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sliot roll off to the left. But at tlie same time one half- — 
supposing tlie bullet to be cut in two, in the opposite direction, 
by the plane of the trajectory — is moving 1 in the same direction 
with, and the other half against, the motion of translation. 
That is, in the case just considered, the left half would be 
rotating in the same direction as the motion of translation, the 
right half in the opposite direction ; consequently, the motion of 
the right half would be assisting, and the motion of the left 
retarding, the air in escaping past it. The bullet, therefore, 
constantly creating- and meeting a denser medium on the left 
than on the right, has a tendency to deflect to the right. This 
tendency, which is in the opposite direction, is stronger than 
the one before mentioned, and the ball is deflected to the right 
— in a curve, as the velocity of rotation diminishes less rapidly 
than the velocity of translation.* In the same w r ay the range 
is increased or diminished according as the rotation of the fore 
part is upwards or downwards. 
From these remarks may be gathered the general causes 
which give uncertainty to the direction of the flight of a 
spherical ball from the smooth-bored gun ; viz., uncertain rota- 
tion and the resistance of the air. In the rifle we have a means 
of counteracting these by imparting a constant and fixed rota- 
tion. The rifled barrel, instead of being a plain cylinder, has 
spiral grooves cut in it. The ball fits these grooves, and in being- 
forced out by the powder, is constrained, in the same way as a 
screw in being withdrawn from a nut, to rotate round an axis 
which is the same as that of the bore of the gun. This rotation 
is constant, and the tendency is for the axis of a rotating- body 
to remain parallel to its original direction,! as may be easily seen 
with a spinning-top ; and, moreover, the axis being coincident 
— at all events, during the early part of the flight — with 
the line of the trajectory, the causes of deflection before con- 
sidered do not come into play, while, at the same time, any 
irregularities of surface are brought first to one side and then 
to the other; and any deflections which might be caused by 
them are neutralized. 
Having- now shown how the inaccuracy is overcome, let us 
examine how the range is increased. The obstacle to an in- 
* Robins showed this very well by an experiment with a bent musket- 
barrel ; the ball, instead of following the direction to which the end of the 
barrel was bent, was incurvated in the opposite direction, and crossed the 
direction of the straight portion. The ball, in fact, rubbing against the bent 
portion, received a strong rotation in the opposite direction, and, as explained, 
was deflected. 
t See Mechauical Summary, p. 277, — “Guns and Iron-cased Ships,”— a 
new form of projectile. 
