STANDARDS OF LINEAR MEASURE. 
361 
caused Mr. Dollond to reduce the thickness of the bar for the distance of an 
inch and three-quarters from its extremities to one-half ; the gold disks and 
plugs were then inserted as before, and the adjustment completed in the man- 
ner which has been described. The plugs being secured and the projecting 
parts removed, the standards were repeatedly compared with Sir George 
Shuckburgh’s scale (the standard being placed upon the scale), when no per- 
ceptible difference could be detected. Pieces of card were now placed under 
the standard as before, without occasioning any appreciable alteration ; and I 
had thus experimental proof of the perfect efficiency of the remedy I had 
employed*.” 
It will now be necessary for me to state some circumstances which led to 
the experiments I am about to detail. 
In the year 1820, I very carefully compared a scale, now in the possession of 
Mr. Dollond, with Sir George Shuckburgh’s standard, when the distance 
from zero to thirty-six inches upon both scales appeared to be precisely the 
same. In the year 1824, I again compared Mr. Dollond’s scale with Sir 
George Shuckburgh’s, upon the occasion of examining a scale for M. Schu- 
macher and another for M. Svanberg. Mr. Dollond’s scale by these compa- 
risons appeared to differ from Sir George Shuckburgh’s only .000181 of an 
inch in defect. I had thus good reason to suppose the value of Mr. Dollond’s 
scale to be well ascertained ; and as it was much thinner than that of Sir 
George Shuckburgh’s, I considered it preferable for determining the value of 
the scales intended for Denmark and Sweden. 
Having recently received from Mr. Dollond a scale which I had instructed 
him to make for the Government of Russia, and also the Imperial standard 
yard from the House of Commons, I commenced the necessary preparations 
for comparing them together. 
Finding by stretching a thread along the table, which I have always used for 
such comparisons, that its surface was concave, I had it carefully planed until 
the thread no longer indicated any irregularity. My first comparisons at once 
showed a considerable difference between the scale intended for Russia (copied 
from Mr. Dollond’s scale) and the Imperial standard yard ; at the same time 
that a scale which I had formerly laid off for my own use from Mr. Dollond’s, 
* Phil. Trans, for 1826, Part II. p. 44-46. 
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