1868. ] Description of the Great Southern Telescope. 459 
The declination circle igs 80 inches diameter, covered with 
the same alloy; it is divided to 10’ of arc, and reads to 10” 
of are. 
The preparation of the specula was carried on at the same 
time and in the, same premises in buildings specially constructed, 
wherein the mounting was both manufactured and erected. 
The alloy used was rather higher than what has hitherto been 
used for such large mirrors. The proportion was copper 82, tin 
14°77. The mirrors were cast on Lord Rosse’s “ Bed of hoops,” 
but with several very important modifications of his process. 
The annealing was conducted in a circular oven and took 
twenty-three days. Immediately on being removed from the oven, 
the specula were placed on the machine, and rough-ground on 
front, back, and edge, during which process they rested on several 
thicknesses of cloth; from this they were removed to their own 
boxes, where they rested on the system of levers before described. 
The finer grinding and polishing was then proceeded with, and the 
specula were never after raised from their supports. 
The Grinding and Polishing Machine, as devised by Mr. Grubb, 
is shown in isometric perspective in Fig. 13, where A is the specu- 
lum in its box, revolving 
on a vertical spindle, which, Fie, 13. 
by a peculiar mechanical 
contrivance, is made to work 
with great ease and steadi- 
ness. B is the grinder 
strongly ribbed at back, and 
supported from its strongest 
points by six triangles of 
wrought iron. Three cou- 
pling levers unite these, 
which are in their turn 
supported by a strong cen- 
tral tripod. In ‘a hole in 
the centre of this tripod is 
inserted a collared cylinder, 
so formed as to allow of a 
rocking motion through this cylindrical piece, and attached to it by 
a cross key, passes the bar a, which is supported at its upper end 
by a lever and weights (not shown in drawing) to balance a portion 
of the weight of the grinder. This vertical bar is laid hold of by 
the double bar b U' linked at c, and to which motion is given by the 
cranks d d’, ‘These cranks receive their motion from the main 
shaft C, which is worked by the wheel pinion and pulley D, driven 
by the belt E. By the adjustment of the lengths of the cranks d d 
of the beam b 0’, and the relative velocities of the crank, shafts, and 
