218 



COMPOSITION AND RESOLUTION OF FORCE. 



tive force, would be taken equal to C D, and the angle C D H would be equal 

 to C D E. This is found by experiment to be the case when light is reflected 

 from a polished surface of glass or metal. 



Motion is sometimes distinguished into absolute and relative. What " rela- 

 tive motion" means is easily explained. If a man walk upon the deck of a ship 

 from stem to stern, he has a relative motion which is measured by the space 

 upon the deck over which he walks in a given time. But while he is thus 

 walking from stem to stern, the ship and its contents, including himself, are 

 impelled through the deep in the opposite direction. If it so happen that the 

 motion of the man from stem to stern be exactly equal to the motion of the ship 

 in the contrary .way, the man will be, relatively to the surface of the sea and 

 that of the earth, at rest. Thus, relatively to the ship, he is in motion, while, 

 relatively to the surface of the earth, he is at rest. But still this is not abso- 

 lute rest. The surface itself is moving by the diurnal rotation of the earth 

 upon its axis, as well as by the annual motion in its orbit round the sun. 

 These motions, and others to which the earth is subject, must be all com- 

 pounded by the theorem of the parallelogram of forces, before we can obtain 

 the absolute state of the body with respect to motion or rest. 



