THE NEW NATIONAL STAND AED OF LENGTH, AND ITS PEINCIPAL COPIES. 658 
think I need give them little trouble for some time. But I think it would be better 
that, for the present, they should confine themselves to the metal. 
“ If experience should show that there is a manifest advantage in some other metal, I 
■wdll apply to them again for direction. 
“ With regard to the authority for length, I must own that I do not think the com- 
parison Avith former measures (except the 10-foot and 6-inch Ordnance bars) of much 
value; or that the Parliamentary Standards Avill be of much use. But I rely upon 
being able to furnish at small trouble and no great cost, measures which shall have a 
given relation to a certain measure within 5- 5 - ooo th of an inch, and which can be per- 
petuated without much risk. I expect to produce a standard yard per day, by devoting 
an hour and a half or two hours to the readings of one bar and the adjustment of a 
second in its box ; which second is to be compared next day, or in 48 hours, and have 
its place taken by a third.” 
The Committee then inspected sundry apparatus under the guidance of Mr. Sheep- 
shanks. 
First, in one of the chambers of the floor next below the ground floor, they in- 
spected the comparing apparatus of the Eoyal Astronomical Society ; being that which 
Mr. Bailt had proposed to use for the Parliamentary Standards, &c., and to which 
attention is called in Mr. Sheepshanks’s communication. 
Secondly, they saw the vacuum apparatus for pendulum vibration, consisting of a 
large brass truncated cone supported by three spreading claws, with proper windows 
below for observations of vibrations, and Avith proper agate planes at the top for the 
support of the knife-edges : the cap closing the top was also exhibited. 
Thirdly, in one of the Auults two storeys beloAv the ground floor, they saw the prin- 
cipal parts. of the neAv comparing apparatus. Upon a large low pier surmounted by a 
stone are planted tAvo stone pillars, carrying a connecting top stone, and two stones 
crossing it ; to these crossing stones the microscopes are attached ; upon the platform 
there travels a large three-Avheeled carriage or moveable platform, upon which is placed 
the large box (intended to contain the Avater, &c. by Avhich temperature will be given), 
in which is a drop-box intended to carry the bars, each in its proper case and supported 
by its proper lever-frame. The illumination is effected by an Argand lamp placed at a 
considerable distance, whose light is directed by proper reflectors, 
The Committee then resumed. It was then 
Resolved, — That the Committee approve entirely of the course followed by Mr. Sheep- 
shanks, and request that Mr. Airy Avill take measures for the examination and discharge 
of the instrument-maker’s accounts. 
The Committee then considered the question of material for the Standard. Mr. Airy 
read a priAnte letter from Mr. Faraday, of which the following is an extract : — “ I do 
not see any reason Avhy a pure metal should be particularly free from internal change 
of its particles, and on the whole should rather incline to the hard alloy than to soft 
copper, and yet I hardly know why. I suppose the labour would be too great to lay 
MDCCCLVII. 4 Q 
