2 Lieut. Col. Miller's description of a percussion shell , 
In a ball fired from a rifle, these imperfections are corrected 
by the spiral or rotatory motion of the ball round its axis, by 
which means any inequalities that may exist in its sides are 
continually shifting round the axis during the flight, and the 
course of the projectile is continued straight forward. 
The spiral motion itself has hitherto been generally sup- 
posed to be communicated to a rifle ball, by the grooves 
which are made in the barrel of the gun. The following 
reasons, however, incline me to think that the spiral motion 
is given to the ball, not only by the direction it receives from 
the grooves of the barrel at the moment of discharge, but 
also by the action of the air upon the indentations in the ball 
itself, which the grooves of the barrel have made, when the 
ball is pressed against them in loading. 
i. The grooves of a rifle run obliquely, making from ~ 
turn or twist, to 1^ in the length of the barrel. The diameter 
of the ball being somewhat greater than that of the bore, the 
ball requires to be driven down with considerable force, fol- 
lowing the grooves, and receiving their impression as it 
descends. The ball therefore is a male, and' the barrel a 
female screw, and unless the ball receive the impression of 
the grooves, it never acquires the spiral motion. This is 
ascertained by firing balls into a bank of hard earth, so as to 
flatten them a little. Unless the indentations made by the 
grooves of the rifle are distinctly perceptible round the edges 
of the balls, proving them to have struck end foremost, they 
never hit the mark with the necessary accuracy. If the balls 
are too small, and it is attempted to remedy this defect by 
using very thick patches, however tight they may be rammed 
down, they never answer the purpose intended. 
