of a Transit Circle. 145 
The interior divisions, or cuts, are also numbered at every 
degree each way, from the eye-end to the object-end of the 
telescope, with the feet of the figures always towards the 
centre. The use of them is likewise very great ; not for read- 
ing off the observations, but for setting the instrument. For, 
at a proper distance from the main pillars, there is a small 
pillar, carrying a compound microscope with a wire in its fo- 
cus ; which being adjustable, and once set to the latitude of 
the place, gives immediately the north polar distance of any 
object seen ; or, by fixing the instrument according to the 
polar distance of an object sought, one is certain of its enter- 
ing, at the proper time, the field of the telescope, near the 
centre wire. This pillar for the polar microscope, is remove- 
able to the other side of the main pillars ; which becomes ne- 
cessary when the instrument is reversed. 
This in general is the form, and these are the peculiarities 
in the construction of this instrument ; which, being de- 
signed for meridian observations, or transits, I apprehend may 
best be named a Transit Circle. 
In the progress of it, when the divisions came to be exa- 
mined in their proper position, as to the truth of the opposite 
dots being exactly in the diameter of the circle, an error was 
discovered, which occasioned a great deal of trouble, and 
much loss of time. When the microscopes had been adjusted 
with care, after turning the circle one way, they continued 
true, and the same dots shewed themselves to be perfectly in the 
diameter, however often the circle were turned the same way 
round : but on one or more revolutions the contrary way, the 
same dots ceased to appear true. This, it was thought, could 
arise only from some deviation in the centre. And, since the 
