Captain Kater’s floating collimator, &c. 31 S 
corrections for the face east differ from those for the face west. 
This is the circumstance alluded to, and requires explanation. 
After the circle is reversed, the plumb-line adjustment is 
made, if necessary ; and then the index correction ought to be 
precisely the same in quantity as before reversal, provided 
the parts of the instrument or the plumb line have suffered no 
derangement in the reversion. I had formerly particularly 
attended to this circumstance, which I was enabled to do by 
means of 4 gold dots placed by the maker on the face of the 
circle. The plumb-line having been made to bisect the ad- 
justing dot on the frame of the circle, a gold dot on the lower 
part of the circle was also bisected by the plumb-line by 
moving the circle. The circle was then reversed ; and if 
when the dot on the frame was bisected by the plumb-line 
the dot on the circle was also bisected by the plumb-line, the 
circle had turned without derangement, at least the plumb- 
line had suffered no derangement as to its point of suspen- 
sion. The examination of this is tedious and delicate ; and 
having satisfied myself that no error was to be feared, it was 
not usually attended to. However, when the observations 
with the collimator were commenced, it was soon found that 
the index correction appeared greater on the east side than on 
the west ; and therefore that probably, in the act of reversing, 
a small derangement about the point of suspension of the 
plumb-line took place, less than a second in its mean quantity. 
This was fully confirmed from the examination by the gold 
dots. The result of this would have been to have made the 
zenith distances south of the zenith, when computed from 
the east and west readings, appear greater than they ought ; 
whereas, not regarding the plumb-line as to reversal, and 
