9® Sir George Shuckburgh's Account 
the error of the centre -j- the sum of the errors of the two 
divisions. 
The 4th column shews the difference of the mean reading 
off from the extremes, and may be considered as the greatest 
actual error in reading off these observations. 
The 5th column contains the numbers in the 3d column 
corrected, by subducting o".g, a quantity which, it was 
found, upon taking the mean of all the numbers in the 3d 
column, the opposite or west micrometer was tco forward 
upon the circle, viz. did not make a perfect diameter, but ex- 
ceeded it in the order of the degrees by o".g. It will be 
observed that this quantity, from the three first series of ob- 
servations of the points 360, and 180°, seemed to amount to 
o".6 ; this difference is inconsiderable. The numbers then 
in the 5th column, thus corrected, will express the true dif- 
ference between the opposite divisions, if the wires in the 
microscopes had described a true diameter. 
The 6th column gives the half of the difference just men- 
tioned, and is = the simple error of the divisions. 
