The morning news. [volume] (Savannah, Ga.) 1887-1900, 1898-04-21
AIMING A BIG GUN.
-ou MOVING TARGET is STRUCK
THOUGH 1T Is NOT SEEN.

Defense.
From the New York Sun.
nh, aiming of a big gun is an exact
..,srre. The nicely of which the iandsman
~ but little. Long-range, high-power
. y.au.d be unable to cope with armor-
r,s,-of-war if gunnery were guesswork.
,ess range and the direction OF the target
romputed by delicate instruments,
Itch make correct allowances for the rise
on5 fall of the tide, the force and direction
- the wind, the curvature of the earth,
"a normal and abnormal refraction of
l,s5, The fight of the projectile is ac-
..r,telv gagged by the aid of range ia
,,.s and delicate machinery for changing
49,. elevation and direction of the gun, and
,...b exact calculations are made of the
tms of the projectile's flight and the speed
anil course of a moving target that an 800
rourd mortar shell, carrying enough gun
.,,ton to sink a ship, may be dropped on
the deck of a ship five miles away. This
&c all done in spite of the fact that the
. sbsu is in the air more than minute, and
.1e shin moves hundreds of yards after
the gun is fired and before the projectile
reaches her An entire fleet of steaming
srmorcads may be destroyed by the guns
,f batteries whose commanders take aim
and fire without seeing a single ship, and
VINO know the position of the fleet only
Il from messages received over telephone
1r telegraph line, from an observer per-
So lie
markable results will be attained at all the
e4scoas. forts of the country when the
20 LeAis depression range finders, re-
| Gently ordered by the government will
have been delivered and set up in the va
| rous coast-defending stations.
| Range finding is merely an application of
the primary principles of surveying. If a
surveyor wishes to find the width of riv
er he lays out a triangle, with a line on
one shore as base and an object on the
opposite shore as the apex. Knowing the
distance between the two points of obser-
ration and the angles made with the base
line by the lines from the observation
points TO the object across the river, he
computes the width of the river. This is
the horizontal system of range finding
adopted by the United States navy, but
now about to be abandoned
Depression finders calculate distance by
uiangulation, the hight of the instrument
above sea level being the base of a right
triangle. Knowing the angle of depression
Of the water line of the object, the dis
lance is a simple matter of computation,
the angle increasing with the approach of
the object. All depression finders are so
made that the movement of the telescope
as the observer follows the target, moves
the range indicator about a graduated
scale, the observer needing only to glance
al the scale TO know the range. No calcu-
lation is necessary The chief advantages
Of the depression position finder are that
but one observing station is needed, and
no use is made of delicate electrical instru-
ments. An instrument giving out, a reserve
instrument may he quickly set up in its
Place. The direction of the target from the
Observing station is shown by a second
Sc IN, graduated into degrees and minutes.
The meridian is the zero point for direction
and horizontal line for depression.
The Lewis depression position finder, the
patents for which have only recently been
ksued, is believed by artillery officers TO
be the most accurate and practical in
arument yet devised for service use. The
English service position finder has maxi
mum error of per cent. up to 1,000 yards,
while the instrument adopted by the war
10 l4,Q00 yards, the normal effective range
ior effective firing from forts. The Lewis
Hsrument consists of a polished iron table
four feet in diameter, resting upon an iron
column, ard supporting a pivoted iron
UaHersing arm, carrying all the other
parts of the instrument In front the arm
clryes upward, furnishing support for the
horizontal axis of a forty-inch telescope,
magnfying twenty-five diameters. The
telescope is, raised and lowered by a crank
operating a finely-adjusted mechanism at
the eye end; and it is swung arounl in
azmuth by a larger crank, turning the
uayersing arm about on its pivot.
The revolving of the traversing arm
nrns the friction roller of the azimuth
Measuring device. Each revolution of the
pller corresponds to 8 degrees of azimuth;
that is, the roller revolves forty times dur
Ing complete revolution of the telescope
about the horizon. The roller has as an
axis vertical shaft, turning two gear
Fneels-one, the degree wheel revolving
Vlth the telescope; the other, the minute
Fhee making a revolution for each de-
gree. This mechanism is behind a square,
|erucal sheet of metal, just under the
eye end of the telescope.
Though one opening in the sheet the
observer reads the degrees and through
dh0lner The minutes of aZimuth. Ine
screw which raises and lowers the tele-
scope meshes with the gear of a horizontal
ahse Wheel a foot in diameter project
Hs from under the table below the ami
~lh reader The wheel has 600 teeth cut
mo Us circumference. A white cellulold
Sheet, one-sxteenth OF an inch thick, is
@t Into the wheel and graduated about
NE circumference IN black. Figures show
hundreds of yards of range, while the
sale reads ten-yard divisions from 1850
'e 900 yards, and 25-yard divisions from
''"p TO 1900 yards. The range wheel is
projected by a cover. a small opening be-
l's made in it to show the range.
Inc instrument is et,up in protected
station on a rise of ground overlooking
'le narbor The higher above sea level
'Ne instrument stands the more accurate
Viii be the readings. The Fort Monroe
instrument has been used at an elevation
u 10 feet While the instrument at the
llf' ~ San Francisco harbor stands 400
..e' above the sea. AT Sandy Hook instru-
menus will be placed on the parapets,
""len average about so feet in hight and
e' in the lighthouse. In setting up the
I"''i great care is taken to make it per-
l'.'y level. so that the travers!ug arm
.AF he swung about without changing the
p'''uon OF the bubble in the spirit-evel it
"IIles When the instrument has beel-
up the azimuth device is fixed
lnat il reads degree when