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VI. On some recent Improvements made in the Mountings of the Telescopes at 
Birr Castle. 
By the Earl of Rosse, F.R.S. 
Received May 14,—Read June 19, 1879. 
[Plates 11-13.] 
For some time in carrying on the work of the Observatory at Birr Castle the mount¬ 
ings of the instruments had been found yearly more and more deficient in the require¬ 
ments of modern astronomy. The six-foot reflector still remained unequalled in 
aperture, and even the three-foot was in two or three cases only surpassed or even 
approached in size, but the mechanical appliances for working them were by no means 
equal in convenience to those fitted to most modern instruments, so that much time 
was unprofitably spent, and in some departments of the science we found it impossible 
to make progress. It was therefore decided, in the first place, to apply a clock-move¬ 
ment to the six-foot, the motion of which in Right Ascension had up to that time 
been given by the hand of an attendant. 
The mounting of the six-foot reflector had proved perfectly successful as to steadi¬ 
ness and ease of working, and with the means available for its execution at the time, 
perhaps, it was the only one which could have been well carried out, but in its limited 
range of motion we latterly found a considerable inconvenience, which increased as the 
re-observation of Herschel’s catalogue proceeded, and when the spectroscope came 
to be applied to astronomical investigation the drawbacks to it were found still 
greater. 
Six-foot refector. 
In the year 1869 the clock-movemeDt was applied to the six-foot instrument. 
This was a matter of some difficulty, as the resistance to be overcome was not only on 
an average greater, but also, owing to the counterpoising being approximate only, 
more variable"' in amount than in any other example; still the clock has been so far 
successful as to have been of immense advantage in working with the filar micro¬ 
meter, t 
* In some positions of the teiescope the unbalanced pressure with or against the motion in Right 
Ascension reaches the large amount of 400 lbs., while the weight to be moved—tube, speculum, &c.— 
probably exceeds 10 tons. 
f The suspending chain admits of a very considerable departure of the telescope from a given declina¬ 
tion, considerably more than the length of the slit of a spectroscope, owing to the variable amount of 
“ sag” of the chain, so it was useless to seek for so accurate a motion in Right Ascension as would keep 
a star’s image within the breadth of the slit. 
MDCCCLXXX. X 
