THE BUREAU OF STANDARDS 



213 



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 Eng- 

 land bv more than 0.005 grain. 



The Coast and Geodetic Survey is engaged in extending its measure- 

 ments 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 stand- 

 ard 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 con- 



A Corner in an Electrical Laboratory. 



