the Mercury Unit with the British Association Unit. 23 



Ebonite plugs were inserted in the ends, and readings were 

 taken in at least four positions, by revolving the tube, in each 

 measurement. 



The temperatures of the tube under observation and bar 

 were given by two thermometers lying against each. Mea- 

 surements were made by both observers. 



Table II. — Lengths of Tubes, in terms of brass Metre (by 

 Bartels and Diederichs, Gottingen). 



Tube 



Length, in 



Length, in 



Average. 



Temp, of 



Temp, of 





centimetres. 



centimetres. 



tube. 



bar. 



I. 



127-7593 



1277610 



127-761 



19° O. 



19-5° C 



II. 



129-8690 



1298726 



129871 



18 



18 



III. 



Broken in 



preliminary 



work. 







IV. 



91-4500 



91-4550 



91-453 



19 



18-5 



V. 



181-0100 



1810147 



181-012 



19 



20 



VI* 



1510951 



1511235 



151.109 



193 



18 



151-1049 



1511034 



151104 



24 



24 



The former value applies to determinations 1 and 2 of this 

 tube, the latter to Nos. 3 and 4. 



The temperature was very constant for the separate 

 measurements on each tube and, as it never differed more 

 than two degrees between corresponding observations, the 

 average length is taken at the average temperature. 



By comparison with the steel Rogers's standard, whose 

 length is accurately known, the brass metre bar was found 

 to equal 10O031 centim. at 25° C. ; a result which agrees 

 perfectly with previous determinations, using *000019 as the 

 coefficient of linear expansion. This would make the bar 

 correct at 8°'7 C. 



Weighing. — Schickert weights and balances were used, the 

 former being compared with the glass standard kilogramme, 

 which has been compared with the Berlin standard. 



In order to avoid errors due to moisture and uncertainty as 

 to temperature, the standard and weights • compared were left 

 standing on the scale-pans for several hours before taking the 

 weighings, which were made without opening the case of the 

 balances. The temperature was kept constant, barometer 

 readings taken, and the air was kept dry by calcium chloride. 

 The brass kilogramme (K) was found to equal 1000'001 

 grammes in vacuo, which agrees well with former determina- 

 tions. Specific gravity 8*3. 



* One end became nicked and was smoothed off. 



