various British Standards of linear Measure. 285 
Third Comparison. Thermometers as before. 
Readings. 
Forty-inch bar | 35-0 
= (Astinterval | 58-7 Div. 
2 | 2d 30:0 Mean of Ramsden’s bar 39-6 
= y 3d 31:0 of forty-inch bar —- 35-6 
Ss } 4th 44:0 
= | 5th 34:0 Forty-inch bar longer 4-0 
ce L6th 40:0 — 
Mean | 39:6 
Forty-inch bar | 36°2 
By the mean of these comparisons, it appeared that the forty- 
inch bar was éoo long 2-9 divizions of the micrometer, or -000124 
of an inch *, 
The preceding measures were taken from the middle of the 
lines on the gold pins; but as it was found that these lines were 
not quite parallel, this accidental circumstance afforded a method, 
of which I availed myself, to attain a greater degree of accuracy. 
The deviation of the two lines was obtained by measuring the 
difference of the distances of their extremities, and by the mean 
of six comparisons was found to be 16°8 divisions. 
Now, as this is the deviation due to the whole length of the 
lines, they will have approached each other 2-9 divisions, at about 
one-sixth part of their length, reckouing from their most distant 
extremities. 
This portion of the line being estimated, transverse lines were 
drawn, indicating the points from which future measurements 
were to be taken. 
On the 14th of April I resumed my comparisons. 
Conceiving that it would be preferable to ascertain the dif- 
ference between some one interval and the mean of all the in- 
tervals of Mr. Ramsden’s bar, and afterwards to compare such 
interval with the forty-inch bar, I now directed my attention to 
this object. 
* Each division of the micrometer is 34,3 of an inch. 
Fourth 
