T1, a point below the target T. Impact would occur at x if the line GT is the surface of the ground. Hence we would get a burst on graze until the corrector had been raised to draw the burst past the point x of the trajectory. This so shortens the range as to make the projectile ineffective.

The effects of the errors are greatly exaggerated in the figures. The nature of the effect from such errors, however, is more forcibly impressed upon the mind. Very slight errors would cause perhaps no appreciable loss of effect from shrapnel.

At medium ranges a change of 4 mils in the angle of site produces a change of about 100 yds. in range. If the angle of site used is too great the range is increased, and if the error is the other way it is shortened.

It must be constantly borne in mind that error in angle of site lengthens or shortens the range, and causes bursts on graze or too high bursts and bursts beyond the target.

(t mes / nick of sett yd:

CHAPTER V

CORRECTOR

Let GOT (Figure 1) be the trajectory for the range GT. The perpendicular O,O to the highest point of the trajectory is the maximum ordinate of the trajectory. Angle d is the angle of departure and angle ƒ is the angle of fall.

If the fuze is set for the range GT by means of the fuze setter, theoretically the projectile would burst at T, and a burst on graze would result; from the very nature of shrapnel the effect of the projectile would be in a great measure lost. Now if the conditions of the atmosphere were such that the powder train of the fuze burned more rapidly than under normal conditions, or if the fuze were so defective as to cause the bursting charge to explode prematurely or before the projectile reached T, the burst could only occur at some point on the trajectory. The highest point at which it could explode would be O. Under normal conditions then, with the fuze set for a given range, the projectile would burst at the point of impact or at T. The fire with shrapnel would be practically ineffective, as it has not the power of impact which an explosive shell has and yet there would be no dispersion of pellets, the very object for which the shrapnel is designed.

In order to secure the desired result a device is attached to the fuze setter by means of which the fuze may be so shortened as to cause the projectile to burst at a point where the maximum effect from the dispersion of the pellets will be had upon the target. In other words the fuze is altered or corrected.

Let GT'T (Fig. 2) be the trajectory for the range GT. It has been found by tests that the maximum effect upon T will be secured when the shrapnel bursts in front of and about 3 mils higher than the target. Therefore, the fuze is corrected so as to cause explosion at T', 3 mils above T. The normal correction then causes a three-mil height of burst. On the corrector scale attached to the fuze setter the normal correc

FIG. 2.

Every

tion is marked 30, the scale running from 0 to 60. point below the normal lowers the burst one mil and every point above 30 elevates the burst one mil on the trajectory. It is obvious that the course of the projectile is not altered, for the trajectory is determined by the elevation of the piece. Nothing that could be done with the fuze would change the elevation of the piece, and however short the fuze is set the projectile cannot burst higher than its trajectory.

In determining the range of a target it is obviously necessary to burst the projectile as near T as possible, for the puff of smoke resulting from the explosion is the indicator or flag which is waved in front of or behind the target, signaling the error in the range. If the signal is given right at the target the accuracy of the range is easily observed, for the flag or puff either obscures or is partially obscured by the target, whereas if the puff occurs in front of and above the objective, as it does at the normal height of burst, it is difficult to tell

whether a high burst, however effective it may be, is just beyond, immediately over, or at the proper interval short. Hence in ranging, or in adjusting the fire, instead of the normal corrector which gives a 3-mil height of burst, a corrector 2 points lower is used to secure a 1-mil height of burst. If the range and the angle of site are absolutely correct, the fuze perfect, and atmospheric conditions normal, corrector 30 gives a mean or average height of burst of 3 mils, and corrector 28 a one-mil height of burst. If, however, conditions are such that a 1-mil height of burst is secured with corrector 30, then 32 gives the 3-mil height of burst and 32 will have to be employed instead of 30 in firing for effect.

Now, if the mean height of burst is adjusted by trial shots at one range the same corrector is used for other ranges, provided no error in angle of site is made for the latter. Of course

3 m.

1 m.

FIG. 3.

the bursts will be higher for longer ranges as the lineal value of the mil in the vertical plane increases with the range. Thus, at 3,000 yards the mean height of burst is 3X3 yards=27 feet; and at 4,000 yards it is 3×4 yards=36 feet. And so a mean height of burst of 1 mil for these ranges would be 9 feet and 12 feet respectively. (Figure 3.)

It is seen then that an increase of n mils in the corrector setting makes a corresponding increase of n mils in the height of burst; a decrease of n mils in the corrector setting makes a corresponding decrease of ʼn mils in the height of burst.

The height of any particular burst may be measured by means of the B. C. telescope. The mean height of a salvo may also be estimated with considerable accuracy, if the bursts as they occur are noted with respect to the horizontal lines in

the field of view of the telescope, and an average is then made. The middle line indicates the normal height of burst; the upper line twice the normal height.

The observer must also be trained to estimate by eye the height of a single burst or the mean height of a salvo.

When the mean point of burst is at the height appropriate during the adjustment (1 mil), from one-half to one-fourth of the shrapnel may, on account of the error of the fuse, be expected to burst on graze. Similarly, when the mean point of burst is at the height appropriate during fire for effect (3 mils), about one-tenth may be expected to burst on graze.

A check is thus afforded on the adjustment of the height of burst, provided a considerable number of rounds fired with the same fuze-setting are observed.

In the accurate adjustment of time fire not only the height but also the interval of burst is important; for projectiles bursting too far in front of the target and those bursting in the air above it produce little or no effect. The interval of burst is correct when both the range and the height of burst are correctly adjusted. Indications that such is the case are: (1) That the bursts on graze bracket the target; (2) that a due proportion of the projectiles burst on graze; (3) that the mean height of burst is about 3 mils; (4) that fragments from the time bursts strike the ground both in front and rear of the target, and that the pattern made by these fragments (as revealed by the dirt and dust knocked up) is close and dense rather than greatly extended; (5) that obvious effect is produced upon the target.

If doubt exists as to the interval of burst it is best to diminish the corrector and get a group of low bursts and bursts on graze.

Observers posted well to the flank of the line of fire may be of the greatest assistance in determining and correcting errors in the interval of burst.