94 
Sir George Shuckburgh’s Account 
quadrant or sextant, for astronomical purposes, are almost in- 
credible to a person that has not considered the properties of 
a circle, and so great, that I am much surprised it has not 
been brought into more frequent and earlier use. For, in the 
first place, the error of the centre, which so constantly takes 
place in quadrants, is entirely done away ; secondly, the small 
error in each individual division is discovered, so that what- 
ever be the skill of the artist, the observer is under no obli- 
gation to rely upon it, but can examine the whole himself. 
The method I proposed was this, to make the index wires in 
the opposite micrometers an exact diameter to the circle, 
and then to observe whether each division corresponded with 
its opposite one, and if any difference, to set it down ; these 
differences I expected to find somewhere = o" o, viz. in the 
diameter passing through the true centre of the circle, and 
the centre of the pivot, round which the machine revolved, 
and at right angles to this they would be at a maximum. 
The greatest quantity of this eccentricity, and the place 
where it lay, being ascertained, it would be easy to deter- 
mine what it was in any other place ; for this eccentricity, in 
any given part of the circle, would be as the co-sine of the 
distance from that point where it was at a maximum ; and 
if, on this principle, a table was constructed, giving the eccen- 
tricity at every degree round the circle, the numbers in this 
table might be compared with the actual observation of the 
eccentricity, by the microscopes all round the circle, and if 
the quantity in the table did not every where agree with that 
found by experiment, the difference would be the actual error 
of that individual division ; and in this manner the whole 
might be examined, every error detected, and a memoran- 
