296 
MINUTES OF PROCEEDINGS OF 
tiles; and the resistance of the air to these being less than to 
spherical projectiles of the same weight, an increased range is 
obtained. 
Many systems have been advocated for imparting this rotatory motion 
—some affecting both the gun and projectile, and some the latter only. 
None of the systems affecting the projectile alone have been practically 
adopted, and all nations use guns and small-arms rifled upon various 
systems. 
All systems of rifling have certain principles in common, and certain 
conditions which must be fulfilled. 
The conditions to be fulfilled are—that the velocity of rotation of the 
projectile must be sufficient to keep it steady to the end of its flight ; 
that the projectile should be centred at the moment of its leaving the 
bore of the gun, so that the rotatory motion may be given accurately 
round the axis of figure of the projectile—or an eccentricity of rotation 
would ensue, which would cause the resistance of the air to act more on 
one side than the other, and cause irregularity of flight; and that the 
axis of the projectile should be stable on leaving the gun. 
In estimating the amount of twist to be given to the rifling in a gun, 
the following principles must be borne in mind. 
Solid shot require a greater velocity of rotation, and therefore a 
sharper twist, than shell of the same weight; because in the latter the 
distribution of the mass is nearer the circumference of the projectile, 
and therefore the radius of gyration is increased. 
With the same calibre, the sharpness of twist of the rifling must 
increase with the length of the projectile; as in a long shot the 
resistance of the air has a greater tendency to turn the shot over on its 
shorter axis, on account of the greater leverage with which it acts. 
The less the density of the material of the projectile, the greater must 
be the twist; as a light projectile loses its velocity of rotation quicker 
than a heavy one. 
The centre of gravity of a projectile should be as near its centre of 
figure as possible, to ensure the greatest steadiness of flight. 
The greater the initial velocity, the greater the velocity of rotation 
that will be required, as the resistance of the air will be greater; but 
with a certain amount of twist, these two velocities bear a constant ratio 
to one another, and as one increases, in the same proportion does the 
other also increase, and no sharper twist in the rifling will be required 
for an additional initial velocity. 
The shape of the head of the projectile will also influence the amount 
of twist that must be given; for in the case of a flat-headed projectile, the 
resistance of the air acts almost at right angles to the flat head, and 
must therefore exert a much greater effect than in the case of a hemi¬ 
spherical or pointed head, when the air has merely to part and flow 
round it. 
When we, therefore, wish to decide upon the amount of twist we 
propose to give to the rifling of a gun, we must consider the most 
unfavourable projectile we wish to throw from the gun, and give a 
sufficient twist to keep it steady in its flight to the extreme range, and 
then we may be sure that it will be sufficient for every case. It would 
