211. =Motion Disposed to be Straight.=--When a body is set in motion, if it be left to itself--that is, if nothing interfere with its motion--it will move in a perfectly straight line. It requires some interference from some force to bend the motion. You will readily see from the views which I have given you that there never is any motion that is, strictly speaking, straight, because every motion is in some measure compound; that is, each cause of motion is modified in its action by other causes of motion. But we can approximate very nearly to straight motion by making one cause preponderate very much over other causes. This I will illustrate. If we fire a bullet horizontally from a gun it is acted upon by three forces: the propulsive force of the powder, the resistance of the air, and the attraction of the earth. The action of the second of these is in direct opposition to the first, and therefore only retards the motion, and does not tend at all to turn it from its straight course. This is seen in the fact that the ball is turned neither to the right hand nor to the left. But the third force tends to make the ball bend its course toward the ground. It does this from the instant that the ball leaves the gun throughout its flight, but so slightly that practically we can consider the ball as going straight for short distances. When we take a long range we must make allowance for this bending down of the motion. Accordingly, for the sake of precision, a double sight is provided in modern guns, as seen at A and B, Fig. 140. This you see secures the pointing of the gun a little above the level of the object aimed at, that level being indicated by the dotted line. [Illustration: Fig. 140.] [Illustration: Fig. 141.] The greater is the propulsive force the more nearly to a straight line is the path of the propelled body. This may be seen very clearly in Fig. 141, representing the issuing of water at different points from a vessel. As pressure in a liquid is as depth, § 121, the force with which the water is thrust out is greater at C than at B, and at D than at C. The issuing stream, therefore, is most nearly straight at the lowest point, D. The motion of projectiles, thus alluded to, will be more particularly noticed farther on. [Illustration: Fig. 142.] [Illustration: Fig. 143.]