Page:Popular Science Monthly Volume 19.djvu/674

656 Barcelona, running through Paris, was very carefully measured by Messrs. Méchain and Delambre, and, by comparing their results with measurements that had been made of other arcs, they were able to obtain 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 the 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 Mètre." 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 character, 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 standard bears to the length of the natural unit that was first proposed. It has been shown, by the more recent investigations of Clark and Shubert, 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 use 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 by pro rata contributions from the signing powers. This bureau is charged with the care of prototype standards, and with the duty of constructing and verifying copies of them.

The work of preparing the prototypes devolved upon the French Section 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 apparatus for weighing in a vacuum. It is so arranged that the