MEASUREMENTS I39 



How do we measure time? 



Tortunately for us,' says Karl Pearson, 'we are not 

 compelled to measure it by a description of the sequence 

 of our states of consciousness. There are certain sense- 

 impressions which in our experience repeat themselves 

 identical to themselves, and which correspond, on an 

 average, to the same routine of consciousness. In the first 

 instance, the alternation of night and day has been employed 

 since the first ages of the history of man to register approxi- 

 mately the same sequence of sense-impressions. A day 

 and a night became the measurement of a certain interval 

 of consciousness. We feel but cannot prove that the same 

 amount of states of consciousness can be approximately 

 contained for a normal human being in each interval of a 

 day and a night. It is a matter of experience rather than a 

 demonstrated fact. 



'Many identical things happen at intervals of identical 

 time,' continues Pearson, from whom we will borrow the 

 following exceptionally clear lines. 'When we say it is four 

 hours since breakfast, we mean in the first place that the 

 large hand of our clock or watch has gone round the dial 

 four times, a repeated sense-impression which we could, if 

 we please, have observed. But how shall we decide 

 whether each of these four hours represents equal amounts 

 of consciousness, and the same amount to-day as yesterday? 

 It may possibly be that our time-keeper has been compared 

 with a standard clock regulated perhaps from Greenwich 

 Observatory. But what regulates the Greenwich clock? 

 Briefly, without entering into details, it is ultimately regu- 

 lated by the motion of the earth around the sun. Assuming, 

 however, as a result of astronomical experience, that the 

 intervals day and year have a constant relation, we can 

 throw back the regulation of our clock on the motion of 

 the earth about its axis. We may regulate what is termed 

 the "mean solar time" of an ordinary clock by "astronomical 

 time" of which the day corresponds to a complete turn of 

 the earth on its axis. Now if an observer watches a 



