...Railway artillery - Personal Page of GENE SLOVER

646
the way of traverse by moving the mount along a straight track.
Since the emplacement is assumed to have satisfactory stability,
there is no objection to the use of' a finite aiming point. If OT,
figure 1, plate 470, is the plane of fire, and the aiming point is P,
and the deflection angle a is known, either from calculation or measurement,
the gun is laid in direction by setting off on the sight an
angle equal to a and traversing the gun until the vertical cross line
of the sight is on P. If it is desired to change the direction of the
plane of fire to OIV, it is only necessary to set the sight at a new
angle equal to a minus h and to bring the sight again on the aiming
point P. The method of infinite aiming point is likewise applicable
to this class of artillery, and the writer recommends its use if time
permits. When the guns are operating at extreme rangelf, and it is
not possible to observe the fire for accuracy, even slight displacements
of the emplacement may throw the plane of fire off the target. Displacement
of the emplacement does not have this effect when an
infinite aiming point is used.
. 581. OPERATION OF CLASSES 2 AND 3.-Guns which are provided
with car traverse usually have so little traverse that it is necessary
to operate them on curved tracks to cover a large target or to enable
them to operate on several targets. Guns which have no traverse
must of course always operate on curved tracks. Any mount which
must be operated on a curved track can not be operated with entire
satisfaction through the use of a finite aiming point. First, it is
inevitable that the track will be displaced from firing, and, second,
it is quite probable that it will be necessary to operate on more than
one target from the same track. When a finite aiming point is used,
a displacement of the track involves a correction in the deflection
angle, and the calculation of this correction is laborious. Further, a
change of target with a finite aiming point, figure 2, plate 470, involves
a calculation in the determination of the new deflection angle
far more laborious than is required when an infinite aiming point is
used, figure 3, plate 470. Although angle e equals angle h, figure 2,
there is no simple relation' between angles a, h, and c, and the computation
of angle c becomes an entirely new trigonometric problem.
In figure 3 it will be seen that the determination of the new deflection
angle c involves only the determination of the angle e. For, angle e
equals angle hi assuming M' N' parallel to MN, angle e then equals
angle d, since angles hand d are equal. (h plus c-o,-c plus d.)
The new deflection angle c therefore equals the old deflection angle a
minus the angle e.
582. PROCEDURE IN DETAIL FOR SLIDING MOUNT ON CURVED
EpI.-Under the head of "Procedure in preparing for action,"
section 5, it was mentioned that in the instructions received by the
commander of heavy artillery to operate on a certain group of targets,
Digitized by Coogle