ELEMENTS OF SCIENCE 



FIG. n. 



dropped from the top of the mast of a ship in rapid 

 motion will fall on the same spot as it would do 

 were the ship at anchor. For it participates in the 

 onward motion of the ship, and this horizontal im- 

 pulse prevents its being left behind by the motion of 

 the ship during the time of its descent. 



It may also be illus- 

 trated by the impulse 

 given by a billiard cue 

 to a ball B, by caus- 

 ing ifc to strike against 

 the cushion of a bil- 

 liard table at the point 

 X (Fig. n). As we 

 have seen, this force, 

 represented by the line 

 X from a to X, may be 

 resolved into two forces 

 represented by a b and 

 a c. Their combined 

 action (represented by 

 the diagonal aX.) would 

 bring the ball to the 

 point X. There the 

 force Xc would, by 

 the third law of motion, cause a reaction by the 

 cushion on the ball, tending to drive it back along the 

 parallel line Xc. For only the force db has acted on 

 the cushion, while the force ac has met with no resist- 

 ance. This last force, then, is still in full operation, 

 and acting together on the impulse Xc, carries the ball 

 to the position d. On comparing the diagonal BX with 

 the diagonal Xc? we see that the angle BXc equals 

 the angle cXd, or, in other words, that " the angle of 



