THE METRIC SYSTEM. 723 



Sir John Herschel claimed, first, that the metre was not ex- 

 actly the ten-millionth of the terrestrial meridian passing through 

 France, which was entirely correct, and that, therefore, it was not 

 a good unit for international use, which does not at all follow. 

 He further attempted to show that the polar radius of the earth, 

 which could never be known except indirectly, was a better unit 

 than the quadrant, a large part of which could be measured 

 directly, and that this radius differed by only eighty-two yards 

 from 500,500,000 English inches. He then proposed to increase 

 the English standard by its one-thousandth part, so as to furnish 

 what he declared would then be " a system of linear measurement 

 the purest and most ideally perfect imaginable." It has always 

 been a surprise that so able a mathematician and astronomer 

 could have overlooked the inherent weakness in such an argu- 

 ment. To those who have followed the history of this subject it 

 is unnecessary to say that for many years no metrologist has 

 thought for a moment of relating the standard of length accu- 

 rately to any terrestrial dimension. The precision of our knowl- 

 edge of the figure and dimensions of the earth, now and for many 

 years to come, is such as to forbid this, even if there were no 

 other arguments against it. In the light of current geodesy Sir 

 John's calculations themselves furnish a curiously interesting 

 proof of this. The argument with which he opposed the metre 

 may to-day be turned with equal force against his proposed 

 " ideally perfect " inch. According to the latest determination of 

 the polar radius of the earth, his eighty- four yards become 

 more than one thousand yards, and if his scheme had been 

 adopted when proposed it would have been as badly "out of 

 joint " with Nature as is the metre. 



The simple facts are that while in the beginning the metre was 

 made to be as nearly as possible one ten-millionth of the meridian, 

 no one imagined that it could be exactly so, or rather that we could 

 ever know that it was exactly so. It is sufficiently near that value 

 to be very convenient in calculations relating to terrestrial dis- 

 tances and areas, but it must always be considered as defined by 

 a material standard, and no metrologist ever thinks of it in any 

 other sense. Within a few years Michelson has devised a method 

 of measuring light waves with an accuracy hitherto unthought 

 of, and has measured the length of several such waves in terms 

 of the international prototype metre at Paris, so that we have 

 the metre related to what we may assume to be an invariable 

 dimension in Nature, with a degree of accuracy extremely satis- 

 factory at the present time. But this does not alter the fact that 

 it is and must be regarded as an arbitrary unit represented by a 

 material prototype. Keeping this fact in mind, it will not be 

 necessary to point out the total irrelevancy of HerscheFs argument. 



