656 THE POPULAR SCIENCE MONTHLY. 



Barcelona, running through Paris, was very carefully measured by 

 Messrs. Mechain and Delambre, and, by comparing their results with 

 measurements that had been made of other arcs, they were able to ob- 

 tain the length of the meridional quadrant of Paris. Four iron bars 

 were then prepared, and their ends were ground and polished until 

 they were of the required length, to represent one ten-millionth part 

 of the quadrant, or one metre. One of these is now in the possession 

 of the United States, and it is supposed to be the only one in existence. 

 One of the original bars was chosen as the standard of France, and the 

 metre of the French Archives was made directly from it, and at the 

 same time two other similar bars were made, one of which is the metre 

 of the Conservatoire and tlie other is the metre of the Observatoire. 

 These bars are made of platinum. 



In the year 1870 a Commission was formed at Paris, which is known 

 as the " Commission Internationale du Metre." This Commission, after 

 mature consideration, concluded that the natural unit which had been 

 assumed was far from satisfactory, for reasons which were well set 

 forth in the scientific journals at the time. The Commission therefore 

 declared, as it had full power to do, owing to its international charac- 

 ter, that the metre of the Archives should be perpetuated for ever as 

 the true metre. It thus appears that the French unit is no more firmly 

 established, so far as any natural basis is concerned, than is the English 

 yard. It may be of interest to know what relation the accepted stand- 

 ard bears to the length of the natural unit that was first proposed. 

 It has been shown, by the more recent investigations of Clark and Shu- 

 bert, that the Archives-metre falls short of being true to the natural 

 unit by one fifty -four-hundredth part. 



The iridio-platinum alloy, which the Commission has decided to 

 nse for standards, is " composed of ninety parts of platinum and ten 

 parts of iridium, with an allowance of two per cent, variation more or 

 less." In 1874 an " International Bureau of Weights and Measures " 

 was established at Paris, to be supported hy pro rata contributions from 

 the signing powers. This bureau is charged with the care of proto- 

 type standards, and with the duty of constructing and verifying 

 copies of them. 



The work of preparing the prototypes devolved upon the French 

 Gection of the Commission, but the International Bureau declined to 

 accept the standards that were submitted, on the ground that the 

 platinum-iridium alloy contained about two per cent, of iron, and was 

 not, therefore, of sufficient purity. The work of the Bureau has been 

 delayed on account of this unfortunate circumstance ; but M. Tresca, 

 the secretary of the French section, does not admit the validity of the 

 objections that have been made to the alloy. The buildings of the 

 International Bureau afford great facilities for its work ; in one room, 

 where standards of weight are compared, there is a very perfect appa- 

 ratus for weighing in a vacuum. It is so arranged that the weigh- 



