Chapter V: Laying

=115.= _General instructions._

1. All guns are so mounted that two motions can be given to the axis of the gun, viz., motion in a vertical direction, termed elevation, and motion in a horizontal direction, termed traversing. Elevation is always given by mechanism, traversing is sometimes carried out by mechanism and sometimes by hand.

A gun is said to be “_laid_,” when, by elevating and traversing, its axis is made to point in the required direction.

2. Accuracy and rapidity are the two main factors in good laying. Accuracy is required against stationary targets, while rapidity becomes of more importance against large and moving targets.

3. Instruction in laying should be divided into four stages:--

_1st Stage._--Instructional target at close distance, explanation of terms and rules, &c.

_2nd Stage._--Laying at well-defined natural objects at effective ranges, and continued at less well-defined objects at long and distant ranges.

_3rd Stage._--Laying at moving targets. This can be practised by sending out men, mounted or on foot, to move at the pace and in the direction desired. In order to check the laying they should be halted by signal on the word “_fire_.”

_4th Stage._--Indirect laying.

4. The instructional target will be found valuable in teaching the men of a battery to lay guns correctly and uniformly, as with it personal errors can be shown, both in elevation and direction, and the practical rules for correcting errors can be demonstrated.

5. When laying with open sights, in order to obtain uniform results, one method should be strictly adhered to. The service method of laying a gun is to direct it so that the centre of the imaginary line joining the two highest points of the notch of the hind sight, the point of the fore sight and the target are in line.

The gun should be approximately laid before looking over the sight; the laying should then be completed by depressing the gun and sights on to the target so as to avoid any error due to play of the elevating gear.

The distance of the eye from the sight should be the same from round to round, and all layers of a battery should be trained to keep the eye at the same distance.

6. After the gunner has attained a thorough knowledge of the sights, and can lay accurately and rapidly on well-defined targets, he should be taught to lay on natural objects, such as troops halted and in motion, hedges, batteries, entrenchments, crest lines, at varying ranges and under varying conditions of light, background, &c. The ground line should always be laid on unless orders to the contrary are given.

=116.= _Direct laying._

1. Direct laying can only be employed when the target is visible over or through the sights.

2. With 13-pr. and 18-pr. Q.F. field guns the elevating mechanism is so designed that, after the sights have been directed on the target, the axis of the gun can be moved independently of the sights, and inclined in a vertical direction at any angle to the line of sight, which remains directed on the target. The angle at which the axis of the gun is inclined to the line of sight, and the range due to it, is shown on the range indicator on the right side of the gun. With the independent line of sight, once the sights are laid on the target any angle of elevation can be given without having to re-lay the gun. To lay the gun, align the sights, or telescope, on the target, and adjust the range indicator to the elevation ordered.

With other field guns the sight is set at the elevation ordered, and the gun is laid by directing the sights or telescope on the target.

3. The advantage of direct laying is that, when the gun is laid, the correct angle of sight is automatically included, and complications will not arise in determining the corrector settings or length of fuze.

The disadvantages are the personal error of the layers, and the difficulties due to light and distance; with the telescopic sight, slowness, over-laying for direction, and difficulty in picking up the target.

4. At medium ranges and against moving targets direct laying, or an aiming point as described in Sec. =120=, should be used, according as the formation of the ground and local circumstances render advisable. The telescopic sight is particularly useful against guns in action, trenches, &c., when great accuracy of line is important.

5. When laying direct the sight clinometer should be set level at the earliest opportunity. This enables the laying for elevation to be checked, or a change from direct to indirect laying to be made, should the target become obscured from any cause.

=117.= _Indirect laying._

1. Indirect laying can be employed whether the target is visible over the sights or not, and _is the normal method employed in the field_.

If a change from direct to indirect laying is desired, the order “INDIRECT” must be given, and the layer must immediately pick up an auxiliary aiming point to lay on for direction.

2. _For elevation._--With sights provided with an adjustable level the level is set at the angle of sight, and the range indicator or drum at the elevation ordered. The gun is elevated and depressed until the bubble is level.

With sights not provided with an adjustable level, or with field clinometer, the angle of sight is added to the elevation due to the range, if it is one of elevation, and subtracted if it is one of depression. The sight or clinometer is set at the elevation ordered, the gun elevated and then depressed until the bubble is level.

3. The advantages of laying in this way are that the personal error of the layer is eliminated, and the accuracy of the laying is not affected by light or distance.

The disadvantage is that, if the angle of sight has not been accurately measured and allowed for, the range shown on the range indicator or drum will not be the true range, and complications will arise in determining the corrector setting or length of fuze.

In Fig. 19 let T be the target, G the gun (18-pr. Q.F.), T being on a higher level than G.

If the angle of sight be underestimated, the range on the range indicator will not be the true range.

Supposing true range GT = 3,900 yards.
” ” angle of sight = 2° 45′ elevation.
Angle of sight ordered = 2° elevation.

It follows that, in order to hit T, the range on the range indicator must be 3,900 yards + 45 minutes, or about 4,150 yards.

The fuze, however, required will be that for 3,900 yards, but the battery commander being unaware that he has wrongly estimated the angle of sight, the fuzes will be set for 4,150 yards with the corrector ordered, and all grazes may be expected. The corrector setting obtained will not be a true one for the day, nor will the range be true.

4. _For direction._--Direction is obtained by laying on an aiming point, or on a line marked by two aiming posts. The method of carrying this out is described in Sec. =198=.

When indirect laying is used in the open with guns not provided with the No. 7 Dial Sight, the layer after the gun has been put on the line by the section commander will pick up an auxiliary aiming point with his rocking bar sight or over the dial sight to lay on from round to round. (_See_ Sec. =198=, b, ii.)

=118.= _Angle of sight._

1. The angle of sight can be measured by the sights on the gun, by the level on the director, or by any other angle of sight instrument. Considerable care should be taken in making this measurement, as on it depends the accurate determination of the fuze.

2. When the target is not visible from the guns, or from a position near the guns, the angle of sight can be found by means of the following formula:--

(_a_¹ × BO + _a_² × OT),
---------------------------------
BT

where T is the target, B the battery, and O the observer.
_a_¹ is the angle of sight from B to O.
_a_² is the angle of sight from O to T.

Angles of elevation are read as plus. Angles of depression as minus.

The No. 3 director is designed to work out this problem automatically, excepting when battery, observing station, and target are approximately in line.

3. When indirect laying is employed, and the sights are provided with adjustable levels, the battery commander should invariably order the angle of sight to the battery, both on first coming into action, and on every subsequent change of target, either by giving out a fresh angle of sight, or by raising or lowering the old angle of sight (_see_ para. =4= below).

The battery commander must also order the necessary correction in the angle of sight to allow for the differences in levels between the guns or different parts of the target.

4. Any alteration in angle of sight will be effected by the order “RAISE (OR LOWER) ANGLE OF SIGHT--MINUTES.” When engaging stationary targets and the correct trajectory has been found, it is unwise to alter the angle of sight to correct the height of burst.

At moving targets, however, it is better to alter the angle of sight rather than to vary the corrector, except for small final adjustments.

=119.= _Deflection._

1. During its flight various causes are at work tending to make a projectile deviate to right or left of its initial direction.

To counteract deviation it is necessary to set the line of sight at an angle to the axis of the gun in a horizontal direction. This angle is called “_deflection_.” If the hind sight is moved to the right (called right deflection) the axis of the gun will point to the right of the line of sight and the shot will fall more to the right in accordance with the amount of deflection allowed. Similarly, if the shot is required to go to the left, left deflection must be given.

2. _Difference in level of wheels._--_See also_ Sec. =106=.

The practical rule for making corrections for difference in level of wheels with gun carriages having a wheel track of 60 inches or thereabouts, is: number of inches or degrees difference in level of wheels × number of degrees of elevation on the range indicator or tangent sight = number of minutes deflection to be given on the side of the higher wheel. When possible, it is better to dig in the higher wheel and level the wheels in this way. This also helps to steady the carriage and improve the shooting.

3. _Drift_ is the deviation of a projectile, due to rotation, that occurs in all rifled guns. The deviation is to the right or left according as the gun has a right or left-hand twist. Drift increases with the elevation and range of the gun, and is counteracted either by:--

i. Permanently inclining the sight (15-pr. B.L.C.) or axis
of the trunnions (13 and 18-pr. Q.F.);
ii. The “_set_” of the dial sight (5-inch B.L. Howitzer);
iii. An automatically actuated mechanism in the sight
carrier (4·5-inch Q.F. Howitzer).

4. _Moving targets._--In the case of a target moving across the line of fire, it may be necessary to give deflection; if so, the following is a useful guide:--

If the target is moving at a walk give 20′.
” ” ” a trot ” 40′.
” ” ” a gallop ” 1°.

This should be given by the battery commander.

5. _Wind._--The wind, especially at long ranges, causes a shell to deviate to the right or left. Although the strength of a wind may vary at different points of the trajectory of a shell, allowance can be approximately made for it by observing its strength and direction.

A rough estimate of the deflection per thousand yards in the range necessary to allow for a wind blowing directly across the line of fire is given below:--

15 minutes when the wind is “Very strong.”
10 ” ” “Strong.”
7 ” ” “Fresh.”
5 ” ” “Moderate.”

In the case of howitzers these amounts should be slightly increased.

These corrections decrease as the direction of the wind approaches the line of fire.

Practically each minute of deflection on the sight gives a difference of 1 inch in every 100 yards of range.

6. All officers and N.C.O’s. should know what angles are subtended by various parts of the hand when placed at arm’s length. Thus:--

Thumb and fingers extended, as in Fig. 21 (about 19 degrees), fist clenched (8 degrees), first and second fingers extended (6 degrees), the various knuckles, &c.

For the purpose of instruction a degree scale calculated for a given distance and painted on a wall will be found useful.

7. In ordering deflection it must be clearly understood that the amount ordered is in addition to any deflection that may be already on the sight, for this reason the word “more” is always to be used. Thus:--sight has 25′ right deflection; if “10′ more right” be ordered the sight will be set at 35′ right. Similarly, if “10´ more left” had been ordered the sight would have been set at 15′ right.

=120.= _Aiming points._[9]

1. Aiming points are conspicuous points capable of easy description on which the gun sight or dial sight is laid. They are used for two specific purposes, namely:--

[9] NOTE.--An aiming point must not be confused with a reference point. (_See_ Sec. =186=.)

i. For obtaining the original lines of fire to a target.
ii. For laying on from round to round.

As regards i. the aiming point should be as distant as possible and, with a view to reducing the necessary correction for parallelism, as much in prolongation of the line of guns as practicable.

It should, however, be noted that a well-defined aiming point not in prolongation of the line of guns is better than an ill-defined one which is in prolongation, provided that the correction for parallelism is not forgotten. (_See_ Sec. =122=.)

As regards ii. when the aiming point cannot be laid on from round to round an auxiliary aiming point must be picked up by the layer.

2. The aiming point or auxiliary aiming points must be made known to the section commander and No. 1, and to the other numbers at the gun as opportunity offers.

3. When the lines of fire are laid out by means of an aiming point, then either two aiming posts can be planted in the line of fire and laid on over the rocking bar sight, or the aiming point or an auxiliary aiming point can be laid on through the No. 7 dial sight, or over the No. 1 dial sight if neither the aiming point nor the auxiliary aiming point are within the field of the rocking bar sight.

=121.= _Clearing the crest._

When guns are in action under cover the section commanders are responsible that the shell will clear the crest. Immediately on coming into action the section commanders must report to the battery commander the lowest elevation which will clear the crest with zero angle of sight, if no angle of sight has been ordered. The various methods of carrying out this operation are fully dealt with in the handbooks of the guns.

=122.= _Parallelism of lines of fire._

1. =It is very important that the lines of fire of the guns of a battery should be parallel when first laid out, and every effort should be made to attain this object.= Its commander will then have a definite condition from which to make his calculations, and should be able to switch his guns from one target to another without losing parallelism, to distribute his fire correctly over a given front, or to concentrate it on a given point.

2. When the guns of a battery are laid off an aiming point with the same angle, their lines of fire will not be parallel unless the aiming point is either at an infinite distance, or in exact prolongation of the line of guns.

In Fig. 22 A and B are the positions of the flank guns and CD is the prolongation of the line of guns.

Suppose P is an aiming point which, for simplicity, is taken as the correct line of fire for the right gun, and is at right angles to the line of guns.

If AE is drawn parallel to BP, it will represent the line of fire required for the left gun.

The correction for parallelism for the left gun is consequently the angle EAP, which is equal to the angle APB, _i.e._, the angle subtending the front of the battery at the range of the aiming point.

This position of the aiming point, namely, at right angles to the line of guns, entails the greatest amount of correction for parallelism for an aiming point at any given distance.

If the aiming point is in any other position at the same distance, such as P¹, the angle AP¹ B becomes smaller, as the front of the battery A¹ B becomes foreshortened when viewed from the aiming point. This foreshortening continues until the frontage of the battery disappears, _i.e._, the aiming point is in prolongation of the line of guns, in which case no correction for parallelism is necessary.

3. The correction necessary to make the line of fire of A parallel to the line of fire of B varies with the distance BP, and the angle PBD.

The total correction, divided by the number of gun intervals, gives the amount of distribution or concentration necessary for each gun.

When the aiming point is in front of the line of guns distribution is required; when in rear the same rule holds good, but concentration is required.

4. The following table gives angular corrections for each gun interval for different positions of the aiming point:--

--------------+------------------------------------------------
Angle between | Range to aiming point.
line to aiming|
point and line+------+------+------+------+------+------+------
of guns. |1000ˣ |2000ˣ |3000ˣ |4000ˣ |5000ˣ |6000ˣ |7000ˣ
--------------+------+------+------+------+------+------+------
|Mins. |Mins. |Mins. |Mins. |Mins. |Mins. |Mins.
10 degrees. | 10 | 5 | 5 | 5 | 5 | 5 | 5
20 ” | 25 | 10 | 10 | 5 | 5 | 5 | 5
30 ” | 35 | 15 | 10 | 10 | 5 | 5 | 5
40 ” | 45 | 20 | 15 | 10 | 10 | 5 | 5
50 to 70 “ | 60 | 30 | 20 | 15 | 10 | 10 | 10
80 ” 90 “ | 70 | 35 | 25 | 20 | 15 | 10 | 10
--------------+------+------+------+------+------+------+------

A good rough rule to ascertain the parallelism required is to divide the angle between the line to the aiming point and the line of guns by the number of thousands of yards range to the aiming point. The result is the number of minutes concentration or distribution for each gun.

5. Another method is as follows:--Two guns (preferably the flank guns) after their sights have been laid on the aiming point at the angle ordered, may be ordered to lay on each other and report to the battery leader the angles read. The battery leader then adds them together, subtracts their sum from 180° and the result divided by the number of gun intervals gives the distribution or concentration required per gun.

Thus in Fig. 23, supposing No. 1 gun reports “my angle 15°,” and No. 6 gun reports “my angle 160°,” the battery leader knows that 5° is the total distribution for the battery and that 1° is the distribution per gun.

=123.= _Displacement._

1. _Single displacement._--When the battery leader is unable to place his director in or quite near to the line of guns a correction known as single displacement becomes necessary to compensate for the difference in the angles between the aiming point and the target, measured at the director and at the battery respectively.

This correction is given to all guns in such a way as to bring the lines of fire towards the director.

In Fig. 24 suppose T is the target, B the battery, D the battery leader’s director and P the aiming point. If no allowance for displacement was made the lines of fire would be as much to the left of the target as B is to the left of D.

2. A simple rule for calculating the displacement angle is as follows:--

6 Base in yards
---- × -------------------------- = Displacement angle.
10 Range in hundreds of yards

_Example._--Suppose in Fig. 24 the base (BD) is 200ˣ, the range (DT) is 4,000ˣ, and the angle to the aiming point (PDT) is 79°.

6 200
Then the displacement angle = ---- × ----- = 3°,
10 40

and the battery leader would give out the angle “82° (_i.e._, 79 and 3) Right” to the battery.

If the aiming point had been to the right of the target, the displacement angle would have been deducted from the battery angle.

3. When it is desired to give the line of fire to the battery leader’s director, or to the flank gun, from the observing station and the distance is not enough to warrant the use of the plotter, the above method of computing the displacement angle will be found useful. (_See_ Sec. =198= (2).)

The battery commander will make the necessary correction before sending the angle to the gun or director.

4. _Double displacement._--When the observing station is some distance from the battery, the switch angle measured to a new target will usually differ from that required at the guns. This difference is known as _double displacement_.

5. In Fig. 25, suppose B is the battery, D the director, A the original target, and T the new target.

The angle measured between the original target and the new target at the director will only coincide with that required at the battery, that is the angle ADT will only be equal to ABT, when all the four points happen to be on the circumference of a circle. If T is in any position such as T¹, outside that circle, the angle at D is less than the angle at B; if in any position T², inside the circle, the angle at D is greater than that at B.

6. In practice it is useless to attempt to calculate the difference of the apex angles at A and T. The most accurate method is to work out the problem on the plotter as described in Sec. =198=. The difference between the battery angle and that for the first target will be the switch angle.

If the line to the first target has been found otherwise than by the plotter, and the observing station is some distance from the battery, the battery leader should measure the battery angle to the first target as soon as possible, and communicate it to the battery commander.

With this information the battery commander, after working out the new battery angle, can ascertain the switch angle to order to the guns.

7. If the tactical situation does not admit of this method, or when, owing to a small difference in range between the two targets, the displacement is likely to be small, an approximation may be arrived at as follows:--

Decide whether the line joining the two targets is more foreshortened to the battery or to your own eye. If the former the switch angle will be less than that measured, if the latter it will be greater.

8. Another method is to switch over one gun through the estimated angle to the new target, correct its line by observation of fire, and, when line is correct, to order parallel lines to that gun.

=124.= _Gun layers and fuze setters._

1. In all batteries of horse, field and heavy artillery, a list of qualified layers and fuze setters is to be kept by the battery commander.

2. In order to qualify to be placed on the list of layers, a non-commissioned officer or gunner is to be examined as described in “Instructions for Practice.” All serjeants should also be tested in laying.

3. Every battery should have at least 3 men qualified as layers and 6 men as fuze setters per subsection, exclusive of serjeants. These should be tested periodically.

Any non-commissioned officer or gunner with good sight should be able to pass this test.