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a strong ring attached to the bar above mentioned. By con- 
necting this bar with a powerful capstan, it was drawn from 
the bed of hoops, along strong beams covered with iron, to its 
place in the centre of the annealing furnace. The ring was 
then removed, and the rest of the chamber filled up with char- 
coal ; the arches with fuel; all the flues and apertures were 
closed carefully with masonry, and it was left to cool gradually 
for sixteen weeks, during the first three of which the exterior 
of the building was sensibly warm. 
In the course of this year considerable progress was made 
with various parts of the mounting ; and when Dr. Robinson 
visited it in February, 1843, he found that the speculum had 
been ground (on a machine similar to the old one, but of 
strength proportioned to its work) ; that the foundations of the 
piers were laid, the tube was in preparation, and the massive 
frame-work and levers by which the speculum is supported in 
the tube, were cast. This elegant contrivance requires some 
explanation. Suppose the back of the mirror divided by two 
concentric circles into three portions, of which the central 
circle is cut by radial lines into six sectors, the middle zone 
into nine segments, and the exterior into twelve, and that all 
of these are equal. If each of these be supported by an equal 
force applied at its centre of gravity, the speculum is obviously 
in the most favourable condition as to flexure. The frame 
mentioned above is rectangular with a cross-piece cast in one, 
and weighs one ton and a half: it bears three strong triangles, 
also of cast iron, supported at their centres of gravity on 
hemispherical bearings. Each angle of each of these bears a 
similar triangle, the angles of which give the twenty-seven 
points of equilibrated bearing for the speculum. They do 
not, however, press directly, but carry platforms of cast iron of 
the shape of the areas which they are to bear, and made exceed- 
ingly stiff by flanches at their edges, and by edge-bars crossing 
them diagonally. A layer of felt is over these; strong up- 
rights from the frame ofa similar character prevent any lateral 
ee 
