focal length. The camera of longest focal length used in the war was the French 120 centimeter (Fig. 41). This was employed with great success in such work as regulating the fire of heavy railway guns brought into range only at night, to fire a few shots at chosen angles. Photographs taken the next day would then show the exact spot where each shell fell, and the damage it did, to serve as a guide for the next night's operations (Fig. 127). The field of these cameras is quite small—8 to 12 degrees—and so not only must sighting be exact but the area covered on the ground must be accurately known. This is to be calculated from the altitude, focal length, and plate size, by the relation— distance on ground altitude —————————————————— = ———————————— plate length focal length Data derived from such calculations may be incorporated in tables, or graphically in diagrams such as Figs. 128 and 129. [Illustration: FIG. 133.—The same subject a few minutes later. Height of smoke shown by shadow. British official photograph.] These calculations and others required in mapping and stereo-work are simply and quickly made by slide-rule devices. One of these, the Burchell Photographic Slide Rule, developed in the English service, is shown in Fig. 130. This consists of two dials, the center one of which is mounted—usually by a pin pushed into a cork behind—so as to turn freely, to permit its being set for altitude, focal length, ground speed, plate size, etc., whereupon the area covered, or the appropriate interval between exposures may be read off. Cameras for spotting work should be capable of exposure at the exact moment desired. For if the camera is ever to catch the gun as it discharges, the bomb as it falls (Fig. 131), or the shell as it explodes (Fig. 132), the photograph must be taken within the instant. Automatic cameras, exposing at regular intervals, while adequate for mapping, are not fitted for many kinds of spotting. CHAPTER XXVI MAP MAKING