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CHAPTER II 



OBSERVATIONS OF CHANGE OF POSITION 



16. Relative Position. The simplest kind of change observ- 

 able in nature is change of position, and the simplest observ- 

 able instance of this occurs when two material particles change 

 their position with regard to each other. A simpler case cannot 

 be observed. We are unable to perceive a change of position 

 in space except as occurring between two bodies at least, or be- 

 tween two parts of the same body. Two bodies are always needed 

 for the perception of movement, rough or exact measurements 

 being made from one to the other. For this cause we are not 

 conscious of the rapid movement of the earth, except by 

 reference to some other body in space. One or more material 

 bodies, coming under any kind of observation for investigation 

 or measurement, will be called a material system. In investi- 

 gation of any change, it is important to exclude from the 

 system all unnecessary members, while including all that are 

 essential. All research proceeds by gradually eliminating the 

 non-essential elements of a change, or by including more and 

 more members within the region of measurable change. 



The relative positions of two material particles may be 

 represented by a diagram (fig. 5), where the length of the 

 straight line A B drawn from A to B represents the distance 

 of B from A. By agreeing to represent a distance in space 

 of one metre by a length of line of one centimetre, or by any 

 similar agreement, the diagram becomes a plan drawn to 

 scale. 



The relative positions of any number of material particles, 

 that is, the configuration of any system, may similarly be 

 represented by ascertaining in this manner the distance of 



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