:J08 THE PRINCIPLES OF SCIENCE. 



the English inch, which is now almost exactly the 

 5 00,5 00,000 th part of the polar axis of the earth, should 

 be made exactly equal to the 5OO,ooo,oooth part, and be 

 adopted as our standard. The first imperfection in such 

 a method is that the earth is certainly not invariable in 

 size; for we know that it is superior in temperature to sur- 

 rounding space, and must be slowly cooling and contract- 

 ing. There is much reason to believe that all earthquakes, 

 volcanoes, mountain elevations, and changes of sea level, 

 are evidences of this contraction as asserted by Mr. Mallet P. 

 But such is the vast bulk of the earth and the duration 

 of its past existence, that this contraction is perhaps less 

 rapid in proportion than that of any bar or other material 

 standard which we can construct. 



The second and chief difficulty of this method arises 

 from the vast size of the earth, which prevents us from 

 making any comparison with the ultimate standard, ex- 

 cept by a trigonometrical survey of a most elaborate and 

 costly kind. The French physicists, who first proposed 

 the method, attempted to obviate this inconvenience by 

 carrying out the survey once for all, and then constructing 

 a standard metre, which should be exactly the one ten 

 millionth part of the distance from the pole to the 

 equator. But since all measuring operations are merely 

 approximate, as so often stated in previous pages, it was 

 impossible that this operation could be perfectly achieved. 

 Accordingly it was shown by Colonel Puissant in 1838, 

 that the supposed French metre was erroneous to the con- 

 siderable extent of one part in 5527, the quadrant of the 

 earth's circumference measuring 10,001,789 instead of 

 10,000,000 of such metres. It then became necessary 

 either to alter the length of the assumed metre, or 

 otherwise to abandon its supposed relation to the earth's 

 dimensions. 



i' ' Proceedings of the Royal Society,' 20th June, 1872, vol. xx. p. 438. 



