IN THE MOUNTINGS OF THE TELESCOPES AT BIRR CASTLE. 
159 
Clock-movement. 
The clock-movement is of the ordinary construction with centrifugal ball-governor, 
bringing into play as a checking force the friction between leather studs, connected 
with the balls and a fixed brass ring. Usually the governor has been so adjusted as 
to give unaided slightly too high a velocity, and its controlling force has been supple¬ 
mented by one brought into play by a bevel-wheel running between one moving with 
the train and one connected with a scape-wheel and seconds pendulum. Any excess or 
defect of velocity of the train produces a motion of the intermediate wheel, accom¬ 
panied by an elevation or depression of a lever in connexion with it, thus bringing it 
into contact with, or withdrawing it from, a rapidly revolving disc worked by the 
train. The arrangement is not novel. 
The method by which the motion of the clock is communicated to the telescope is 
peculiar. On the lower end of the fork is fitted a sector of six feet radius. It has an 
arc of nearly 40°, giving a run of over two hours. Its face is smooth. The motion of 
the clock is imparted to a square-threaded screw (V X), and to the nut working on it 
are attached two straps of sheet brass, which wrap round the face of the sector and 
are fastened to it at its eastern end. A third brass strap fastened to the western end 
of the sector and wrapping round it between the other two keeps them in a proper 
state of tension, and a bow (j k) connected with the nut by a universal joint at p and 
at q fastened to the strap gives the tension to the straps, so that the sector and nut 
are practically rigidly connected with one another. A coupling at V connects the 
screw with a handle for winding back, and a pulley, round which passes an endless 
rope up to the gallery, and a pinion geering into the spur-wheel enables the observer 
to move the telescope forward or backward in Right Ascension by turning the nut. 
The mounting has now been completed about three years; but as our attention has 
been up to this specially directed to the clearing up of doubtful points in former 
observations of the Nebulae which came to light on preparing them for publication, 
that the work of the six-foot since its erection might be brought out in as complete 
a form as possible, comparatively little work has been engaged in as yet with the 
three-foot; however, it has been fairly tested as regards its steadiness, and no 
tremor through the action of the wind was noticed. The fine motions both in Right 
Ascension and Declination, the two motions of the arm of the gallery, and the clamps 
are within easy reach of the observer when in the gallery. It is intended to add an 
arrangement for reading the Declination circle from the gallery, and it is hoped that 
some sufficiently simple means to enable him to read the Right Ascension circle may be 
devised, and if these additions can be satisfactorily carried out they will, of course, be 
of immense service to the observer, as now time is lost in descending for the purpose of 
making the set. 
* The late Mr. Cooke, of York, and Mr. Howard Grubb, of Dublin, have employed similar devices for 
the control of equatorial clock-movements. 
