Fig. 1
This curved line is called the trajectory.
The resistance of the air and the force of gravity (the force that pulls all bodies toward the earth) are the two things that make the path of the bullet a curved line, just the same as they make the path of the baseball thrown by the player a curved line.
The resistance of the air holds the bullet back and the force of gravity pulls it down, so that the two acting together make the bullet's path curved.
The longer the range the more will the path of the bullet (the trajectory) be curved, as shown by the following drawing:
Fig. 2
[1350]. Sighting or aiming. Now, on the rifle there are two "sights,"—the front sight and the rear sight,—which enable the rifleman to regulate the path of the bullet, as the ball player regulates the path of the ball.
If the ball player wants distance, he throws the ball high (raises the path, the trajectory), using his eye and guesswork, and likewise if the rifleman wants to shoot at a distant target, he, too, shoots the bullet high (that is, he raises the muzzle of his rifle), but he doesn't have to depend upon guesswork. It is all worked out for him by experts and all he need do is to set the rear sight for the proper range,—that is, for the distance the object is from him.