542 



Transactions of the Society. 



filar micrometer with an invar screw having a pitch of £ mm. With 

 tins combination one division of the micrometer equals 0-4725 /a. 

 Twenty readings were made on each end of each space, with the 

 following results : — 



The arithmetical mean of the corrections arising from these 

 thirtv measurements 1 >v the writer is as follows : — 



Finally, combining the results of the measurements by Mr. 

 Keeley and the writer, as above recorded, giving them equal 

 weights, we have : — 



These differences are again insensible. 



From a series of sixteen comparisons, comprising eighty readings 

 on each end of the first fa mm. of " Powell and Lealand No. VIII.," 

 with the mean of 400 readings on the first five ^ mm - of " Zeiss 

 No. II.;' a glass millimetre ruled similarly to " Powell and Lealand 

 No! VIII.," which (" Z. IL") in turn had been carefully compared 



