i88 THE GRAMMAR OF SCIENCE 



how should we know it was losing time, and how measure 

 the amount ? It might be replied that we should find 

 that the year had fewer days in it ; but then how could 

 we settle that it was the day that was growing longer and 

 not the year that was growing shorter ? Again, it may 

 be objected that we know a great number of astronomical 

 periods relating to the motion of the planets expressed in 

 terms of days, and that we should be able to tell by com- 

 parison with these periods. To this we must answer that 

 the relation of these periods expressed in days, and in 

 terms of each other, appears now indeed invariable ; but 

 what if all these relations are found to have slightly 

 changed a thousand or five thousand years hence ? Which 

 body shall we say has been moving uniformly, which 

 bodies have been gaining or losing ? Or, what if, the 

 ratios of their periods remaining the same, they were 

 all to have lost or gained ? How shall we, with such a 

 possibility in view, assert that the hour to-day is the 

 " same " interval as it was a thousand, or better perhaps a 

 million, years back ? Now certain investigations with 

 regard to the frictional action of the tides make it highly 

 probable that the earth is not a perfect time-keeper, nor 

 are we able to postulate that regularity of motion, by 

 which alone we could reach absolute time, of any body in 

 our perceptual experience. 



Astronomy says it is not in me, nor do we get a more 

 definite answer from physics. Suppose an observer to 

 measure the distance traversed by light in one second ; 

 can this be for all time a permanent record of the length 

 of a second ? Another observer a thousand years after 

 measures again the distance for one of Jiis seconds, and 

 finds it differs from the old determination. What shall 

 he infer? Is the speed of light really variable, has the 

 planetary system reached a denser portion of the ether, 

 has the second changed its value, or does the fault lie with 

 one or other observer ? No more than the astronomer 

 can the physicist provide us with an absolute measure 

 of time. So soon as we grasp this we appear to lose 

 our hold on time. The earth, the sole clock by which we 



