the distance is 20 × 36⁄0.5 = 1440 ft. Using such rough-and-ready "apparatus," the degree of accuracy to be expected is not great; however, the author has found it quite sufficient to determine the altitude of a machine to within 5 or 6 per cent. of the truth. If for the observer's arm and foot-rule we substitute a low-powered telescope or binocular of, say, 2 or 3 diameters magnification, with micrometer cross-wires, with which to follow up the apparent reduction in span of a receding aeroplane, until some prearranged signal is given, the range could undoubtedly be determined easily within 2 or 3 per cent. At 1 mile distance this means a degree of accuracy represented by a maximum error of about 40 yards, or sufficient to enable shrapnel to be dropped right on the mark. Parenthetically, it may be pointed out that the same method will enable the range of a hostile aeroplane to be determined, provided the type be identified, and its leading dimensions are known; it also suggests the importance of not flying exactly towards or away from, or exactly broadside to, any position of the enemy guarded by counter-aircraft artillery; flying end on to the enemy is also to be deprecated on the ground of fixity of direction.
§11. Attack by Gun-Fire. An aeroplane operating in a hostile country is liable to attack by rifle and machine-gun fire, also by shell-fire from special anti-aeroplane artillery. It has comparatively little to fear from field artillery owing to the want of handiness of the ordinary field-gun. The "laying" of a field-piece is far too clumsy a business to permit of its effective use on so small and rapidly moving a target as presented by an aeroplane in flight, though it may be effective when used against a dirigible. With regard to rifle or machine-gun (small bore) fire, calculation shows that aircraft is abso-
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