FAVOURABLE TO THE INVESTIGATION OF PARALLAX. 
335 
The instrument employed was an equatoreally mounted telescope of 10 feet 9 inches 
focal length, with an object-glass of 7f inches clear aperture, of which the flint-glass 
is by Guinand, and the crown glass English, the whole having been finished and 
perfected by Dollond. 
This telescope is mounted in the manner usually adopted by the opticians of this 
country in a fixed observatory, in the immediate vicinity of my residence in Stafford- 
shire; the polar axis is formed of four mahogany planks 14 feet 3 inches long and 
10 inches square in the middle, the pivots of which are of hard bell-metal, and rest 
above and below on Y® attached to large and solid stone piers, which are supported 
by a foundation of brick-work, joined with cement, and formed into a solid mass of 
great extent by filling up the space enclosed by an outward circuit of brick with 
stones and mortar pounded together : this mass of brick-work and stone extends 
beyond the equatoreal room, and forms also the support of the piers of a 5-foot 
transit. 
The Declination and the Hour Circle are each 3 feet in diameter ; the two verniers 
of the former read off to 10" in space, and that of the latter to seconds of time. To the 
telescope there is attached a parallel-wire micrometer with one equatoreal fixed, and 
two moveable wires; the screw heads are divided into 100 parts; the micrometer is 
provided with the usual position circle, graduated on silver, with its vernier which 
reads to 6' in space. The value of one part of the micrometer first employed was 
0"* 15628, but from the 1st of January 1847 the value 0"* 15641 was used, the first 
having been determined by Mr. Beaumont, the former possessor of the instrument, 
and the latter by myself. There are six eye-tubes that can be used with the micro- 
meter, with powers varying from 85 to 820, but the power almost invariably used in 
the observations about to be described was 450 ; one of 320, and sometimes a lower 
power, was occasionally, but very rarely, employed, and when this occurs a notice to 
that effect will be found in the Table containing the results of observations subjoined. 
The telescope is provided with a clock-work motion ; the performance of the object- 
glass, so far as its powers have been tested, leaves nothing to be desired, but the 
mounting of the telescope, notwithstanding the precautions above mentioned, does 
not appear to be equally steady with that of some other recently erected equatoreals. 
Whenever the night admitted of it, ten measures both in position and distance were 
obtained of each object observed ; when in the sequel I speak of &.set of observations, 
the term is to be understood as applying to all the observations of a single night, 
which almost always comprised that number of measures both of position and 
distance. To each individual measure, whether of position or distance, an arbitrary 
weight was assigned, and registered at the time of making it, the number 10 being 
supposed to express a result with which the observer was entirely satisfied, and 
smaller numbers to denote a less degree of confidence in the value obtained ; in 
point of fact, however, no higher number than 8 seems to have been used, and the 
weights employed generally range from 4 to 7- The sum of the weights of the indi- 
2x2 
