THE NEW NATIONAL STAND AED OF LENGTH, AND ITS PEINCTPAL COPIES. G27 
on the upper surface will be greater for a thick bar than a thin one, he proposes as the 
best form for a scale a very thin bar carried by a stout one. 
Philosophical Transactions, 1831, page 345, &c. Captain Kater gives an account of 
a Copy of the Imperial Yard made for the Koyal Society. The principle of construction 
is that just described, the thickness of the thin bar being inch. The Copy (0 — 36) is 
compared microscopically with the Imperial Standard itself. The Copy and the Imperial 
Standard were laid upon a marble slab, carefully made plane. The apparatus used is 
the same as that described in the Philosophical Transactions, 1821. 
This is the last of Kater’s papers on the subject of standard-comparisons. The same 
subjects however had been taken up by others both in England and on the Continent ; 
and results of considerable importance, both as to the methods employed in the practical 
apphcations of standards, and as to the evaluations applicable to the perpetuation of the 
Standard, had been obtained. The following account of these will probably suffice. 
In various parts of Europe, bases have been measured by the use of end-measures. 
But in every instance, the principle has been adopted of not permitting the successive 
measuring-rods to come in contact, but of measuring the interval between their ends. 
In some instances, this has been done by inserting a glass wedge ; in others, by use of a 
fuhl-hehel or multiplying-lever : in all cases, reference is ultimately made in some degree 
to optical observation. 
In the extension of the British survey, bases have been measured with a new appa- 
ratus : of which the following peculiarities deserve notice. First, the Ordnance Survey 
Office have used, as standards of reference for field-measures, two 10-feet iron bars ; each 
bar carrying points for microscopic observation on surfaces sunk to the level of the middle 
of the bar’s depth ; and supported, not on a table or on immoveable supports, but on two 
rollers placed each at the distance of one-foui’th of the bar’s length from one end ; the bar 
being so fixed at the middle as to prevent it from running endways. Secondly, that the 
measuring-bars (which were so constructed as to be sensibly invariable in length under 
different temperatures) carried points for microscopic observation ; and the space between 
the point on the following end of one bar and that on the preceding end of the next 
bar was measured by means of a pair of microscopes firmly connected, but so arranged 
that the distance between their focal points is sensibly independent of changes of tempe- 
rature. In this apparatus therefore the measure is purely optical without any reference 
whatever to contact. 
Subsequently to the introduction of this apparatus, new standard-bars and scales were 
formed (of considerable importance in the present inquiry) and were extensively com- 
pared. First, two iron 3-feet bars marked lA and 2A, bearing points at the middle of 
the depth of the bars, and usually supported on semicircnlar blocks of brass, each placed 
at one-fourth of the bar’s length from one end, were prepared for the Department of the 
Ordnance Survey. These bars were compared immediately with the Imperial Standard 
in the year 1834: the comparisons were not published till the printing (in 1847) of the 
Account of the Measurement of the Lough Foyle Base (see pages 82 and [28] of that 
