UNITS AND STANDARDS OF MEASUREMENT. 369 



The French Government and the present International 

 Metrical Commission have for obvious reasons decided in 

 favour of the latter course, and have thus reverted to the 

 first method of defining the metre by a given bar. As 

 from time to time the ratio between this assumed 

 standard metre and the dimensions of the earth becomes 

 more and more accurately known, we have the better 

 means of restoring that metre by actual reference to the 

 globe if required. But until lost, destroyed, or for some 

 clear reason discredited, the bar metre and not the globe 

 is the standard. Any of the more accurate measurements 

 of the English trigonometrical survey might in like 

 manner be employed to restore our standard yard, in terms 

 of which the results are recorded ^ 



The Pendulum Standard. 



The third method of defining a standard length, by 

 reference to the seconds pendulum, was first proposed by 

 Huyghens, and was at one time adopted by the English 

 Government. From the principle of the pendulum (p. 353) 

 it clearly appears that if the time of oscillation and the 

 force actuating the pendulum be the same, the length 

 must be the same. We do not get rid of theoretical 

 difficulties, for we must practically assume the attraction 

 of gravity at some point of the earth s surface, say 

 London, to be unchanged from time to time, and the 

 sidereal day to be invariable, neither assumption being 

 absolutely correct so far as we can judge. The pendulum, 

 in short, is only an indirect means of making one physi 

 cal quantity of space depend upon two other physical 

 quantities of time and force. 



The practical difficulties are, however, of a far more 



q Thomson and Tait s Elements of Natural Philosophy/ Part i. 

 p. 119. 



Bb 



