REPORT ON ASTRONOMY. 
It had long been thought that an instrument might be con¬ 
structed which would enable an observer to take at once the 
transit and the zenith distance of a celestial body. Several such 
had been made in England ; one is particularly described in 
Wollaston’s Fasciculus ; and one is well known as the instrument 
used by Mr. Groombridge. It consists of two parallel circles, 
firmly braced together, and fixed to an axis similar to the axis 
of a transit instrument: the telescope also passes through this 
axis and between the two circles, and it rests near both its ex¬ 
tremities on the braces connecting the two circles. The 
graduations on the circles are read (as in the large instruments,) 
by microscopes with moveable wires. This I should conceive 
to be an excellent instrument. In Germany, however, a very 
different instrument has been constructed with the same object, 
and (principally through its use by Bessel,) has obtained con¬ 
siderable celebrity. Reichenbach’s circle * was first constructed 
(I believe) about 1820, and a considerable number of instruments 
of this construction have since been made for observatories in 
all parts of the Continent. In a journey in the North of Italy 
in the year 1829, I saw and examined several exactly similar to 
Bessel’s. To the extremity of the axis of a transit instrument, 
a graduated circle is fastened ; this circle turns accurately 
around another circle carrying on its circumference four verniers 
(the line of separation between the two circles being almost 
imperceptible) : and this vernier-circle has a fixed as well as a 
reversible spirit-level, to show how much it deviates from a fixed 
position. One pivot of the graduated circle passes within that 
of the vernier-circle, and the latter rests upon the Y : at the 
opposite end the pivot of the graduated circle rests immediately 
on the Y. To prevent friction, each extremity of the transit- 
axis is supported by a lever-counterpoise, and the vernier-circle 
is supported by an independent lever-counterpoise: and, to 
prevent flexure of the telescope, each end is supported by a 
lever-counterpoise whose fulcrum is at the transit-axis. An in¬ 
strument of this kind would I conceive be below mediocrity 
unless the workmanship w r ere most exquisite (the German 
workmanship is very fine) ; and when made in the best possible 
way, I cannot but think that its mechanical structure is extremely 
weak. The first thing to be provided for in an instrument for 
measuring zenith distances is, that the circle and the telescope 
shall move together. In Troughton’s mural circle this is en¬ 
sured by firmly fixing the telescope by its extremities to the 
limb of the circle, without any close connexion at the centre. 
* I have seen several circles by Reichenbach constructed on different plans: 
that which is used by Bessel is generally known by this name. 
