OP ARTS AND SCIENCES. 



339 



Comparison of the Tresca Meter T^'^ with the Line-Meter 



1881. 



F, with Comparator No. 1. 



TH—Fe 



6 = 3.40 /u. 



atO°G. 



T^z — Fe 

 +104.2/* 



+101.1/* 



+ 103.4/* 



+ 102.9/* 



+102.8/* 



+101.5/* 



+102.0 /* 



+102.4 /* 



+101.4/* 



+100.9/* 



(t — 0°)C. 



+16.5 

 +12.3 

 +13.1 

 +15.7 

 +11.5 

 +13.6 

 +10.8 

 +18.7 

 +10.2 

 +12.5 



"We have, therefore, for 0" C, 



T'^ — F, 

 But T'''- — A 



Hence F^ — 0.5 /x 



This relation has an importance far beyond any ordinary comparison 

 of standards, since the centimeter derived from the Froment line-meter 

 is the unit upon which Angstrom's wave lengths depend. It is be- 

 sides the basis of nearly all the later physical investigations under- 

 taken in France. If, therefore, the transfer of this meter to the surface 

 of bar F is assumed to be without error, the correspondence of the 

 whole unit with the Metre des Archives is nearly perfect. 



Comparison between Meters T and R^. 



After my return from Washington, two series of comparisons be- 

 tween these standards were instituted, one series with Comparator 

 No. 1, and the other with the Universal Comparator. In the first 

 series two microscopes were used, being attached to the carriage. The 

 bars wei'e placed at a distance 2 x centimeters apart, and observations 

 were made for the positions -\-x and — x. 



For X ■=. -(-3.0 cm. 



