364 
CAPTAIN KATER ON THE ERROR IN 
For this purpose I prepared sets of wires, which by careful measurement I 
found to be, .012, .02, .03, .04, .05, and .10 of an inch in diameter. 
In order to subject the surface of the marble slab to a more rigid examina- 
tion, I prepared some fine copper wire by annealing it, and then stretched it 
along the slab, applying as much weight as it would bear at each end with- 
out breaking. This detected a curvature of one end of the slab, which the 
thread did not indicate, and which was removed by placing a thick card 
between it and the table. 
The Imperial Standard Yard. 
I commenced with the Imperial standard yard. The bar upon which this 
yard is laid off, is 1 .07 inch square. 
On placing a wire of .012 of an inch diameter under the middle of the 
bar, the two extremities were raised above the marble ; a thin wedge was 
placed under one extremity of the bar, the other being then in contact with 
the marble, in order to prevent the effect which would arise from the weight 
of the apparatus carrying the microscopes when placed upon the bar. This 
precaution, I shall content myself with remarking, was used in every instance 
where necessary. 
The following comparisons were then made between the length of the yard 
when the bar lay flat upon the marble, when it was curved upwards by a wire 
placed under its middle, and when curved downwards by wires placed under 
the dots marking the yard. In the latter position the middle of the bar 
touched the marble. 
With wires of .012 of an inch diameter. 
Without Wires. 
A Wire under 
Wires under 
Error when curved 
Error when curved 
Sum of 
the middle. 
the ends. 
upwards. 
downwards. 
Errors. 
div. 
div. 
div. 
div. 
div. 
div. 
95.5 
89-5 
112.0 
96.5 
91-7 
111.5 
95.7 
88.0 
111.7 
79-0 
72.5 
94.5 
Mean 91. 7 
85.4 
107.4 
6.5 
15.5 
22.0 
0.2 Reduction to the Chord. 
91-9 
