694 
ME. AIET’S ACCOUNT OF THE CONSTEUCTION OF 
the amount of the expansion of a metallic bar for a given change of temperature, that 
the effect of a change of 0°-01 or 0°-02 Faheexheit is sensible in the measure of the length 
of a bar ; and it was therefore necessary to employ methods of ascertaining the tempe- 
rature, which should be certain to that degree of accuracy. Xo thermometer in 
England, at the time of commencing these experiments, could be trusted to such a 
nicety. Mr. Sheepshanks, therefore, commenced his labours by forming a system of 
original thermometers ; determining independently the freezing-point, the boiling-point, 
and the volume of every section of the tube of each. This process occupied a consider- 
able time. To these new thermometers the thermometers employed in the former com- 
parisons of the Imperial Standard with the Royal Society’s scale and the Ordnance bars 
were referred, and the comparisons were reduced anew ; and the relations between the 
lengths of the adopted authorities and the Imperial Standard were ascertained with all 
the accuracy which the comparative rudeness of construction of the Imperial Standai-d 
permitted. The thermometers to be used in the details of the new comparisons were 
also referred to the new thermometers ; the thermometrical expansions of the various 
bars employed for the new standards, or of similar bars, for temperatm-es not differing 
materially from 62° Faheenheit, were ascertained ; and the whole of the thermometrical 
determinations were placed on a satisfactory basis. 
“ 18. Very numerous comparisons were made by Mr. Sheepshanks of the adopted 
authorities with bars which he called “ Brass 2 ” and “ Split-plug A ” (the dirision or 
trait in the latter being not a cut on a flat face of metal, but the line produced by 
forcing two small surfaces of metal into contact, and then cutting by a ti’ansversal sec- 
tion through both pieces of metal ; the section of the contact-plane then exliibits a deli- 
cate line). There were, however, discordances in the latter comparisons which pre- 
vented him from giving full confldence to the results. At length a better mode of 
illuminating the lines was devised, and then the whole of the preceding work was 
rejected, and new observations were begun. By means of these, the lengths of bars 
called Bronze 12 and Bronze 28 were determined in relation to the lost Imperial 
Standard, and the latter was used as a basis of reference for other bars. 
“19. The Report of 1841 recommended (Article 5) that, besides the legal standai'd. 
there should be prepared four copies, to be deposited in places to be aftem’ards deter- 
mined. Mr. Sheepshanks therefore proceeded to make a series of comparisons of 
Bronze 28 with numerous bars, in order to select from them flve bars which, in respect 
of the distinctness of their engraving, their floating evenly in quicksilver, and their near 
approach to the length of the lost Imperial Standard, might seem well adapted to be 
taken as Parliamentary Standard and Parliamentary Copies. When the observations 
had been carried to an extent which, it was supposed, would be perfectly satisfactory, it 
appeared that the results of comparison by different observers were sensibly different. 
An extensive series of new observations by numerous observers was at once commenced. 
The discussion of these observations showed clearly, as had been suspected before, that 
there is a difference among the results of different observers far exceeding the uncertainty 
