H 7 
of a Transit Circle. 
trials so disturbed by the shaking of carriages, while it was 
at his house, that I could make no satisfactory examination 
there myself. When the instrument was in its place, I tried 
every experiment I could contrive to discover the cause of 
this error; whether it could be in the microscopes themselves; 
any shake in them, or in the pillars, or in the hanging of the 
Ys. Finding none of these to be in fault ; and, upon trying 
the instrument at every 10 degrees all round, perceiving the 
axis thrown backward instead of forward upon turning either 
way, it occurred to me, that any grease or other particles 
would have it more in their power to produce that effect in a 
sort of pivot-hole, (which the hollowed sides really are) 
than between two fair flat surfaces. I thereupon took out the 
Y s, and had them formed to an exact right-angle, with the 
whole sides perfectly smooth, and flat, and well finished : 
and, since that has been done, I ready can discover no dif- 
ference which ever way the circle be turned ; but think I may 
now say that deviation is quite removed. 
Yet, I apprehend, it would have been of no consequence if 
it had continued, or been greater than it was. For, since the 
readings are as it were in a line above and below the centre, 
and both of them positive ; any motion of the centre towards 
the right hand, would give the dots, both above and below, 
the appearance of being more to the left than they ought to 
be ; and thence would give the measurement too small, and 
that in an equal degree in each ; so that the sum of zenith 
distance given by one microscope, and of altitude by the 
other, would thereby be less than go degrees, by just double 
the error. And if the axis be moved towards the left, the 
contrary would be the result ; the sum would exceed go de- 
