274 Mr. L. Bell on the Absolute 



subject — " All exact measures of length made now must wait 

 for their final correction until the establishment of the new 

 metric prototype." Short standards of length are in some 

 respects peculiarly liable to error, since they must be com- 

 pared with the subdivisions (often not sufficiently well deterr 

 mined) of secondary standards ; and small sources of uncer- 

 tainty, such as poor defining-lines, slight changes in the 

 apparatus and the like, of course are much more serious as 

 the length is less. 



Fortunately, there were available for the measurement of 

 the gratings two standard double decimetres, which have been 

 determined with almost unprecedented care by Professor W. 

 A. Rogers. They are upon speculum metal; were graduated 

 and determined by Professor Rogers early in 1885, and were 

 purchased by the University late in the same year. They are 

 designated respectively S® and Sjj, and are discussed at length 

 in the ( Proceedings ' of the American Society of Microsco- 

 pists for 1885. 



The bar S x is 23 centim. in length. Near one edge is the 

 double decimetre Sj divided to centimetres, the 5 -centim. 

 lines being triple. S 2 is 27 centim. in length, and graduated 

 in the same way. The defining-lines in both are fine and 

 sharp, and the surfaces are accurately plane. They are stan- 

 dard at 16°' 67 C. ; and from an elaborate series of compari- 

 sons with four different standards, the coefficient of expansion 

 was found to be 



17*946 fju per metre per degree 0. 



Sj and Sg depend for their accuracy on a long series of inde- 

 pendent comparisons with Professor Rogers' bronze yard and 

 metre R 2 , and steel standards whose relation to R 2 was very 

 exactly known. R 2 has been determined by elaborate com- 

 parisons with various standard metres and yards, and is 

 described and discussed at length in the ( Proceedings ' of 

 the American Academy, vol. xviii. The length of the metre 

 was determined, both directly and through the yard, by com- 

 parison with the following standards : — 



I. The metre designated T, copper with platinum plugs, 

 traced and standardized by Tresca in 1880 from the Conser- 

 vatoire line-metre No. 19, which bears a very exactly known 

 relation to the Metredes Archives. 



II. The yard and metre designated C. S. ; brass with silver 

 plugs, belonging to the Stevens Institute. The yard was 

 compared with the Imperial Yard in 1880, so that it is directly 

 and exactly known. It was afterwards sent to Breteuil ; and 



