684 T1A-RT. OF EOSSE ON THE CONSTETJCTION OF SPECULA OF 6-FEET APEETUEE, 
back in the shape of a cross. To prevent metallic contact, the bars had been bound 
round with woollen cloth. Strong planks were placed against the face, and screw bolts 
were passed through the planks and projecting ends of the bars. The speculum was thus 
encased, and was easily raised by the crane face up. In the mean time a strong wooden 
platform had been made with three iron pillars securely fixed in it, about 2 feet long 
each, and so disposed as to support the speculum with the least strain. The frame carry- 
ing the supporting levers, to be hereafter described (Plate XXIV. fig. I), was placed 
upon this platform, the three iron pillars passing through interstices in the levers ; and 
the speculum was lowered till it rested upon the pillars, the levers being considerably 
below it. The bars and planks encasing the speculum were then removed, and the 
frame and levers raised by the crane till the speculum was completely supported by 
them. It now rested on its levers, and was taken to the grinding-machine. I have 
been thus minute in describing the means we had employed in removing the speculum 
from the oven, turning it over, and placing it on its bed of levers, as in the arts they have 
never to deal with a material at once so heavy and so brittle ; and we were guided by 
long experience, which others may not have had. 
This speculum had been more than a month upon the grinding-machine, and was just 
ready to be polished, when it was broken by an accident. Immediate preparations were 
made for recasting it. While the speculum had been in the annealing-oven we had 
finished a powerful lathe for turning the grinding-tools, with a slide-rest moving in the 
proper curve. The bed of hoops was placed upon that lathe, and its radius of curvature 
adjusted : the floor of the oven also was cut roughly to the same curve. As we were 
anxious to guard, as. far as possible, against contingencies, and to secure a working 
speculum with the least delay, we were satisfied to employ an alloy somewhat lower 
than on the former occasion, and an ingot of speculum-metal was added which contained 
more than the proper proportion of copper. A little additional copper diminishes the 
brittleness considerably, while it increases the liability to tarnish. 
The speculum was successfully cast, but the surface was covered with minute fissures, 
about the breadth of a horse-hair. These we resolved to grind out. The grinding was 
very tedious, partly oufing to the metal- being a little below standard, and partly to the 
deepness of the fissures. After the first day’s grinding, the fissures, which previously 
were scarcely perceptible, became much enlarged, owing to the edges chipping away ; 
and the whole surface thus became, as it were, covered with large wrinkles. The pro- 
cess of abrasion is necesspily extremely slow, as both the velocity and the pressure are 
kept within very narrow limits, to prevent the evolution of heat, which would crack the 
speculum. The grinding continued for nearly two months, the machinery working for 
part of the time at night ; and a few of the fissures were so deep that even then the traces 
of them were perceptible. The speculum was then polished ; and its performance fully 
equalled our expectations. 
A telescope intended to be constantly employed requires two specula. We had now 
leisure to encounter delays and difficulties in endeavouring to procure a second speculum 
