r to drift is observable at tennis, golf, base-ball and
cricket; but this effect is explainable by the inequality of pressure due
to a vortex of air carried along by the rotating ball, and the deviation is
in the opposite direction of the drift observed in artillery practice, so
artillerists are still awaiting theory and crucial experiment.

After all care has been taken in laying and pointing, in accordance with
the rules of theory and practice, absolute certainty of hitting the same
spot every time is unattainable, as causes of error exist which cannot be
eliminated, such as variations in the air and in the muzzle-velocity, and
also in the steadiness of the shot in flight.

To obtain an estimate of the accuracy of a gun, as much actual practice as
is available must be utilized for the calculation in accordance with the
laws of probability of the 50% zones shown in the range table (see
PROBABILITY.)

II. INTERIOR BALLISTICS

The investigation of the relations connecting the pressure, volume and
temperature of the powder-gas inside the bore of the gun, of the work
realized by the expansion of the powder, of the dynamics of the movement of
the shot up the bore, and of the stress set up in the material of the gun,
constitutes the branch of interior ballistics.

[Illustration: FIG. 3.]

A gun may be considered a simple thermo-dynamic machine or heat-engine
which does its work in a single stroke, and does not act in a series of
periodic cycles as an ordinary steam or gas-engine.

[Illustration: FIG. 4. Pressure Curves, from Chronoscope Experiments in 6
inch gun of 100 calibres, with various Explosives.]

An indicator diagram can be drawn for a gun (fig. 3) as for a [v.03 p.0276]
steam-engine, representing graphically by a curve CPD the relation between
the volume and pressure of the powder-gas; and in addition the curves AQE
of energy e, AvV of velocity v, and AtT of time t can be plotted or
derived, the velocity and energy at the muzzle B being denoted by V and E.

After a certain discount for friction and the recoil of the gun, the net
work realized by the powder-gas as the shot advances AM is represented by
the area ACPM, and this is equated to the kinetic energy e of the shot, in
foot-tons,

(1) e = {w /2240} (1 + {4k^2 / d^2} tan^2 [delta]) {v^2 / 2g},

in which the factor 4(k^2/d^2)tan^2[delta] represents the fraction due to
the rotation of the shot, of diameter d and axial radius of gyration k, and