...Railway artillery - Personal Page of GENE SLOVER

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to compute the gisement of the gun-target line. (The gisement of
a line is its angular direction, measured clockwise, with reference to
a north and south line through the zero point of the system of quadrillage.
Azimuth is a somewhat analogous term, but is not exactly
the same, since it is the angular direction of a line also measured
clockwise, but with reference to a north and south line through the
point at which the line is beil.lg considered. Since north and south
lines are not parallel, obviously the azimuth of a line running, e. g.,
north 45° east, is not the same at points 5 miles or even I mile apart.)
577. FINITE AIMING POINT.-If the sighting apparatus is directed
on a visible and fixed reference point, either improvised or natural,
at a finite distance, this point is called a finite aiming point Under
such circumstances the goniometer, or panoramic sight, is always
attached to the gun and is set at the calculated angle. This scheme
of laying is most generally applied to field mounts and to railway
artillery having fixed emplacements.
578. INFINITE AIMING POINT.-If instead of a visible and fixed
reference point an aiming rule is used, we have what is called an
infinite aiming point. An aiming rule is a bar of iron L-shaped in
cross section about a meter and a half in length, which is mounted
on two heavy iron stakes set about a meter apart (pI. 469). Provision
is made in the mechanism on the heads of the stakes for adjusting
the bar both vertically and horizontally. In the outside of one
leg of the bar there is a V groove in which two of the legs of the
small iron tripod holding the goniometer, or panoramic sight, slide.
When mounted on this rule a sight may be moved through a distance
of nearly a meter and a half, all positions of the line of sight
being parallel to each other. The use of an aiming rule, of course,
always involves the use of either another sight or a mirror, adjustable
for horizontal angles, mounted on the gun. The use of the
aiming rule will be described in detail later.
APPIJCATION OF FINITE AND INFINITE AlMlNG·POINT METHODS TO
RAILWAY ARTILLERY.
579. On the basis of methods of traversing and schemes of fire
control railway artillery may be divided into three classes:
Class I: Guns provided with traverse and firing from fixed or
semifixed emplacements on a straight track.
Class 2: Guns provided with limited traverse and firing from
a curved track.
Class 3: Guns having no traverse and firing from a· curved
track.
580. OPERATION OF CLASS I-The emplacements of such mounts
are so designed as to have satisfactory stability throughout the range
of traverse provided. It is obvious that nothing can be secured in
Digitized by Coogle