The majority of the photographs made in the British service were obliques, taken by short focus (6 to 10 inch) hand-held cameras. This type was employed partly because of difficulties to be noted presently, in using other forms of cameras, but more especially because such pictures sufficed for the kind of information desired. A hand-held camera formed part of the outfit of each flying boat and dirigible, but, unlike land reconnaissance, planes ascending primarily for picture taking were unknown in their naval service. Usually no photographic objective was predetermined—photographs were made only if objects of interest were come upon. Mapping also formed no part of the seaplane's work. Four plates would be carried, instead of as many dozens in the land machine, and often these would come back unexposed. There were of course some photographic flights planned out beforehand, for the purpose of photographing lighthouses and other landmarks whose appearance from the air should be known to the naval aviator (Fig. 175). Among the accidental and record types of photograph come convoys (Fig. 176), whose composition and arrangement were made a matter of record, particularly if any ship was out of its assigned position. Photographs of oil spots on the sea surface, or other results of bomb dropping, were necessary evidence to establish the sinking of a submarine (Fig. 179). Pictures of all types of ships friendly, neutral, and where possible, enemy—were a much needed part of naval equipment, in particular pictures of friendly destroyers and submarines, which should not be bombed by mistake. For safe navigation it was essential to have photographs of uncharted wrecks (Fig. 181), of buoys out of place and of ships failing to return signals or otherwise to comply with rules. The great majority of the pictures were taken from altitudes of not more than 300 meters. [Illustration: FIG. 177.—Submarine coming to the surface. U. S. Naval Air Service photograph.] [Illustration: FIG. 178.—Dropping depth bombs.] [Illustration: FIG. 179.—The submarine destroyed. Destroyer on tell-tale oil patch. British official photographs.] [Illustration: FIG. 180.—A convoy at anchor in port.]