various Brilish Standards of linear Measure. 283 
The 40-inch bar was laid near Mr. Ramsden’s bar, on the 12th 
of April 1820, and a thermometer placed upon it. Three ther- 
mometers were also arranged at equal distances along Mr. Rams- 
den’s bar. 
On the 13th of April I commenced my examination. ‘The 
intersection of the wires of the one microscope being placed on 
the centre of the left hand dot, the intersection of the wires of 
the other microscope was brought, by means of its micrometer 
screw, to the centre of the right hand dot, and the reading of 
the micrometer registered. In this manner the six intervals of 
Mr. Ramsden’s bar were compared in succession. It may be 
necessary to remark, that as the microscopes invert, the readings 
are to be taken in a contrary sense, the higher number indi- 
cating defect, and vice versa. 
Readings. | Thermometers. 
1st interval. 29°5 54:0 
2d 10-0 | 53°5 namie’ bar. 
3d 10:0 53:5 
Ath 16°5 | 
5th 10:0 53:0 Forty-inch bar. 
6th | 19-0 | 
Mean | 15°9 
The difference of temperature of the two bars, being so small, 
may safely be neglected. 
The micrometer microscope was now set to 15-9) divisions, and 
the apparatus being laid on the 40-inch bar, the intersection of 
the wires of the left hand microscope was brought to the middle 
of the line on the gold pins, and the piece of latin brass was 
moved along the bar, till the middle of the line drawn upon it 
appeared in the intersection of the wires of the micrometer mi- 
croscope. The whole having been carefully examined, the mi- 
crometer microscopes were withdrawn. 
The tracing microscope was next brought over the 40-inch 
bar, and placed so that the intersection of its wires appeared upon 
the middle of the line traced upon the brass; the brass was then 
slid away, and a line drawn with the cutting point upon the gold 
surface. 
I had next to compare the distance thus obtained, with the 
mean of the six intervals on Mr. Ramsden’s bar. 
Nn2 First 
