THE NEW NATIONAL STAND AED OF LENGTH, AND ITS PEINCIPAL COPIES. 693 
“ 14. It was intended that the permanent rest of each bar, after the termination of 
the comparisons, should be upon a system of eight roller-cylinders, permitting the free 
thermal expansion of the bars, these rollers being carried by lever-frames, so arranged 
that the pressure received by each roller is necessarily one-eighth part of the whole 
weight of the bar, and the distances between the rollers being nearly those which are 
defined by the theory of the fiexure of bars as proper to ensure that the measure between 
two lines (even if cut upon the surface of the bar) shall not be sensibly altered by the 
fiexure of the bar. During the comparisons, however, the bars were floated in quick- 
silver. 
“ 15. Under the direction of Mr. Sheepshanks, and with our approval, the following 
apparatus was prepared for the comparison of the bars. A massive foundation of 
masonry was established in the cellar beneath the apartments of the Royal Astronomical 
Society, in the block of buildings east of the entrance-gateway of Somerset House. 
Upon one part of this foundation were supported stone piers and transoms, which 
carried the comparing microscopes. Upon another part there travelled a large carriage, 
bearing the trough of water which contained the trough of quicksilver in which the bars 
floated. It is not necessary here to describe the places of the thermometers, the con- 
trivances for illuminating the divisions, &c. We desire, however, confidently to express 
our opinion, that no apparatus possessing equal firmness and delicacy with this, or 
ensuring equal immunity from ii'regular thermal expansion, had ever before been used 
for the comparison of standards a traits. 
“ 16. The selection of authorities for the reproduction of the Imperial Standard was a 
matter of considerable difficulty. A careful collation of all the direct comparisons of 
different scales with the Imperial Standard, and of cross comparisons between the 
different scales, presented anomalies which it was impossible to explain. The scale 
which at a late date had been compared with greater care than any of the others, 
namely, the scale of the Royal Astronomical Society, was proved by Mr. Bailt to have 
undergone considerable change. This discovery necessarily took away all authority from 
other scales of a similar construction. The results obtained from Shuckbuegh’s scale, 
according as Katee’s or Baily’s measures are followed, are exceedingly discordant. 
Indeed, Baily himself rejected the result of his own comparisons through Shuckbuegh’s 
scale, as being quite incompatible with the result of direct comparisons with the Impe- 
rial Standard. After an elaborate examination of all these measures, Mr. Sheepshanks 
arrived at these conclusions : — FAst, that no scales ought to be used for restoration of 
the value of the Imperial Standard, except those which had been compared directly with 
the Imperial Standard. Secondly, that among these the only admissible scales are, the 
Royal Society’s brass scale No. 46, and the two 3-feet iron bars of the Ordnance Depart- 
ment. After examination of the reasons adduced by Mr. Sheepshanks, we assented to 
these conclusions. 
“ 17. The definition of a length, as represented by a standard bar, requires a statement 
of the temperature at which the standard bar is to be used. It appears, upon examining 
MDCCCLVII. 4 X 
