UNITS AND STANDARDS OF MEASUREMENT. 3G5 



spring could be observed with the requisite accuracy. We 

 thus appear to be devoid of any hope of establishing a 

 sure standard of the efflux of time. 



The Unit of Space and the Bar Standard. 



Next in importance after the measurement of time is 

 that of space. Time comes first in theory, because pheno- 

 mena, our internal thoughts for instance, may change in 

 time without regard to space magnitude. As to the phe- 

 nomena of outward nature, they tend more and more to re- 

 solve themselves into the motion of molecules, and motion 

 cannot be conceived or measured without reference both 

 to time and space. 



Turning now to space measurements, we find it almost 

 equally difficult to fix and define once and for ever, a unit 

 magnitude. There are three different modes in which 

 it has been proposed to attempt the perpetuation of a 

 standard length. 



1 i ) By constructing an actual specimen of the standard 

 yard or metre, in the form of a bar. 



(2) By assuming the globe itself to be the ultimate 

 standard of magnitude, the practical unit being a sub- 

 multiple of some dimension of the globe. 



(3) By adopting the length of a simple pendulum, 

 beating seconds as a standard of reference. 



At first sight it might seem that there was no great 

 difficulty in this matter, and that any one of these methods 

 might serve well enough ; but the more minutely we 

 inquire into the details, the more hopeless appears to be 

 the attempt to. establish an invariable standard. We must 

 in the first place point out a principle not of an obvious 

 character, namely, that the standard length must be defined 

 by one single object 1 . To make two bars of exactly the 



1 See Harris' 'Essay upon Money and Coins,' part ii. [1758] p. 127. 



