116 Mr. Cooprr’s Observations on Comets. 
a transit is in no way disturbed by the clamping of the circle. This is cast of 
bronze, in one piece, which, I conceive, gives it a decided advantage over those 
which have been made in England, of several pieces of brass joined by screws, 
rendering them more liable to disturbance from changes of temperature or strain. 
The German scarcely ever exceed three feet in diameter, French measure; the 
English range in great observatories from five to eight, and, as a consequence ot 
these greater dimensions, are more liable to irregularities of expansion and 
flexure. It may be said that the larger circle can be divided and read more ac- 
curately, but this is not the fact. The divisions of Ertel, Gambey, and Repsold, 
are of astonishing sharpness, and will bear any magnifying power in the reading 
microscopes that can be required. My circle, of which I annex engravings, 
is three feet two inches, English measure, in diameter, and divided to two 
minutes ; these are subdivided by eight microscopes, achromatic, and magnifying 
about thirty-five times, which are supported by a frame, representing the vermier 
circle of the earlier instruments, and, like it, connected with a fine level, by means 
of which any deviation can be measured and corrected. The scale of the level is 
one second of space, = 34, of a French inch. In the transverse, or axis level, the 
scale is the same. The continental astronomers seem now to prefer the micrometer 
microscopes ; and in the meridian instrument constructed for Pulkova they are 
used, four of them being applied to each of the two circles which are carried by 
the extremities of the axis. I have retained the two circles for the sake of sym- 
metry and equilibrium, but one is merely used as a finder, and for giving the 
degrees and minutes, while all the microscopes are applied to the other, as I am 
strongly persuaded that one circle with eight readings gives a better result than 
two with four. The telescope is of unusual power, having seven inches aperture, 
and ten feet focal length, being specially destined to determine the places of very 
small stars. The tube is composed of two frusta of cones, sufficiently strong and 
uniform to be secure from injurious flexure, without the counterpoises formerly 
applied by Reichenbach ; but as a means of guarding against its influence, the 
object glass and eye apparatus can be applied indifferently at either end. Of 
course, the circles and micrometer frame are counterpoised. The field of view 
can be illuminated in the usual way, by a light placed at the end of the axis 
which carries the setting circle, and which is perforated for that purpose ; or the 
lines can be illuminated, in a dark field, by openings in the opposite sides of an 
