Mr. Pond on the 'Declinations 
428 
peculiar to the instruments made by Mr. Troughton : it 
regards the vertical axis rather than any other part, and is, 
in fact, exactly analogous to the usual verification of a zenith 
sector. 
During the period in which I was engaged in making ob- 
servations with circular instruments, I was led to consider the 
advantages and inconveniences of the usual method of adjust- 
ing them ; and it appeared to me, that the essential part of 
their construction, which relates to their adjustment, was 
capable of being improved. 
In order to render the nature of the improvement, which I 
wish to propose, more intelligible, I ought previously to 
remark, that there are, at present in use, two modes of ad- 
justing these instruments, which are founded on different 
principles. 
In the one, two points are taken on the limb of the circle ; 
and when these are brought into a given position, by means 
of a plumb-line passing over them, the microscope or index 
is made to coincide with the zero point of the divisions : by 
this method, the error in collimation remains constant ; and, 
if the adjustment is by any accident deranged, it can easily be 
rectified, and there will be no absolute necessity for frequently 
reversing the instrument ; so that this method seems well 
adapted for large instruments, particularly if placed on stone 
piers. But it is liable to this defect, that the adjustment can- 
not be examined at the moment of observation ; and if any 
change should take place in the general position of the frame 
work, the observation will be erroneous without the means of 
detection. It was probably to avoid this inconvenience, that 
