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cables to the " fuze indicator and time rheostat " and thence to the " deflection voltmeters."

As the body of the " double telescope " traverses about a vertical axis, but laying is dene in the plane of sight, it is necessary to multiply the angular velocity of the body of the instrument by the cosine of the angle of sight in order to obtain the angular velocity of the target. This is effected electrically by passing the current from the lateral magneto through a rheostat, whose resistance is varied by a rubbing contact passing along it, as the telescopes are elevated or depressed.

Another rheostat and an accumulator (connected to the fuze indicator and time rheostat) cause an angle of sight needle in the fuze indicating voltmeter to move to the same angle of sight as that of the telescopes ; this needle is controlled by another circuit.

2. The fuze indicator and time rheostat consist mainly of the time rheostat, a fuze indicating voltmeter, a microphone and an external accumulator of three cells.

As explained above, the currents generated by the magnetos pass through rheostats on their way to the deflection voltmeters. These rheostats are situated beneath the time adjusting dial, and their resistance is altered as the dial is turned. The setting of the dial is dependent upon the height of the aircraft and the setting of the fuze, and is effected as follows: A graduated height arm is moved by means of a milled head until it reads the height obtained from a height-finder. On its right-hand upper edge is a reader for reading the fuze curves on the time adjusting dial. The latter is turned until the reader of the height arm is on the fuze curve representing the length at which the fuzes have been set.

I JAM/ftM

ru INDICATOR AND TIMC RHEOSTAT

DOUail TELESCOPE

FIG. 4. Arrangement of Brocq apparatus.

The angle of sight needle in the fuze indicating voltmeter is con- trolled by two circuits, viz,: that referred to in (i) which tends to set it at the angle of sight of the telescopes, and another in which are the vertical magneto armature in the double telescope, the rheostat beneath the time adjusting dial and another rheostat which auto- matically adds eight seconds to the time of flight. This eight seconds is an allowance for the time taken to set the fuze, load, lay and fire the gun. The angle of sight needle therefore makes with its zero or horizontal line an angle equal to the angle of sight to the predicted position of the target at which the shell will burst. When the height arm is moved, a height strip inside the fuze indicating voltmeter is also moved. Its height above the zero line of the angle-of-sight needle represents, to the scale of the instrument, the height of the target. The intersection of the needle and strip therefore represents the position of the target at the moment of the shell burst. Fuze curves are marked on the glass cover of the voltmeter, and the curve which is nearest to the intersection of the needle and strip will indicate the length at which fuzes are to be set. This fuze length is called down the microphone to the fuze setters, and is transmitted to the sight setter by the man taking up the shell.

3. The deflection voltmeters are of the dead-brat type and read to 10 on either side of zero. Two are provided for each gun ; one for lateral and the other for vertical deflection. As a rule, two guns can be worked by one Brocq equipment, four deflection voltmeters being provided. The required deflection is read by the upper pointer. Corrections for wind are applied by moving the scale by means of a knob beneath the voltmeter, the amount of correction being indi- cated on the scale by the lower pointer.

Stereoscopic range-finders were extensively used by the Cen- tral Powers for anti-aircraft work. (A. C. W.)

SOUND-RANGING

The method of locating hostile guns by the sound, or sounds, consequent on their discharge was introduced on the British front in France during 1916. It had at that time already been in use in the French army for many months. It speedily proved its usefulness, especially in circumstances which rendered other methods of location very difficult or impossible. The system

of concealment known as " camouflage " added considerably to the difficulty of finding the position of gun-pits on photographs taken from the air, and, further, these photographs offered no certain method of deciding whether a gun position, once identified, were occupied or no. The locations given by sound- ranging frequently enabled well-concealed positions, which had previously been missed on air photographs, to be detected, and offered a sure index as to whether known positions were active at a given time. Although air photographs always offered valuable confirmation of the sound-ranging locations, and were, when available, consulted with this object in view, the method is, of course, quite independent of such support. It works as well at night, or when, owing to fog, mist, or smoke, the visibility is poor, as on clear days; it can detect batteries so well hidden as to be invisible from the air or on air photographs; it is always ready when once the apparatus has been installed; and a location can be obtained, under favourable conditions, within a minute or two of the arrival of the report of the piece. On the other hand the instalment of the apparatus necessitates the laying of several miles of wire, and involves considerable preliminary labour in other ways; the method will not work during a heavy bombardment; and certain weather conditions, to be discussed later, render locations almost impossible. The difficulty first mentioned will quite possibly be surmounted or diminished; the other two seem, at present, insuperable.

The method has been elaborated to permit the directing of fire on a hostile piece by comparing the record of the sound of discharge of the piece with that of the burst of the shell directed against it. With iz-in. and o-2-in. howitzers destructive shots have been directed very successfully by sound-ranging.

Principles. The method generally adopted in the British, French, and American armies is to record the instant of the arrival of the sound made by the hostile piece at certain fixed and carefully sur- veyed posts, spaced at intervals varying from 1,000 to 2,000 yards. If it be assumed that the sound spreads out from the source with a known velocity, the same in all directions, then a known interval between the arrival of the sound at two fixed posts will determine a curve on which the source must lie. This curve is a hyperbola with the two posts Pi and Pi as foci, for the determining condition is that the difference of the radii vectores SPi, SPi be constant. If, in addi- tion, the time of arrival at a third post be known, then the interval between this and the time of arrival at either Pi or P 3 will fix a second hyperbola on which the source must lie, and so determine the posi- tion of the source. In practice three posts are not sufficient, since any uncertainty caused by the recording of a spurious sound at a post would falsify the location. In general six posts are used, which, taken consecutively in pairs, give five lines which should all inter- sect. Any accidental selection of the record of a spurious sound at one or more of the posts is then at once detected by the non-inter- section of the curves. Records of the sound at five, or indeed four, of the posts are generally sufficient for the experienced sound-ranger, even when several guns are being recorded at short intervals, so that the use of six posts allows for the sound not being successfully re- corded at one or two of the posts.

Nature of Sounds from High- Velocity Guns. In the preceding argument it has been assumed that the sound spreads out with uniform velocity in all directions from the source. There is little doubt that this is true, in a still atmosphere of uniform temperature, of the sound of the discharge of the piece. With the modern high- velocity gun, however, a second sound, originating in the motion of the shell through the air, always accompanies the sound of dis- charge. This second sound is due to a pulse of compression ^set up by the shell, known as the " shell-wave," or " onde de choc." It is perceived by an observer in front of the gun as a sharp crack, which is followed after an interval depending on the type of gun, the elevation of the gun, and other factors, by the duller, heavier sound of the discharge, or gun-wave. To examine the formation of the shell-wave by the passage of the projectile, consider the resultant disturbance produced by the pulses of compression travelling out with the velocity of sound from every point of the path of the shell. For simplicity take in the first case a projectile travelling horizon- tally with a uniform velocity greater than that of sound; let G be the position of the gun, Si, Si,. . . SH> be the positions of the projectile at the end of the 1st, 2nd,. . . loth second (fig. 5). When the shell is at Sio the compression originating at G has travelled out as a spherical shell with G as centre for 10 seconds, that originat- ing at Si has travelled out as a spherical shell with Si as centre for o seconds, and so on. The envelope of all these spheres is a cone with its apex at Sio ; if the shell be travelling close to the surface of the earth the trace of this cone on the surface is ASuC, which represents the pulse of compression under discussion. If the velocity of the shell be considered to decrease with time, as in any actual case, the