706 EAEL OF EOSSE ON THE CONSTEUCTION OE SPECULA OE 6-EEET APEETUEE, 
Explanation of PLATE XXIV. 
Fig. 1. The speculum upon its supporting levers, the apparatus by which the levers 
themselves are counterpoised having been previously removed to prevent 
confusion, 1. The lime-boxes, connected with the cover by sliding tubes. 
2. The cover, which fits nearly air-tight. Before the cover is taken off, the 
lime-boxes are removed, all communication between the lime-boxes and 
speculum having been first intercepted by valves. Without this precaution, 
lime dust would make its way to the speculum. 3. Bing in which the specu- 
lum is suspended. 4. Supporting levers, which are shown in plan in fig. 10, 
where A represents a primary triangle, B a primary and three secondary 
triangles, and C one-third of the system complete, the dots being the balls. 
5. The frame upon which the levers rest: a single casting. 6. A massive 
casting, which is bolted to the bottom of the table as soon as the speculum 
is in its place: it bears the wrought-iron girder (7) which holds the suspend- 
ing-ring. 8. Turn-table for changing the specula : it is at the north of the 
telescope, close to the elevating-windlass. 
Fig. 2. The same, with the weights and levers for counterpoising the secondary triangles. 
9. The weight. 10. The common fulcrum of the levers. The levers are 
connected with the secondary triangles by slight straight rods. 
Fig. 3. Grinding- and polishing-tool, seen at the back. It is suspended by gimbals ; the 
tool can therefore be turned over easily from time to time, which is necessary 
in applying the pitch and resinous composition. 
As soon as the tool is prepared the gimbals are removed; and it is then 
managed by the shackle in the centre. The shackle carries a triangle, with a 
lever at each angle. Each lever carries similarly two levers, connected at 
their extremities with T-shaped levers, which carry the tool by thirty-six 
points. The chain through which the counterpoise acts during the progress 
of grinding and polishing is hooked to the shackle, and the strain is thus 
distributed so equally that there is no sensible distortion. 
Fig. 4. The same tool, seen in front. The straight grooves are produced by casting ; 
and in this there is no difficulty, provided the pattern is nicely made. The 
little square blocks are kept in their places in the usual way by pins, and, 
remaining in the sand, are removed separately. The circular grooves are of 
course cut in the lathe. 
Fig. 6 represents the telescope seen from the south-east, the stage of the first gallery 
being slightly raised. 
Fig. 6, from the south-west, showing the machinery of the second and third galleries. 
Fig. 7, from the south, showing the position of the telescope when a man enters the 
tube to fix the small speculum, and remove the cover of the large one, in 
preparing for the night’s work. 
Fig. 8, as seen from the north. 
