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.