1826.] Philosophical Transactions for 1826, Parts I. and I L 61f 



inches from the centre of the disk, and being made slightly- 

 conical, a plug of brass was ground in the hole so as to fit it 

 perfectly. A gold disk was let into the top of the plug, and 

 reduced to a level with the surface of the scale. The other end 

 of the plug projected beneath the scale, and had a small hole 

 through it to admit a wire, by means of which it might be turned 

 round. A very fine deep dot was then made by Mr. Dollond 

 upon each of the gold disks, as nearly as it could be done at the 

 distance of thirty- six inches from each other, the dot upon the 

 moveable disk not being exactly in its centre. 



'* Before the plug was ground in its place a small hole was 

 drilled through the side of the scale into the conical aperture. 



" The microscopical apparatus employed on the present occa- 

 sion, has been described in the paper upon the comparison of 

 various British standards of linear measure, before quoted. 



" The cross-wires of the microscopes being brought respec- 

 tively over zero, and thirty-six inches upon Sir G. Shuckburgh's 

 scale, the apparatus was transferred to the new standard, and the 

 intersection of the cross-wires of one of the microscopes placed 

 upon the centre of the fixed dot. The moveable dot was then 

 brought, by turning the brass plug, to the intersection of the 

 cross-wires of the other microscope. 



" The distance of the dots was repeatedly compared with Sir 

 G. Shuckburgh's standard upon different days, in order to ascer- 

 tain tliat no perceptible error remained. A drill was passed 

 through the hole in the side of the scale, and the brass plug 

 carefully pierced through ; a pin was then driven into the plug 

 so as to render any change of position impossible, and the pro- 

 jecting part of the plug was cut off. 



*^ The standards being thus finished, they were again compared 

 with Sir G. Shuckburgh's scale, and it was with surprise and 

 disappointment that 1 found the whole of them apparently too 

 short. They had been adjusted upon a board of mahogany care- 

 fully planed, and the table upon which they were now placed was 

 so flat as to occasion little alteration in a spirit-level passed 

 along it. The error of the standards was, however, far too con- 

 siderable to be attributed to any curvature which on this occasion 

 could take place, and it was not until after several days that I 

 discovered the cause of this perplexing circumstance. 1 found 

 that by placing a card, the thickness of which was accurately 

 one-fiftieth of an inch, under the middle of the standard, the 

 distance of the dots was much increased, and by placing a card 

 of the same thickness under each of the extremities, and with- 

 drawing that which was under the centre, the distance of the 

 dots was considerably diminished. The total difference 

 amounted to no less than -0016 of an inch, whilst the double of 

 the error which would have arisen from mere curvature under 



