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CAPTAIN KATER ON THE ERROR IN 
and which might be taken as equal to it, differed very little from the Russian 
scale. I now procured Sir George Shuckburgh’s scale and Mr. Dollond’s, 
and set myself carefully to investigate the cause of this difference. 
It was immediately evident that Sir George Shuckburgh’s scale still nearly 
agreed with the Imperial standard yard, and that there was no great difference 
between Mr. Dollond’s scale, my scale, and that intended for Russia. But the 
difference between Mr. Dollond’s scale and Sir George Shuckburgh’s was so 
great, as to leave no doubt on my mind that in determining the value of the 
former, I had committed some considerable error ; and as other scales are 
dependent upon this, it became an object of importance to ascertain its origin 
and amount. The origin of this error I could not for a moment hesitate in 
pronouncing to be the thickness of Sir George Shuckburgh’s scale ; the very 
great care I have been accustomed to bestow in comparisons of this kind, 
leaving me no other cause to which it could with probability be referred. 
I have said that a thread stretched along the table indicated no irregularity 
of surface. I now examined the surfaces of the Imperial standard yard by 
stretching a thread along that surface which was perpendicular to the table. 
Every face of this bar was perceptibly concave ; yet when laid upon the table 
and pressed horizontally at one end, it moved about its centre, proving that 
the surface of the table was convex, though the thread was not capable of 
indicating it. 
I next procured a marble slab nearly sixty- four inches long. This I preferred 
in its unpolished state ; as the operation of polishing being performed upon 
small portions of the marble in succession might destroy the plane surface 
procured by grinding. Upon this slab I placed the Imperial standard yard, 
Sir George Shuckburgh’s scale, my own, and Mr. Dollond’s scale; the 
Russian scale being laid, for want of room, upon the table. The Imperial 
standard yard seemed now to rest with nearly its whole surface in contact 
with the marble, and this, in addition to the test of the thread, I considered 
to be a sufficient indication that the marble was plane. 
The following comparisons were then made between the different scales, in 
which the same microscopes and apparatus were employed as are described in 
an account of the comparison of various British standards of linear measure, 
published in the Philosophical Transactions for 1821 . It is to be observed that 
