44 Captain Kater's account of the 



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 

 occasion 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 s6 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 intersec- 

 tion 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 ascertain that 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 impossi- 

 ble, and the projecting part of the plug was cut off. 



The standards being thus finished, they were again com- 

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

 surprise and disappointment that I found the whole of them 

 apparently too short. They had been adjusted upon a board 

 of mahogany carefully 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 considerable to be attributed to any 

 curvature which on this occasion could take place, and it was 



