TIME AND MOTION 



63 



A was stationary, the bead B was moving 

 toward A with a constant velocity, while the 

 bead C was moving away from A with a con- 



Figure 11 

 A Space-Time Map of Beads on a String 



stantly increasing, or, as we say, with an accel- 

 erated velocity. The collision of beads A and B 

 is an event,^ or a point in space-time, which 



3 It will avoid confusion in our further discussion if we 

 agree to use the word event for any episode which has no 

 considerable extension in space or time. For example, if we 

 were dealing with the modern history of Europe the French 

 Revolution must be treated as affecting a great many 

 square miles of territory over a number of years; but if we 

 were considering the whole history of the solar system, its 

 spatial and temporal extension could be ignored and it 

 could be regarded as an event in this technical sense; or, in 

 other words, as a mere point upon our space-time map. 



