iCOMPOUND MOTION. 37 



by the stroke of its wings, is able to support its weight in the 

 air. If the force with which it strikes the air below it, is 

 equal to the weight of its body, then the re-action of the air^ 

 upwards is likewise equal to it, and the bird being acted 

 upon by two equal forces in contrary directions, will rest be- 

 tween them. If the force of the stroke is greater than its 

 weight, the bird will rise with the difference of these two 

 forces ; and if the stroke be less than its weight, then it will 

 sink with the difference. In the act of rowing, the water is 

 struck with the oars, in a direction opposite to that in which 

 the boat is required to move ; and the boat, is driven along 

 by the reaction of the water on the oars. 



Questions. — 1. When is a body in motion ? 2. What is force ? 

 3. What are the motive powers ? 4. In what direction is the motion 

 of a body acted upon by a single force ? 5. What is velocity ? 6. To 

 what is the velocity of a moving body proportioned ? 7. How do you 

 calculate the velocity of a moving body ? 8. What is uniform motion ? 

 9. Accelerated? 10. Retarded.'' 11. Why cannot perpetual motion 

 be produced by art ? 12. When a stone falls from a height, how does^ 

 gravity accelerate its motion .' 13. What is said of the distances 

 through which heavy bodies fall in successive seconds of tim* ? 14. 

 What is an instance of retarded motion .'' 15. What is the momentum 

 of a body.' 16. Of what composed.'' 17. Why is it so important 

 with respect to mechanics ? 18. Wliat is meant by the term reaction ? 

 19. To what is reaction equal ? 20 . Explain the manner in which 

 birds support themselves in the air. 



LESSON 19. 



Compound Motion. 



Projec'tile, impelled forward in a right line. 

 Horizon'tal, parallel to the horizon, on a level. 

 Oblique', not direct, not perpendicular, not parallel. 



If a body be struck by two equal forces in opposite direc- 

 tions, it will not move at all ; but if the forces, instead of 

 acting on the body in opposition, strike it in two directions 

 inclined to each other, it will follow the direction of neither 

 of the forces, but will move in a line between them. There 

 are many instances in nature, of motion produced by several 

 powers acting at the same time. If a ship at sea sail before 

 the wind directly east, and a current set from the north, it 

 will be driven in a direction between the south and east. 

 4 



