64 THE REALITIES OF MODERN SCIENCE 



In speaking of motion we refer the position of one 

 body to that of another. Consider the case of two 

 railroad trains at rest on parallel tracks. We know 

 that it is difficult for a passenger in one train to 

 tell by looking at the other whether his own is starting 

 or not. He may see the other apparently sliding by, 

 but he cannot decide, on visual evidence alone, as to 

 his own motion unless he can see the ground. Even 

 if he is told that he is actually in motion he cannot 

 determine the direction. Visually he can decide only 

 as to whether or not there is a relative motion of the 

 two adjacent trains. This fact may be illustrated by 

 placing two pencils side by side, making a mark on 

 each, and seeing in how many ways one or both may 

 be moved so as to produce the same motion of one mark 

 relative to the other. 



The idea of motion is best expressed more abstractly. 

 Let two points be always connected by a straight line ; 

 if this line changes in length or direction there is a 

 relative motion 1 of the two points. It is always with 

 relative motions that we deal, as in the case of a body 

 falling with reference to the earth. If we knew two 

 fixed and intersecting lines in the universe we could 

 refer to these the positions and motions of all bodies. 

 But what do we mean? fixed with reference to what? 

 We reply, "Why, absolutely fixed," and use "abso- 

 lutely " in its technical sense. It is, however, preferable 

 to consider all motion as relative. 



Recognizing that motion is relative, let us imagine 



1 An interesting exposition of the historical and philosophical 

 aspects of mechanics is that of Mach, "The Science of Mechanics," 

 Open Court Publishing Co., 1907. 



