How are cars prevented from running off the track, while turning a curve? The outer rail is made higher than the inner rail, so as to overcome the tendency of the cars to run off the track. What grand example have we of the action of these two forces? The earth and other planets that move around the sun, are kept in their places by the action of these two forces. When a ball strikes a solid body and bounds back, what is that motion called? It is called _reflected_ motion. At what angle does the ball leave the solid body? It bounds off from the solid body, so that the angle of reflection is always equal to the angle of incidence, as is shown in Fig. 39. [Illustration: Fig. 39.] How may a stone be skipped on a smooth pond? A stone may be skipped, by throwing it so obliquely that the water will cause it to bound off from its surface. When a boat moves across a river at the rate of four miles an hour, and the current runs four miles an hour, in what direction will the boat go? The boat will go diagonally across the river. Why will the boat go diagonally across the river? Because the boat will be carried as far down the river as the river is wide. If the river be one mile wide, then the boat, while crossing, will be carried down it one mile. How do boatmen overcome this difficulty? They overcome this by rowing the boat towards a point higher up the stream. Does it take longer to cross a stream when the current is strong? Yes; because the boat is carried farther down the stream by the current, and it must, therefore, go a greater distance in crossing. Why is a person apt to fall when he jumps from a wagon that is moving? Because his body partakes of the motion of the wagon and moves on after his feet are stopped by the ground, so that he falls forwards. How can he prevent falling? He can prevent falling by jumping in the same direction as that in which the wagon is moving. When a stone is dropped from the top of a mast of a vessel, where does it fall? The stone falls at the bottom of the mast. Why does the stone fall at the bottom of the mast? Because it partakes of the motion of the vessel, and is carried forward while falling. When a ball is thrown perpendicularly into the air by a person on a boat, where does the ball fall? The ball falls back into the person’s hand, although the boat may have carried him several feet while the ball was in the air. Why does the ball fall back into his hand? Because it partakes of the motion of the boat, and moves as far as the boat moves. When a stone thrown from a car is aimed at a post, where will it strike? The stone will strike ahead of the post. How far ahead of the post will the stone strike? It will strike as far ahead as the car moves while the stone is going to the post. Where must we aim so as to hit a bird that is flying? We must aim ahead of the bird. Why must we aim ahead of the bird?