Page:Popular Science Monthly Volume 79.djvu/217

Rh made of brass, for example, could be trusted to withstand the effect of oxidation during a long period of years, though happily a recent test at the bureau of the celebrated brass "troy pound of the mint" shows that it has not changed in the 83 years since it was received from England by more than 0.005 grain.

The Coast and Geodetic Survey is engaged in extending its measurements across the continent by means of triangulation. Formerly the survey had only about twelve base lines, each from five to ten miles in length, extending 3,000 miles from the Atlantic to the Pacific. These base lines had been measured with the greatest precision and at night to avoid temperature changes. The department is at present engaged in filling in intermediate base lines, which can now be measured readily by means of modern steel and "invar" tapes to one part in a million. Moreover, the work can be done daytimes and with steel tapes under known tension. The bureau compares these tapes, under the same tension and at the temperature of melting ice, with the primary standard of length, the national prototype meter. Thus it comes to pass that our continental surveys from Maine to California, and even to Alaska, are on the same basis of measurement as those made in other parts of the civilized world.

The bureau is charged with the duty of furnishing to the several states commercial standards of weights and measures in common use. It offers its services free to the state authorities and invites to a conference