rifling. It serves two purposes. The bullet—you use only one—has to screw round and round to get out, and that gives the explosion time to act upon it. It increases the muzzle velocity. Then it gives the bullet a rotary motion, and anything spinning on its axis travels very much straighter than it would do otherwise. It's the twisting motion that keeps a top from falling over."

Frank could readily understand this, and he remembered what he had read about the gyroscope.

"Now," continued Mr. Oliver, "we have to consider the pull of the earth upon the bullet, which would bring it down, and to counteract this you have to direct it rather upward. The slight curve it makes before it reaches its mark is called the trajectory, and it naturally varies with the distance. You arrange it by the sights. There are two of them, one on the muzzle and one near the breach. The last one slides up and down like this. The farther off the mark is the higher it must go. As you have to get them both in line, it's evident that pushing the back one up will raise the muzzle. You can understand that?"

Frank said that he could, and Mr. Oliver pushed the rearsight down and snapped a lever.

[Pg 46]"It's cocked, though it hasn't a shell in it. At a hundred yards or less the sight goes down about the limit." He handed Frank the rifle. "Stand straight, left foot a little to the left and forward—that will do. Now bring the rifle to your shoulder—left hand under the barrel near the rearsight, elbow well down, right hand round the small of the butt, thumb on the top. Try to hold it steady."

Frank found it difficult. The rifle was heavy and the muzzle seemed to want to drop, but Mr. Oliver stopped him when he let his left elbow fall in toward his side.

"Bring it down and wait a moment before you throw it up again," he advised.

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