UNITS AND STANDARDS OF MEASUREMENT. 367 



be sure that even the most solid metallic bars do not 

 slightly contract by age, or undergo variations in their 

 structure by change of temperature. M. Fizeau was in- 

 duced to try whether a quartz crystal, subjected to several 

 hundred alternations of temperature, would be modified in 

 its physical properties, and he was unable to detect any 

 change in the coefficients of expansion 11 . It does not 

 follow however, that, because no apparent change was 

 discovered in a quartz crystal, newly-constructed bars of 

 metal would undergo no change. 



The only principle, as it seems to me, upon which the 

 perpetuation of a standard of length can be ultimately 

 rested, is that, if a variation of length occurs, it will in 

 all probability be of different amount in different sub- 

 stances. If then a great number of standard metres were 

 constructed of all kinds of different metals, alloys ; hard 

 rocks, such as granite, serpentine, slate, quartz, limestone ; 

 artificial substances, such as porcelain, glass, &c., &c., care- 

 ful comparison would show from time to time the com- 

 parative variations of length of these different substances. 

 The most variable substances would be the most divergent, 

 and the true standard would be furnished by the mean 

 length of those which agreed most closely with each other, 

 just as uniform motion is that of those bodies which agree 

 most closely in indicating the efflux of time. 



The Terrestrial Standard. 



The second method assumes that the globe itself is a 

 body of invariable dimensions. The founders of the me- 

 trical system selected the ten-millionth part of the dis- 

 tance from the equator to the pole as the definition of the 

 metre, and the late Sir John Herschel proposed that 



11 'Philosophical Magazine/ (1868), 4th Series, vol. xxxvi. p. 32. 

 ' Familial' Lectures on Scientific Subjects,' (1866) p. 191. 



