652 
ME. AIET’S ACCOTIN'T OF THE CONSTErCTIOX OF 
that the freezing-point rises in time to its old place, to be again displaced by a fresh 
boiling. 
“This difficulty I propose to meet thus. I take properly graduated thermometers 
which are of some age, and of which the freezing-points are carefully ascertained. I 
compare these with standard thermometers which have just been verified, at tempera- 
tures as high as I can get by natural means (say as high as 70° or 80°). Then those 
(^. e. the old thermometers) are good so long as the freezing-point does not alter ; and 
I expect that it will not alter, for these thermometers will not be exposed to severe 
changes of temperature. If it should alter, the correction of the freezing-point will 
be common to all the readings. I intend to make these comparisons by putting the 
standard and other thermometers into the iron trough, and reading with the telescope. 
“ The experiments for expansion will be made for those temperatures which naturally 
occur for such experiments, and not much higher. 
“ What I have now to ask is, that you approve or disapprove of what I have done, 
and sanction the payment of the expense incurred, except so much as refers to the ther- 
mometers. This is not very large, and is as yet incomplete ; nor can it be said whether 
some portions of the apparatus may not belong rather to the artist than to Her Majesty’s 
Government. 
“ I now come to the point on which I particularly require direction. 
“ I intend to inquire into and to leave means for inquiring into the best material for 
a standard bar. But this will be a work of time. The question which presses is, what 
material will you adopt nowl Mr. Baily wished to have bronze, and a number of 
bronze bars are made. I have had half a dozen bars of several materials, \iz. cast steel, 
cast ii'on, Swedish iron, and Low Moor ii’on : I ordered some of copper, but the casting 
failed; Mr. Donkin has however given me a copper bar. I propose to keep all the 
bases of any measure which may be made in the hands of the Committee, so as to have 
the whole subject accessible, and to preserve the power of completing it. 
“ If the Committee will specify the material, my mode of proceeding will be as fol- 
lows. Suppose that cast steel were proposed I should take a steel bar, mark the diHsions 
determining a yard pretty nearly, find its expansion, compare it with other authorities, 
and then define the yard in terms of it. 
“ Having thus done, I should direct the artist to make me five copies, juutatis 
mutandis, as near as might be to the definitive yard. These copies would be the 
Standard and four Parliamentary copies A, B, C, D. I should now make a 's ery careful 
comparison of the primary Standard with my generator, and also ufith A, B, C, D. 
The Standard and A, B, C, D may now go to the Government for legislation or adop- 
tion, and for conservation. The generator I should retain, and upon this should found 
all my future inquiries, and in fact all copies for publication. The process is the same 
whatever metal is employed, and in each metal I propose to have a scale and a pen- 
dulum, for perpetuation and for comparison Avith other measui’es. 
“ If the Committee will determine as to the metal and the authority for the length, I 
