158 
THE EARL OF ROSSE ON SOME RECENT IMPROVEMENTS MADE 
in the usual manner, and is brought within reach of the observer by means of a wire 
rope passing over pulleys. Each worm is cut across at the centre of its length and 
one portion is free to slide lengthwise, so that any shake can be removed by tightening 
the nut and lock-nut at the outer side of one of the bearings.* 
Gallery. 
The construction of a gallery to enable the observer conveniently to reach the eye¬ 
piece has been considered as one of the greatest difficulties to be overcome by the 
designer of an equatorial mounting for a Newtonian, and it seems to have led to the 
Cassegrain form being preferred for the great Melbourne reflector. However, I was 
deterred from adopting the latter construction by what appeared to me the greater 
difficulties and inconveniences inseparable from it, as also by the risk of fracture of the 
speculum in boring out its centre. The following form of gallery was adopted as being 
apparently liable to the least objections. 
A circular wall of brick and cement, whose centre nearly coincides with the centre 
of motion of the mounting, carries a wrought-iron rail (L). On this a wooden, iron- 
bound framework (O P, Q N) provided with wheels at M, N, 0, runs and carries an 
arm (T U) provided with worm-wheel horizontal and altitude motions. From the 
extremity of this arm is suspended the platform on which the observer stands. It 
will then appear that the gallery has three motions, one in altitude and two in 
azimuth, and one only of them, that of the frame upon the railway, has not been 
brought under the easy control of the observer. By means of them and the power 
of turning the ring at the mouth of the tube, the observer can reach the eye-piece in 
comfort in every position of the telescope, except at low altitudes, where a light inde¬ 
pendent ladder is used. The arm which carries the platform is constructed of four 
deal rods sprung out, connected by pairs at their upper ends, and firmly held together 
at each cross strut by hoop iron. A cast-iron counterpoise (W"), weighing about 
1650 lbs., is fastened immovable on the prolongation backwards of the arm, which it 
balances when 400 lbs. weight is placed upon the platform. In observing, equilibrium 
is restored by keeping between the bottom (which is of sheet copper) of the gallery 
and the floor just so many lead weights as will, with the weight of the observer or 
observers, just make up the 400 lbs. Two counterpoises (W 7 , W') serve to bring the 
centre of gravity of the gallery within the wheel-base, as otherwise the wheels at N 
and O would not rest on the rail. 
* Since the erection of the instrument it has been found impossible so to adjust the nuts and lock-nuts 
that the worms shall be quite free from shake and yet turn with sufficient freedom, but one-sixth of a turn 
of a helical clump, since fitted to each and working between the end of the worm and its bearing, and 
under the control of the observer, meets the difficulty. 
