photographic chronograph at a "central" or calculating station so situated as to entail the minimum amount of wire connection to the listening instruments. Each section had two advanced posts at which observers were on duty day and night in order to start the automatic recording mechanism at the "central" when necessary. The listening instruments were equi-spaced on a straight line, or more generally on an arc of a circle which was concave toward the enemy, situated a short distance behind the line of the advanced posts mentioned above. The distance between the listening instruments was generally about fifteen hundred meters so that the entire length of the sound-ranging base was about seventy-five hundred meters or slightly less than five miles. The employment of a regular base, generally an arc of a circle, was a highly important innovation which was introduced by the British; it rendered the interpretation of the records easy even when there was considerable artillery activity because the indications on the record which were caused by any one of the many guns which might be firing at about the same time were spread out on the record in a simple geometric pattern if the listening instruments were arranged on the ground in a simple curve. Owing to this it was possible to locate several guns, firing practically simultaneously, without a loss of time in correctly interpreting the photographic record delivered by the instrument at the central station.

The recording mechanism at the "central" consisted of an accurate timing device arranged so as to photograph on a moving strip of sensitized paper a series of lines about onefiftieth of an inch apart; these lines were the shadows cast by a set of spokes of a wheel which was kept spinning in the path of a beam of light which fell on the sensitized paper; the rate of spin of the spoked wheel was governed by a tuning fork so that the shadows were cast on the paper with the greatest attainable regularity; the rate chosen was one hundred shadows per second so that the photographic paper had recorded on it across its entire width an extremely accurate time scale the

smallest interval of which was one one hundredth of a second; the tenth second mark and the entire second mark were made so as to be easily distinguishable from the others in order to permit rapid counting. The photographic paper employed was of the width of the standard moving picture film as this could be obtained quickly and at low cost both in Europe and America.

Superimposed on the time scale on the paper were six shadows evenly spaced across the width of the paper; these shadows were cast by six tiny moving elements of a specially constructed galvanometer. One of each of these elements was electrically connected to a corresponding one of the listening instruments of the sound-ranging base. When the sound of a gun arrived at listener No. I there occurred a slight twitch in the element No. 1 of the galvanometer and this twitch was photographically recorded on the moving paper strip. The time the twitch occurred could be read with an accuracy of at least a hundredth of a second because the record of the twitch and of the time were superposed on the same piece of photographic paper. When the sound arrived at listener No. 2 the element No. 2 responded and recorded the exact time as just described for No. I and the same was true for the other four elements. Thus if the mechanism were set in motion before the sound of the gun reached the listener nearest to the gun and was allowed to run until the sound reached the listener furthest from the gun the photographic record which was delivered, automatically developed and fixed, contained all the information necessary to calculate the gun's position; i. e., it contained the five intervals between the times of arrival at the six listening instruments. If, as generally, the recorder was run for twenty or thirty seconds, the burst of the enemy's shell was also recorded and its position could be reported to the artillery in one to two minutes after the gun had fired.

Figure 5 illustrates the type of record obtained from various types of German guns variously located with reference to the listening instrument. Figure 5 A shows the "twitches on

all six recording elements due to the sound from a 15 centimeter howitzer situated very nearly equidistant from all six of the listeners. The vertical lines represent tenth second intervals. The hundredth second intervals which appear on the actual film have been omitted from the drawing. Figure

5 B is similar to A except that a wind was blowing from the left flank which caused the sound to be so loud on the right flank that the twitches of the lower elements are too rapid to photograph. Figure 5 C is the record from a high velocity gun situated towards the right flank of the sound-ranging base. The portions of the record marked S are due to the bow wave of the shell as it passes over the various listening instruments while the portions marked G are due to the muzzle wave of

the gun. It is, of course, this latter portion of the record that is used to calculate the position of the gun. The time intervals between S and G serve to identify the caliber of the gun.

The listening instruments were grids of very fine wire electrically heated and mounted in the narrow neck of bottleshaped containers. When the sound from a gun arrived at the container, air was forced in and out by the pressure changes existing in the passing sound wave; the air rushing in and out cooled the hot wire mounted in the neck of the bottle and this cooling disturbed the flow of the electric current used to heat the wire, and the variation in the flow of current was what actuated the moving part of the galvanometer at the "central and caused the twitch in the shadow recorded on the moving photographic paper. The listeners were rendered purposely insensitive to loud but high-pitched noises like rifle fire, etc., but purposely very sensitive to grave and sometimes almost inaudible sounds like heavy caliber artillery fire; in fact, for the purpose for which they are designed the listeners were superior to the human ear and were able to pick up German guns as far in the rear as guns were likely to be placed. Very often a gun - the report of which had not been audible found on the same record with a nearer and audible gun. The timing device at the "central" station was run continuously day and night but the remainder of the apparatus was run only when firing was taking place; for this reason the apparatus was electrically controlled by observers stationed near the front line trenches; these observers had certain groups of enemy artillery assigned to them for surveillance and they were instructed to start the recording mechanism whenever they heard firing from their assigned areas. There were generally two or more forward observation stations (marked O. P. on Figure 4) to each sound-ranging section, so chosen with reference to the lay of the land, that no enemy-firing on the five mile front could take place without attracting the attention of at least one of the groups of forward observers.

was

A typical record not only contained data from which the

position of the enemy gun could be located but also contained data from which the location of the burst of the shell could be calculated; thus both the range and the time of flight of the shell were known. In the case of guns employing fixed ammunition charge a knowledge of these two quantities was sufficient to determine the caliber of the gun since the values of the muzzle velocities of many of the German guns were well known to the Allies. Even in the case of guns not employing fixed ammunition charge the fact that the burst of the shell could be located enabled the officer in charge of the section to recover fragments of the shell on which to base an estimate of the caliber.

The possibility of an estimate of the caliber of the enemy guns was one of the unique features of sound-ranging. Another important feature was the ability to locate guns which were brought up by the enemy in preparation for an attack and which were therefor not used in the period preceding the attack in order to insure an element of surprise. Such guns usually fired but one or two ranging shots and if they were well concealed usually escaped detection by ordinary means; many such guns were located by sound-ranging when they fired their first, and often only, ranging shot.

The location of the enemy artillery formed only one part, though the more important part, of the routine work of a soundranging section. When, because of bad weather, aerial or other visual observation was impossible the sound-ranging sections were called on to correct the fire of the friendly artillery on enemy objectives either to silence the fire of batteries or to harass the enemy traffic in the back areas. In the case of silencing the fire of an enemy battery which had just fired, sound-ranging was very effective. The following consideration will show why this was so suppose an enemy gun has just fired and that a record has been obtained by the soundranging section; to obtain an accurate location it is necessary to apply temperature and wind corrections to the observed data and it is the lack of accurate knowledge of the wind and