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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 



