February 13, 191 5 
LAND AND WATER 
HOW NAVAL GUNS ARE AIMED 
By SIDNEY GRAVES KOON 
THE marvellous accuracy of our naval gunners has 
been time and again a cause for self-congratula- 
tion on the part of those of us who never saw a 
naval gun fired. But how many of us know the 
intricate process by which that success is 
achieved ? How many know the complex relations that 
exist between the enemy's speed, his distance from our gim, 
the weight of our shell, the velocity with which it leaves the 
muzzle, the rolling of our ship as it tears through the heaving 
billows ? The certainty that, sooner or later, a dreadnought 
action must take place in European waters lends point to a 
brief study of this subject. 
"M 
FIG. I. 
When a battleship A, Fig. i, fires a shell at a hostile 
ship B, that shell takes a curved path C-C-C, called its 
" trajectory." If the gun is properly aimed the shell lands 
on the target, explodes with a horrid noise, spreads destruction 
round about, and sometimes sets fire to the ship B. If the 
ships are very close together, as was often the case a century 
ago, the path of the shell may be practically a straight line, 
like that shown below the trajectory. Unfortunately for 
this ideal condition of shooting, however, the attraction of 
gravitation acts so persistently upon the shell in its flight 
that the gun has to be aimed well above the point to be hit, 
under penalty of falling far short and burying the shell 
harmlessly in an inoffensive ocean. So the shell starts on a 
course such as that of the upper straight line, from which it 
is gradually pulled farther and farther down as it wings its 
flight across the roUes of water between its gun and the 
enemy's ship. The angle D between the straight line above 
the trajectory and that below it is called the " angle of 
elevation " of the gun. The distance between gun and target 
is the " range." And it is the correct determination of this 
range which is the most difficult part of accurate naval 
gunnery. The greater the range the greater must be the 
angle of elevation ; the Ughter the shell the greater the 
elevation for a given range ; the greater the velocity with 
which the shell starts its journey the smaller may be the 
angle of elevation and, consequently, the flatter will the 
trajectory be. 
But there is another important element, and that is the 
location of the exact target from right to left. If it be 
desired to hit the enemy's mast and the shell actually hits 
something a hundred feet away on either side that is not good 
gunnery. So we have the two things to look out for — the 
gun's elevation to correspond with the exact range and its 
" traverse " to correspond with the location " sideways " of 
the point to be hit. The range is determined simultaneously 
from several positions on the ship, of which one may be K 
in Fig. 2. The method will be described later. 
FIG. 2. 
The traverse is in many cases adjusted by an officer in a 
narrow place in the turret G H. This is the man who fires 
the gun. He stands between the gun F' and the armoured 
wall of the turret, and looks at the enemy through the 
peculiar-shaped telescope E'. What he sees is indicated in 
Fig. 3, where the " cross-hairs " of his telescope are shown 
to be on the forward funnel of the hostile ship. The axis of 
this telescof)e is very accurately parallel to the horizontal 
axis of the gun. And the telescope pierces the heavy armour 
of the turret in the manner shown in order to avoid having a 
small shot or piece of shell come right through into the 
turret if it should chance to land just where the telescope is. 
If, now, our shell could cover instantaneously the distance to 
the enemy, a shell fired from this turret, with the correct 
elevation, would strike the forward funnel. But it takes a 
modern shell 16 seconds to travel the six miles now considered 
a moderate battle range. During 16 seconds the enemy, if 
steaming at 20 knots speed, would have moved ahead 540 
feet. Consequently the shell would strike 540 feet behind 
FIG 3. 
FIG. 6. 
FIG. 7. 
the point at which it was aimed, or perhaps 200 feet behind 
the stern of the target ship. So we see that, just as in 
shooting at a wild duck on the wing, we have to estimate the 
enemy's speed and anticipate him — in this case by 540 feet. 
Now to get the range. Several instruments are in use 
for this purpose, all based upon a simple principle of 
trigonometry. If we know the angle Z, of a right-angled 
triangle. Fig. 4, and know the side M opposite that angle, 
FIG. 4. 
then the side N can be readily computed. In this case M 
is the distance between centres of the mirrors P and P' 
in the instrument, Fig. 5, while N is the range sought. The 
FIG. 5. 
side M is known to the thousandth part of an inch. So it 
remains to measure the angle and thus determine the range. 
The mirror P is fixed at exactly 45° to the axis of the tube 
PP'. A ray of light, entering the instrument at P and 
reflected to the mirror R, is again reflected into the eye- 
piece S, where it forms the lower half of the image in Figs. (> 
and 7. Similarly, a ray of light, entering at P' and reflected 
to R' and thence into the eye-piece, forms the upper half of 
the image. The mirror P' (or sometimes R' instead) is 
adjustable. The amount of movement of that mirror 
necessary in bringing the two halves of the image in Fig. 6 
into correct mutual position, as shown in Fig. 7, may be 
measured to the fraction of a minute of arc. And this 
measure, shown on an ivory scale, tells the officer using the 
" telemeter " — or " stadiameter," as it is variously called — 
just how far away his target is. 
The arc of movement of the mirror is almost inappreci- 
ably small. With a 6-foot " base line " PP', an angle of 
40 seconds of arc (one ninetieth of one degree) represents a 
range of 10,300 yards, or about six miles. An angle of 
30 seconds shows the range to be 13,750 yards. As an angle 
of 29 seconds indicates 14,225 yards it is evident that an 
error so small as one second of arc (the thirty-six -hundredth 
part of one degree) will produce, in estimating this 8-mile 
range, an error of 475 yards, or a quarter of a mile. To 
correct such errors, and at the same time those variations 
due to the action of atmospheric conditions upon the powder 
used and upon the refraction of rays of light passing over 
long distances at sea, " spotters " are employed. Men with 
powerful glasses, stationed in elevated positions from 
which they can watch the fall of shells in the water, verify 
or correct the range as determined by stadiameter and 
telephone their observations to the ordnance officer below. 
In this way it takes only a few shots to locate the exact 
range required, after which hitting the enemy is a mere matter 
of the precision with which these various elements may be 
continued in their several combinations. 
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