UNITS AND STANDARDS OF MEASUREMENT. 359 



lutely no mode of defining a length, but by selecting some 

 physical object exhibiting that length between certain 

 obvious points as, for instance, the extremities of a bar, 

 or marks made upon its surface. 



Standard Unit of Time. 



Time is the great independent variable of all change, 

 that which itself flows on uninterruptedly, and brings the 

 variety which we call life and motion. When we reflect 

 upon its intimate nature, Time, like every other element of 

 existence, proves to be an inscrutable mystery. We can 

 only say with St. Augustin, to one who asks us what is 

 time, ' I know when you do not ask me/ The mind of 

 man will ask what can never be answered, but one result 

 of a true and rigorous logical philosophy must be to 

 convince us, that scientific explanation can only take place 

 between phenomena which have something in common, 

 and that when we get down to primary notions, like those 

 of time and space, the mind must meet a point of mystery 

 beyond which it cannot penetrate. A definition of time 

 must not be looked for ; if we say with Hobbes c , that it 

 is ' the phantasm of before and after in motion,' or with 

 Aristotle that it is 'the number of motion according to 

 former and latter ; ' we obviously gain nothing, because 

 the notion of time is involved in the expressions before 

 and after, former and latter. Time is undoubtedly one 

 of those primary notions which can only be defined physi- 

 cally, or by observation of phenomena which proceed in 

 time. 



If we have not advanced a step beyond Augustin's acute 

 reflections on this subject d , it is curious to observe the 



c ' English Works of Thos. Hobbes,' Edit, by Molesworth, vol. i. p. 95. 

 d ' Confessions/ bk. xi. chapters 2028. 



