THE NEW NATIONAL STANDAED OE LENGTH, AND ITS PEINCIPAL COPIES. 647 
mentary Standard representing the Imperial Standard Yard. The composition of this 
bar is not known. The defining points were large and irregular dots (upon gold pins, 
melted out by the fire of the Houses of Parliament), which did not admit of accurate 
observation. This bar, and other bars compared with it (Sir George Shuckbuegh’s, the 
Eoyal Society’s, and Colonel Colby’s), were usually laid for comparisons (it is believed) 
on a flat surface, in which arrangement, as is now known, the metal is in constraint, and 
the comparisons are liable to considerable error. On the completion of the Eoyal Astro- 
nomical Society’s scale, it, as well as other scales of similar construction, were compared 
with the Parliamentary Standard: though these comparisons were more accurately 
made than those akeady cited, yet the mounting of these scales is liable to some objec- 
tion, and sufficient attention was not given to their thermometric expansions at the 
temperatures in which they were commonly used. A more serious difficulty is this : 
when, for the scales of the construction similar to that of the Eoyal Astronomical 
Society, the same method of supporting is used as was used in their first comparisons, 
the difference between the lengths of the different scales is not found to be the 
same as it was at those ffi’st comparisons ; the discordance bemg 5-wowf^ inch, or 
perhaps twice the probable error of one micrometer-reading. Moreover, the intervals 
between divisions engraved on four sides of the Eoyal Astronomical Society’s scale do not 
preserve the same relation as formerly, the discordance being about r o ^ooo th of an inch. 
The difference between the lengths of Colonel Colby’s bars appears to be sensibly the 
same as it was formerly ; but there are other objections to the accuracy of comparisons 
derived from these bars : they are of different metal from the Standards ; theh expan- 
sions have not been ascertained ; their thermometers do not correspond ; they were not 
properly supported. It is (in Mr. Sheepshanks’ opinion) an advantage that they are 
made of a pure metal. Eeferences to the determinations of the length of the pendulum 
may be rejected at once, not only because they are inaccurate, but also because they are 
in fact expressed in terms of Shuckburgh’s scale, and not in terms of the Parliamentary 
Standard. Mr. Sheepshanks considers therefore that our materials for restoration are 
generally unsound ; and that with our endeavours to give the best evidence in our power 
as to the past, we must combine, what is now more important, the discussion of methods 
for avoiding similar difficulties for the futm’e. For this purpose it appears necessary to 
investigate anew the questions of stability and thermometric expansion of standards, and 
to devise better methods of comparing standards than have hitherto been used. And in the 
pursuit of these inquhies Mr. Sheepshanks proposes to reject all considerations but those 
of accuracy and permanence ; to adopt the most complete and convenient mode of experi- 
menting, regardless of time, trouble in preparation, and (to a certain degree) of expense. 
Mr. Sheepshanks then gave the following sketch of his proposed plan of operations : — 
“ The place of experiments to be in the lower cellars of Somerset House, in the apart- 
ments of the Eoyal Astronomical Society. 
“ The supports of the microscopes to be built of large squared stones. 
4 p 2 
