OF ARTS AND SCIENCES. 287 



XXII. 



STUDIES IN METROLOGY. 



FIRST PAPER. 



By William A. Rogers. 



Presented May 9th, 1883. 



It will be the aim of the present paper to present a critical study of 

 certain standards of length, which have been compared either directly 

 or indirectly with the original standards, which are now recognized 

 throughout the civilized world as the ultimate and supreme authority 

 in all matters relating to units of length ; viz. with the " Imperial 

 Yard" at London, and with the "Metre des Archives" at the Inter- 

 national Bureau of Weights and Measures, situated at Breteuil, near 

 Paris. 



The second paper will contain a discussion of all the standards of 

 length which have been constructed by the writer from the prototypes 

 investigated in this paper. These prototypes are designated and de- 

 fined as follows : — 



I. The Tresca Meter, having the designation T. 



The bar upon which the defining lines of this meter are traced is 

 composed of pure copper, and has narrow strips of platinum inserted at 

 each end. The bar has the X shape proposed by Professor Tresca, 

 which allows the graduations to be placed nearly in the plane of the 

 neutral axis without interfering materially with their examination under 

 a microscope having considerable magnifying power. 



The platinum surfaces are in the same plane with the surface of the 

 copper, not only at each end, but throughout the entire length. These 

 surfaces are fairly well adapted to receive sharply defined graduations ; 

 but this material is far inferior to platinum-iridium in this respect. 



This bar was placed upon the tracing comparator of the Conserva- 

 toire des Arts et Metiers, on the morning of February 4, 1880. After 

 remaining at a constant temperature for about forty hours, M. Gustave 



