SPACE AND TIME 187 



matter rather of experience than of demonstration ; it 

 cannot be proved, — it can only be felt. 



Very much the same holds for the smaller intervals of 

 time. 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-face 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 conscious- 

 ness, 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 

 regulated by the motion of the earth round its axis, and 

 the motion of the earth round the sun. Assuming, how- 

 ever, 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 so-called circumpolar star, or one that remains 

 all day and night above the horizon, it will appear, like 

 the end of his astronomical clock-hand, to describe a circle ; 

 the star ought to appear to the observer to describe 

 equal parts of its circle in equal times by his clock, or 

 while the end of the clock-hand describes equal parts 

 of its circle. In this manner the hours on the Greenwich 

 astronomical clock, and ultimately on all ordinary watches 

 and clocks regulated by it, will correspond to the earth 

 turning through equal angles on its axis. We thus throw 

 back our measurement of time on the earth as a time- 

 keeper ; we assume that equal turns of the earth on its 

 axis correspond to equal intervals of consciousness. But, 

 all clocks being set by the earth, how shall we be certain 

 that the earth itself is a regular time-keeper? If the 

 earth were gradually to turn more slowly upon its axis, 



