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);