232 CONTEMPORARY SCIENCE 



tion concerning the "real" length of the rod. It may have 

 changed, or it may not. It must always be remembered 

 that measurement of the length of a rod is simply a process 

 of comparison between it and an arbitrary standard, e.g., 

 a meter-rod or yard-stick. In regard to the problem of 

 assigning numbers to intervals of time, it must be borne 

 in mind that, strictly speaking, we do not "measure" such 

 intervals, i.e., that we do not select a unit interval of time 

 and find how many times it is contained in the interval 

 in question. (Similarly, we do not "measure" the pitch 

 of a sound or the temperature of a room.) 



Our practical instruments for assigning numbers to 

 time-intervals depend in the main upon our agreeing to 

 believe that a pendulum swings in a perfectly uniform 

 manner, each vibration taking the same time as the next 

 one. Of course we cannot prove that this is true, it is, 

 strictly speaking, a definition of what we mean by equal 

 intervals of time ; and it is not a particularly good defini- 

 tion at that. Its limitations are sufficiently obvious. The 

 best way to proceed is to consider the concept of uniform 

 velocity, and then, using the idea of some entity having 

 such a uniform velocity, to define equal intervals of time 

 as such intervals as are required for the entity to traverse 

 equal lengths. These last we have already defined. What 

 is required in addition is to adopt some moving entity as 

 giving our definition of uniform velocity. Considering 

 our known universe it is self-evident that we should choose 

 in our definition of uniform velocity the velocity of light, 

 since this selection could be made by an observer anywhere 

 in our universe. Having agreed then to illustrate by the 

 words "uniform velocity" that of light, our definition of 

 equal intervals of time is complete. This implies, of 

 course, that there is no uncertainty on our part as to the 

 fact that the velocity of light always has the same value 

 at any one point in the universe to any observer, quite re- 

 gardless of the source of light. In other words, the pos- 

 tulate that this is true underlies our definition. 



