PHYSICS IN WARFARE 95 



fleeted each from the inside edge of a right-angled prism, into 

 a cinematograph camera, the positions of the shadows on 

 the film being controlled by small adjustments of the prisms. 

 This camera was worked by a small motor, which was in turn 

 connected to a relay arrangement under the control of the 

 forward observers in the observation post just behind the 

 front line, who could therefore start or stop the film at will. 

 In practice they started it as soon as they heard the report of 

 a hostile gun, so that the first indication of artillery activity 

 that the people at Headquarters had was the hum of the motor 

 as it was switched on. A few seconds later the strings were 

 seen to kick, and then the report was heard. 



The time-wheel consisted of a disc, around the edge of which 

 a number of little cogs were placed. It was controlled by an 

 electro-magnet, being pivoted excentrally to it. The electro- 

 magnet was switched on and off automatically by a tuning- 

 fork of frequency 50, which therefore made a hundred half- 

 vibrations per second. Matters were so arranged that the 

 beam of light from the lamp was intercepted by the cogs one 

 hundred times per second, so that, at these small intervals, the 

 shadows of the cogs were photographed and gave the vertical 

 time-markings on the film as shown in Fig. i , which were there- 

 fore put on as the film was run through. Had they been 

 marked on the film before it was required for use, it would have 

 been almost impossible to have adjusted the rate of the motor 

 sufficiently accurately to have passed the film through exactly 

 at the necessary rate. 



Sound-ranging was able to give not only the actual position 

 of the hostile battery in the field, but was able to determine 

 its nature, i.e. whether gun or howitzer, and also its calibre, i.e. 

 the size of the shell it fired. We shall consider these separately. 



The essential distinction between howitzers and guns is 

 that the former throws a shell very high but to no great distance, 

 whilst the latter fires at long range, but comparatively to no great 

 height. 



Moreover, the howitzer shell travels more slowly than sound, 

 whilst the gun shell travels far more quickly. It is this 

 latter distinction between the two that is utilised in sound- 

 ranging and is actually recorded on the film. 



Consider the case of a howitzer shell. Suppose the shell 

 to be fired from the point O, Fig. 3, and let us consider the state 

 of things at the end of, say, five seconds. The sound-wave 

 of the discharge will have travelled to the extreme right posi- 

 tion indicated, the numbers under the horizontal line giving 

 the position of the sound-wave at the end of successive seconds, 

 whilst the numbers above give the positions of the shell at 

 the same times. 



