420 Mr. Woodhouse's account of the 
passages of stars from their tabulated, or computed right 
ascensions. 
The clock's error, as it was to be expected, was found, 
after allowing for its rate, different with different stars ; 
which is a sign of the instrument being out of adjustment in 
some of its parts. The error might be in the line of colli- 
mation ; in the axis not being horizontal ; or, which was pro-^ 
bably the chief cause of error, in the transit deviating from 
the plane of the meridian. Any one, or two, or all of these 
circumstances might occasion the noted difference in the 
clock's errors. 
For instance, the clock being before sidereal time, its error 
from a Cygni was found to be less than from Aquilce; This 
might arise from the western end of the axis being too high, 
or from the line of collimation deviating to the east, or from 
the transit deviating to the west. A single observation such 
as this, or any number of the same stars, would leave us in 
doubt respecting the causes of the want of adjustment ; but 
a third star would lessen this doubt. Thus, if, the clock's 
error after allowing for its rate being from a: Cygni 4*. 721, 
and from u Aquilse 4^.854, we attributed the difference of 
errors to a defect of horizontality in the axis, the quantity of 
such defect would become known. Let it be expressed by 
H, the clock's error by e ; then for the latitude of Cambridge 
we should have two simple equations between H and e, from 
which both may be found 
— 4.721 + e = 1.39 H, ^ Cygni 
— 4.854 s = .75 H, ct Aquilce ; 
and, accordingly, H = o®.2, nearly. 
With this value of H, the error of time for a Urs. maj. 
