666 
ME. AIET’S ACCOUJvT OF THE COXSTEUCTIOX OF 
in. 
“ Eoy’s scale, 36 inches =GKAHAii’s Tower Yard E— 0‘00047, which is 
pretty well. 
“ But a mean yard of Eamsden’s bar=GEAH.oi’s Tower Yard E+0-00114 
“ Eoy’s yard, according to Katee, is about 0“"0009 longer than the Imperial Standard. 
“ In fixing the Imperial Standard, ILa.tee very A^isely, I think, dismissed aU the sup- 
posed units of the Survey. Eamsden’s scale is lost ; and I have not used Eoy because 
it has no independent value, — professing merely to be a copy of Geaha]m’s Tower Yard ; — 
because the comparisons made of it by Shuckbuegh are very indifferent and inaccui-ate ; 
and because those by Katee in 1821 are inferior in number, and very inferior in quahK, 
to those made by him in 1831 between the Imperial Standard and the Scale of the 
Eoyal Society, which I consider his most complete work in linear measurement. 
“Appendix No. 2. 
“ The Boyal Astronomical Society's Scale and the Tubular Scales discussed. 
(The purport of these remarks has already been giverr.) 
“ Appendix No. 3. 
“ Comparisons between Brass Bar 2 and other Scales." 
(These are now unnecessary.) 
Mr. Sheepshanks rrow commerrced comparisons of Brass 2 with several Bronze bars, but 
Avas much distressed by the imperfect definition and varying position of the defining lines, 
as viewed under the nricroscopes. Numerous attempts Avere made to correct these faults ; 
at lerrgth, recourse Avas had to arrother construction Avhich he called “ Split-Plug.” 
Instead of cutting the line upoir the end of a gold pin, a plug of gold, included in one 
of bronze, Avas split longitudinally ; the surfaces of separation Avere smoothed ; the tAvo 
hah'es of the plug (each half consistiirg of tAvo metals) Avere then rrnited by pins ; the 
compound plug was forced from beloAV into a hole of the bar ; and its errd AA'as ground 
off level Avith the bottom of the Avell, and Avas polished. The diAision of the gold plug 
presented a well-defined dark line, scarcely affected by change hr the dkection of the 
light. It appears to have been fh-st used on 1849, March 12. But about the same time 
arrother improvement Avas made, namely, the use of pierced glass prisrrrs instead of 
annular mirrors, for reflecting the light of the lamps upoir the defining lures. The 
custom Avas also introduced of interchanging the lamps after every series of obseiwations. 
These innoA^ations seem to have been so far satisfactory, that no desh’e aaas afterwards 
entertained to extend or retain the use of the Split-Plug. 
Split-Plug A was however compared fourteen times (fiA-e observations in each series) 
Avith Brass 2 between 1849, March 12, and May 14; the mean gave 
At temperature 49°T, Split-Plug A = Brass 2-f 0"®''"1044. 
One revolution =0‘'''-00370. 
