COPY.

NOTES ON THE LOCALIZATION OF HOSTILE BATTERIES.

            It having been decided that the Survey Department of each Army is to have an Artillery Observation Section, the following notes are written with a view to suggesting how information obtained by this Section can be rapidly utilised, in conjunction with information from other sources, so as to enable G.O.C. R.A. Division or Heavy Artillery Group to engage hostile batteries without delay.

It is assumed that there will be a sub-section of the observation Section, under an officer, for each frontage of approximately two Army Corps with from 4 to 8 outlook posts.

The accompanying diagram represents two Corps, each with two Divisions in the front line and the position of the sub-section officer and 6 look our posts.

Owing to many reasons the proportion of bearings from outlook stations that produce intersections are very small compared to those that are indeterminable in themselves, and yet these latter in many cases would intersect with flash bearings taken from battery O.P.s.

It would be impossible for the officer in charge of a sub-section of the Observation Section to deal with Hostile Artillery reports of two Corps, but as these reports are compiled in Divisions, information obtained by outlook posts should also be communicated to Divisions.

Referring to attached diagram, the subaltern officer cannot communicate each flash from station A to F to the R.A. of each Division even supposing he had direct lines to them, but in most if not every case, there is a Battery O.P. or a battery in action fairly close to each of these stations to which they should be linked up by telephone and by this method information could be passed on to the Divisional R.A. in the shortest space of time.

Information for the Heavy Artillery Groups would be communicated by the sub-section officer either direct or through the Corps.

2. COMPILATION OF REPORTS ON HOSTILE ARTILLERY FIRE.

            Hitherto this has been undertaken by Brigade Majors, R.A. but the writer after 3 months experience is of opinion that the other duties of Brigade Majors are so important and arduous that he has not the necessary time to thoroughly work out and dovetail together the various reports collected from many sources.

For this work the Staff of an R.A. Division should have an Intelligence Officer selected from one of the Batteries of the Division.

The sources of information at his disposal would be as follows:-

1. Reports from outlook stations.
2. Daily Battery Reports.
3. Daily Infantry Reports.
4. Reposts from R.F.C. communicated by Corps.
5. Aeroplane photographs.

All bearings to flashes or smoke spurts would be telephoned in to him; other reports should reach him by 9 a.m. and include the period from 6 a.m. to 6 a.m.

From these reports he should first make a compilation of areas shelled, the times they were shelled and the directions from which the shells came.

He would then see what intersections (if any) coincided with time and direction.

In the event of a flash bearing without any intersection, he should take the aeroplane photos and follow the direction of the ray noting down likely or possible positions and see if they coincide in any way with Battery or Infantry Reports.  If time from “flash to report” has been correctly noted, by allowing 370 yards per second as the velocity of sound he should be able to determine the position of a howitzer within 5%.

In the case of a gun the “ondu du choc” has to be considered, corrections for this vary with the different natures of guns *

* The “sound expert” might be asked to write a short note on this.

He should keep a book with the “History” of each hostile battery showing how and where its position was suspected or determined, the dates it fired and the objectives.

It is very necessary to note positions suspected even on vague information as the “History” will show on what date it was suspected and a reference to the daily log book will give full details.

Having located the battery, the next thing is to determine nature and number of guns and by degrees work out the zone of fire.

The Intelligence Officer should then mark out on a tracing – for the use of G.O.C. and Brigade Major – all hostile batteries located and the direction of their zones of fire, different coloured chalks being used for the different natures of guns.

He should copy on to his map aeroplane photos, hedges, trees, entrenchments, tramways and other detail not already shown.  The study of these photos and comparing those taken recently with older prints means several hours work, but is time well spent.

Battery Daily Reports.

            The information contained in them should be as a rule fuller and more definite; the trouble arises from most battery commanders trusting solely to F.O.O.s instead of having in addition “Look out” men.

With a battery of four officers, one of whom may be temporarily absent, the manning of an O.P. from 5 a.m. to dusk is no light work, the presence of an officer is necessary in case the battery has to fire but he ought to have look out men to relieve him of the continuous strain in watching for flashes, etc.

Both the French ad the Germans make use of Senior N.C.O.s for this purpose and must we acknowledge that our N.C.O.s are less capable?

In most batteries there are aspirants for commissions and part of their training to qualify them for officers should be the manning of O.P.s.

During a “Strafe” the F.O.O. cannot fire his battery and look out hostile batteries at the same time and yet this is the very time when most flashes might be seen.  On page 319 F.A.T. the duties of “Look out” men are clearly laid down but it has apparently been assumed in some batteries that their services are not required during Trench Warfare.  Occasionally there is firing at night when flashes are most visible but few batteries report them as there is no one observing from the O.Ps.

As the number of men qualified and available for look out duties in any one battery is limited, the best plan would appear to be for the batteries of a Brigade or Group to take over this duty in turn for 24 hour spells.

It must be distinctly borne in mind that the observation stations have not been organised to supplant battery look outs but rather to supplement them, and the more flash bearings that can be obtained from different stations, the greater the facilities for locating hostile battys.

How to locate a flash.

The difficulty of locating hostile batteries has been greatly increased by the introduction of smokeless and flameless powders.  A great deal can be done by sound bearings and, absurd as it may at first seem, a telescope is of great assistance in locating a battery by sound.  First point the telescope in the direction from which the sound is thought to come, subsequent reports may make one think it is more to the right or left, ultimately the sound is laterally bracketed and a sharp look out can then be kept on this area.

During sunlight flashes from guns are seldom seen but by careful watching one may observe a “smoke spurt” or so called “air concussion” and one is lucky a premature may be discovered.

With guns the smoke spurt rises but little and merely shows as a light bluish haze, with howitzers it appears to be projected upwards in the shape of a candle flame.  With dummy flashes by day the flash is as a rule brighter than that of a howitzer and the smoke is in the form of a roundish cloud.

At night the detection of dummy flashes is far more difficult, but as it is almost impossible without electrical apparatus to fire a dummy flash simultaneously with a gun, they can generally be detected by variations in the time from flash to report when the flashes are all on the same bearing.  All dummy flashes ought, however, be reported with a note to this effect.

When a flash is seen the best method of recording it is for the battery look out man to use a director clamped at zero on some distant object clearly recognisable on the map, and give a bearing X^o right or left of this.

(The writer has had experience of angles Right or Left of chimneys of haystacks which the observer himself has been unable to locate on the map).

When a flash is seen, a record of the time from flash to report of gun is most important, particularly in the case of howitzers. Every battery has a Government stop watch but no not one in ten ever uses it for this purpose.

Again the noting of times of flashes and fall of shell leaves much to be desired.  “Several flashes between 10 and 11 a.m.” might mean anything from 10 to 100.  Again “20 flashes at 11 a.m.” is obviously false.  It may be impossible to note the exact time of every flash but the statement that about 30 flashes were seen between 10 and 11 and 5 noted at the following times would be far more valuable and very little more trouble, or again “20 flashes commencing at 10.37 and continuing till 11 a.m.”.  Battery commanders in many cases fail to realise the importance of these returns and seem to think that they are merely required for the compilation of statistics which though of academic interest to higher authorities are of no particular value, and it should be the duty of the I.O., R.A. to inform B.C’s of each flash reported by them can be traced to a located hostile battery.  Once B.C’s begin to see tangible results and appreciate what is being done, there will be an improvement in their daily reports.

Daily Infantry Reports.

On first undertaking this work the writer found that but few of the reports contained information that was of any value, to some extent due to but little trouble being taken with them but chiefly because junior officers had very vague ideas of what was required, why it was required, and how it should be reported.

It is impossible when Divisions are in the front line to give instructions on these points but from the lectures under Lt.-Col. Koebel at Bailleul it is evident that the Infantry are anxious to learn how they can help the Artillery, and a system of lectures followed by a discussion while Divisions are in reserve appears to be most desirable.

When the forward trenches are being badly shelled and call for retaliation, the Artillery can best retaliate on the offending batteries if they know their zone of fire, and the determination of this depends greatly on daily reports sent in by the infantry.

It is realised that in the trenches, with a limited outlook through a periscope and no trench maps, the infantry officer has not the same facilities for observing or reporting as an F.O.O. in a comfortable O.P., but his report ought to be edited and amplified if possible by the Infantry Brigade Intelligence Officer, before being forwarded to the R.A.

As an example:-

“Flash seen near a farm-house between 11 and 12 bearing 73^o from “Bay 6 trench 256, shells falling near cemetery several blinds lying about” .

In this form it is of but little value for the following reasons:-

1. The time is too vague.
2. Nothing to show whether the bearing is True. Sheet Line or Magnetic, or how it was taken.
3. The R.A. do not as a rule possess maps showing trench numbers much less bays.  The map square point is required before the angle can be correctly plotted.
4. There may be 3 or 4 cemeteries near the Divisional front.
5. Although “blinds” were handy, no attempt was made to describe the nature of the gun.

An example of a valueless report hardly creditable to an N?C?O. much less an officer, the following was actually received:-

Time of firing …        …        …         …        All day.

Nature of gun …       …        …        …        All sorts.

Direction from which hostile fire came … Everywhere

Objective if known …          …        …        All round me.

The only heading left blank being “The position of observer”.

            The chief points to which attention should be paid are:-

1. Accurately noting time and direction of every flash or smoke spurt.
2. If possible note time from flash to report of gun.  (Time from report of gun to arrival of shell is useful only when the shells fall near observer and their calibre is known)
3. Time and place of shells that fall, with if possible direction from which they came, stating how estimated, such mean line of successive shell holes, the line between a short and over-ranging rounds, shell scoops &c.
4. It is more important to know the calibre of the shell than whether it was H.E. or Shrapnel.  The former can be measured from blinds or from curves of fragments of the base.  All fuzes should be sent on for inspection and labelled for return.
5. Avoid local names and always give map squares.

The following apply equally to Artillery and Infantry Reports:-

Should fire take during the night after daily reports have been made up, send in a supplementary report either by telephone or special messenger as soon as possible the next morning.

Certain batteries have been located as firing at night, but as no reports were received of areas shelled during these hours, it has been impossible to fix their zones of fire, Compass bearings should be given only in such cases as no better method is available for recording direction for the following reasons:-

1. Every compass has an error of its own.
2. There may be iron in the neighbourhood which deflects the needle.
3. Unless carefully placed on a level rest, it is almost impossible for the average observer to get a correct reading.
4. An officer was found who believed that a “True Bearing” was the true reading of his compass, and there may be others like him.

In the event of an F.O.O. in the forward trenches becoming a casualty while observing the fire of his battery, the nearest Infantry Officer should be able to carry on till a relief is sent down, and this has been done on more than on one occasion, but only where there had been complete co-operation between the F.O.O. and the Company officers.

The above notes are written on the basis of trench warfare, but afford excellent training for field warfare when the difficulties of collecting information will be increased and the facilities for plotting angles, &c greatly lessened.

The next page in the original has the chart referred to which I cannot produce here.

Frontage of two Corps in line each Corps with two Divisions in Front Line.

ONDE DU CHOC.

The Onde du Choc is the name given to a phenomenon which occurs when a projectile is fired from a gun with an initial velocity greater than the velocity of sound in air, which is 1134 feet per second.

When a gun is fired, it is a matter of common experience that when one is in front of the gun and within a certain angle of the line of fire, a very distinct double report is heard.  There is a sharp crack of great intensity, followed at a short interval by a heavy thud.  This effect is very noticeable with guns with a high muzzle velocity, such as the six inch Mark VII.  The first crack is very loud and often distressing to the ear.  The thud which follows has a much deeper note, does not distress the ear in the same way, but seems to have a much greater volume behind it from the way it rattles the windows, shakes buildings etc.  With the six inch Mark VII an Observer about 1500 yards in front of the gun hears the two sounds at about one second interval.

With an 18 pr. gun the two sound follow so quickly that it is often hard to separate them, although they can sometimes be clearly distinguished.  If one is in a building in front of an 18 pr. battery which is firing, the report of the gun is heard a few instants before the building is felt to vibrate.  This indicates the existence of the two reports, only here they are separated by a much shorter interval of time.  The double report is often very noticeable when a German 7.7 c.m. battery is firing.

The two sounds are caused in the following way:-

The second report is in each case caused by the discharge of the gun, it is the true gun report.  It consists of a sound wave of great length and amplitude, so that it is accompanied by big pressure changes, and this explains its effect in shaking buildings, rattling windows, etc.  The first report on the other hand is caused by the shell moving through the air.  It is called by the French the “Onde du Choc” or “Wave of Concussion”.  Just as a boat, which is moving through the water, creates a bow-wave shaped like the letter V, so a shell moving through the air creates a conical bow-wave in the air.  It is this wave which is heard as the heard as the first report, it is a very short wave of great intensity, like a whip crack.  The shell only makes this bow-wave when travelling with a velocity greater than of sound.  As its velocity falls, the vertex of the cone becomes blunter, until finally the velocity of the shell drops below that of sound and it ceases to make a bow-wave or “Onde du Choc”.

Since the shell moves initially with a velocity greater than that of sound, the shell gets well ahead of the wave spreading from the gun, which represents the report of the discharge, and so the Onde du Choc is set up in the air in front of the gun report.  Both then travel together with the velocity of sound.  In this way an Observer in front of the gun hears first the “Onde du Choc” or “Shell wave”, then the report of the gun.  The Onde du Choc is always very much louder than the true gun report; in fact it is only occasionally that the actual discharge of a German gun can be distinguished at all.  What is generally taken for the gun report is in reality the “Onde du Choc”.

This must be considered when an Observer tries to get the range to a German gun by measuring the time between seeing its flash and hearing its report, or timing the interval between the report and shell burst.  When an Observer is directly in the line of fire of the gun the Onde du Choc precedes the shell burst by intervals which vary from ½ second for 7.7 cm. gun firing at a range 4000 yards to six seconds for a 13 cm. gun firing at a range of 8000 yards.  These corrections must be made to the time recorded before exact deductions can be made as to the distance of the gun from the Observer.

All the above remarks apply only to guns whose muzzle velocity is greater than the velocity of Sound.  They do not apply to Howitzers which have a muzzle velocity of less than 1134 feet per second.  The report heard when a howitzer fires is the discharge itself.

The Onde du Choc is only heard by an Observer in front of a Gun within a certain angle of the line of fire.  An Observer, to a flank, or behind does not hear it.  When the wave is set up it moves through the air in a direction at right angles to its wave front, and so is not propagated to a flank or behind the gun.

(Sgd) W.L. Bragg, 2/Lieut.

O.C. Sound Ranging Section.

4/3/1916