THE NEW NATIONAL STANDAED OF LENGTH, AND ITS PEINCIPAL COPIES. 641 
of the apparent difference, it becomes necessary to define the temperature near which 
any numerical value of the modulus is supposed to apply. Mr. Sheepshanks, after dis- 
cussing these experiments, fixed upon the following as representing the linear expansion 
for 1° of Faheenheit at temperatures differing little from 62° Faheenheit, the length 
of the bar being 36 inches : — 
Metal No. I (Brass) .... 
inch. 
0-000343 
Gun-metal No. 2 
0-000351 
Gun-metal No. 3 
0-000354 
Gun-metal No. 4 
0-000339 
Brass Tube 
0-000367 
Brass Bar 
0-000350 
Copper 
0-000326 
Steel 
0-000212 
Glass 
0-000178 
Brass of the Imperial Standard . 
0-000355 
I now proceed with the Comparisons of Standards, made by Mr. Baily at his house, 
37, Tavistock Place, with the purpose of ascertaining the state of the materials available 
for the restoration of the length of the lost Imperial Standard. 
Besides the Shuckbuegh Scale (to which, on account of its imperfections and the 
doubts and discordances in its comparisons, little importance was attached), it was 
kno^vn that five existing standards, of superior construction, had been compared with 
the Imperial Standard ; namely, the Eoyal Society’s 3-foot brass scale (constructed by 
Captain Katee), the Royal Astronomical Society’s brass tubular scale, two iron bars 
belonging to the Ordnance Department and preserved in the office of the Trigono- 
metrical Survey, and a 3-foot brass scale (constructed by Captain Katee and similar to 
that of the Eoyal Society), in the possession of a private person. The loan of the last- 
mentioned standard was refused to the Commission ; but I had obtained the four first- 
mentioned from their owners and custodiers, and delivered them on 1843, December 5, 
to Mr. Baily. 
After some prehminary observations, Mr. Baily made the following comparisons : — 
The Royal Society’s scale compared with the middle 3 feet of the Royal Astronomical 
Society’s scale. The Royal Astronomical Society’s scale supported on its rollers, at 
quarter-length distance from the centre. The Royal Society’s scale lying fiat on the 
T-bar of the frame, and nearest to the observer. There appears to have been no difficulty 
in the observation, except from the size of the dots on the Royal Society scale, the 
breadth of each being 77 divisions of the micrometer. The Royal Astronomical Society 
scale is longer than the Royal Society’s scale, — 
