366 THE PRINCIPLES OF SCIENCE. 



same length, or even two bars bearing a perfectly defined 

 ratio to each other, is beyond the power of human art. If 

 two copies of the standard metre be made and declared 

 equally correct, future investigators will certainly discover 

 some discrepancy between them, proving of course that they 

 cannot both be the standard, and giving cause for dispute 

 as to what magnitude should then be taken as correct. 



If one invariable bar could be constructed and main- 

 tained as the absolute standard, no such inconvenience 

 could arise. Each successive generation as it acquired 

 higher powers of measurement, would detect errors in 

 the copies of the standard, but the standard itself would 

 be unimpeached, and would, as it were, become by degrees 

 more and more accurately known. Unfortunately to con- 

 struct and preserve a metre or yard is also a task which 

 is either impossible, or what comes nearly to the same 

 thing, cannot be shown to be possible. Passing over the 

 practical difficulty of defining the ends of the standard 

 length with complete accuracy, whether by dots or lines 

 on the surface, or by the terminal points of the bar, we 

 have no means of proving that substances remain of in- 

 variable dimensions. Just as we cannot tell whether the 

 rotation of the earth is uniform, except by comparing it 

 with other moving bodies, believed to be more uniform 

 in motion, so we cannot detect the change of length in a 

 bar, except by comparing it with some other bar sup- 

 posed to be invariable. But how are we to know which 

 is the invariable bar? It is certain that many rigid 

 and apparently invariable substances do change in di- 

 mensions. The bulb of a thermometer certainly contracts 

 by age, besides undergoing rapid changes -of dimensions 

 when warmed or cooled through 100 Cent. m Can we 



m "Watts' 'Dictionary of Chemistry/ vol. v. pp. 766, 76?. Dr. Joule 

 has recently confirmed the statements concerning the contraction of a 

 thermometer-bulb. 





