to be fired horizontally from a common gun. 3 
2. When a rifle is fired at a moderate range, say from 100 
to 300 yards, it is found that the accuracy of fire is maintained 
without any perceptible diminution ; from which it may be 
inferred, that the spiral motion is maintained undiminished 
also. If, again, a rifle is fired at an elevation producing its 
greatest range, which is about 37 j°, its fire will not be so 
accurate as that of a plain barrel. The reason I conceive to 
be, that when the ball is at the summit of its flight, where it 
changes the direction of its course the most rapidly, the 
grooves have not sufficient length to enable the air to act 
upon them while the ball is forming the curve, so as to keep 
the axis and line of flight in one, by which means the position 
of the ball is suddenly changed, the spiral motion lost, and it 
continues to fly afterwards with great irregularity. 
3. According to the calculation made by Mr. Robins, 
vol. i. p. 133, ed. 1761, the resistance of the air to a shot' 
passing through it with a velocity of 1550 feet in a second, 
is equal to 120 times the weight of the shot. Dr. Robison, 
when treating of projectiles, in his Elements of Natural Phi- 
losophy, supposes that this resistance may be increased to 
138 times the weight of the shot. But supposing it to be 
much below what it has been estimated at by either of those 
gentlemen, it is difficult to believe that a quarter of a revolu- 
tion in the barrel can communicate to the ball a rotatory 
motion, which it has to maintain for hundreds of yards 
against this enormous pressure. And as this pressure falls 
obliquely on the grooves of the ball, I believe it to be the 
sustaining cause of the spiral motion. 
4. So easy is it to communicate the spiral motion, that a 
grooved leaden ball will acquire it, in falling perpendicularly 
