FIG. 168.—Bethune, August, 1918, illustrative of interpretation by shadows.] [Illustration: FIG. 169.—Typical trench photograph showing first and second lines, communicating trenches, listening posts, machine gun emplacements, and barbed wire.] A few illustrations of the more ordinary and obvious objects whose detection is the subject of aerial photography are shown in accompanying figures. Fig. 169 pictures a typical trench system, with barbed wire. The trenches show as narrow castellated lines, from which run the zigzag lines of communicating trenches, saps, and listening posts. The minute pockmarks behind the main trench lines are shell holes and machine gun pits. The barbed wire shows as double and triple gray bands, intricately criss-crossed at strategic points. Another form of defence, intended for the same purpose as the barbed wire of the western front, is that furnished by overthrown trees in forest regions. Fig. 170 reveals a mountain fortress surrounded by a zone of felled trees, and indicates in striking manner the value of the information a single aerial photograph may furnish to an attacking force. Fig. 123 shows on a comparatively large scale opposing trench systems in which a natural obstacle—a river—separates the adversaries. Nicks and dots indicate machine guns to the skilled eye, and several rectangular structures are revealed as concrete buildings which have survived unscathed the shell fire which has obliterated, and caused to be rebuilt, nearly every other element of the trench system. [Illustration: FIG. 170.—A mountain fort surrounded by felled timber.] Isolated battery emplacements (Fig. 171) must be carefully studied to learn if they are in use. The chief indication is given by the paths the men make in going and coming; these show as fine light lines, obliterated by growing vegetation if long disused. Another indication is the blast marks in front of the gun muzzles; occasionally the sensitive plate will catch the actual puff of smoke as the gun is discharged. [Illustration: FIG. 171.—Three stages in the life of a battery.] Railways of various gauges show as thin lines, crossed by ties, and exhibiting the characteristic curves and switches. They are particularly important to detect because they naturally lead to guns or supplies of importance. Abandoned railways from which the rails and ties have been removed leave their marks on the ground and must be carefully distinguished from lines in actual operation.