THE NEW NATIONAL STAND AED OF LENGTH, AND ITS PEINCIPAL COPIES. 631 
contained in the Act 5 Geo. IV. cap. 74, sect. 1, ... by which the standard yard ... is 
declared to be a certain brass rod therein specified, is the best which it is possible to 
adopt. With respect to the provisions for restoration, we are not prepared to recommend 
the adoption of that prescribed by section 3 of the same Act, whereby it is dhected that, 
in case of the loss of the metallic standard above-mentioned, the yard shall be restored 
by taking the length which shall bear a certain proportion to the length of the 
pendulum, vibrating seconds of mean time, in the latitude of London, in a vacuum, at 
the level of the sea Since the passing of the said Act, it has been ascertained that 
several elements of reduction of the pendulum-experiments therein referred to are 
doubtful or erroneous : thus it was shown by Dr. Younu, Philosophical Transactions, 
1819, that the reduction to the level of the sea was doubtful ; by Bessel, Astron. Nachr., 
No. 128, and by Baily, Philosophical Transactions, 1832*, that the reduction for the 
weight of air was erroneous; by Baily, Philosophical Transactions, 1832, that the spe- 
cific gravity of the pendulum was erroneously estimated, and that the faults of the agate 
planes introduced some degree of doubt; by Katee, Philosophical Transactions, 1830, 
and by Baily, Astronomical Society Memohs, vol. ix., that very sensible errors were 
introduced in the operation of comparing the length of the pendulum with Shuckburgh’s 
scale, used as the representative of the legal standard. It is evident, therefore, that the 
course prescribed by the Act would not necessarily reproduce the length of the original 
yard Several measures, however, exist, which were most accurately compared ■with 
the former standard yard (in particular the Boyal Astronomical Society’s Scale, described 
in their Memoirs, vol. ix., and the iron bars belonging to the Board of Ordnance, in the 
custody of Colonel Colby,) and by the use of these the values of the original 
standards can be respectively restored without sensible error. And we are fully per- 
suaded that, ■with reasonable precautions, it -will always be possible to pro^vdde for the 
accm’ate restoration of standards, by means of material copies which have been carefully 
compared with them, more securely than by reference to any experiments referring to 
natural constants. With these ^iews we recommend the following course : — 
1. That so much of the Act 5th Geo. IV. cap. 74, as pro^vddes for the restoration of 
the standards in the manner therein mentioned, be repealed ; and that the standard of 
length be defined, in subsequent enactments of the legislature, either by the whole 
length of a certain piece of metal or other durable substance, supported in a certain 
manner, at a certain temperature (to the construction and dimensions of which, further 
allusion will be made in this Eeport) ; or by the distance between two points or hues 
engraved upon the surface of a certain piece of metal or other durable substance, sup- 
ported in a certain manner, at a certain temperature ; but that the standard be in no 
way defined by reference to any natm'al basis, such as the length of a degree of meri- 
dian on the earth’s surface in an assigned latitude, or the length of the pendulum 
\ibrating seconds in a specified place. 
* This reference appears to have been transcribed by mistake from that wbicb follows. It ought to have 
been Sabixe, Philosophical Transactions, 1829. 
4 N 2 
