68 



THE ANATOMY OF SCIENCE 



3. Likewise the spatial distance between two 

 events does not depend upon the particular 

 bases from which a survey is made, or, as geome- 

 ters would say, it is independent of the choice 

 of particular axes of reference. This postulate 

 may be called relativity of position in space. 



0' 



4. If two bodies are in relative motion we 

 may equally well say that either is at rest and 

 the other in motion. This postulate may be 

 called relativity of motion. 



It will be noted that the last three postulates 

 deal with relatives, only the first wdth absolutes. 

 From these postulates certain theorems arise. 

 For example, if B is moving with respect to A, 

 and C in the same direction with respect to B, 

 then the velocity of A with respect to C is the 

 sum of these two velocities; for in the left half 

 of Figure 14 let us represent as before a string 

 with three beads, at two different instants sepa- 



