95 
of the Equatorial Instrument. 
dum made of it. With this intention, and a full expectation 
of finding the eccentricity = 8" or 10", I set to work to exa- 
mine the circle all round, having previously determined the 
diameter of the points in the arch to be about 21", and the 
thickness of the wire = 12". It is true, the points were not 
all exactly of the same size, nor could it be expected, but in 
general it might be concluded, that when the wire equally 
bisects a point, the segment on each side of the wire is about 
= 4" : so that an error of 1" in the bisection can never be 
committed, with a tolerable light and reasonable care, the mi- 
croscope magnifying 16 or 18 times. I placed the moveable 
wire of the east microscope so as to bisect the division 360°, 
and then, by repeated trials, made the moveable wire of the 
west microscope bisect the opposite point of 180° ; in which, 
taking a mean of three or four observations, I could not err 
more than a few tenths of a second. The index of the mi- 
crometer screw being then carefully adjusted to zero (0) of 
the divisions of the head, I made every 10th degree of the 
circle pass under the micrometer wire of the east microscope, 
which wire might now be considered as fixed, and then noted 
whether the opposite division was under the moveable wire 
of the west microscope ; if not, I wrote down the difference, 
after three or four times reading off. The result of these ex- 
periments may be seen in the adjoined table. Where 
The 1st column shews the point, or division, that was 
brought under the wire of the east microscope. 
The 2d column shews the want of coincidence, or how 
much the opposite point disagreed with the wire in the oppo- 
site microscope, at each reading off. 
The 3d column gives the mean difference, and is = double 
