404, Dr. Herschel's Description of a 
face. The thickness, which is equal in every part of it, re- 
mains now about 3^ inches ; and its weight, when it came 
from the cast, was 2118 pounds, of which it must have lost a 
small quantity in polishing. 
An iron ring, 4 yj- inches in diameter within, 4 inches 
broad, and i-|- thick, has at the face of it on the inside a strong 
bead or rim added to its thickness, which fits the offset in the 
speculum, but is not quite so deep as that. A cross of the 
same substance of iron as the ring, goes over its back, and 
when the speculum is placed into the ring, so as to rest upon 
the offset, the cross over the back confines it in the ring. By 
the addition of a thin cover of sheet iron on the back, and 
another of tin on the face, the rim makes a complete case for 
the mirror. 
Three strong handles are fixed against the sides of the ring, 
by which the speculum may be lifted horizontally, or using 
only one of them, vertically, as occasion may require. 
To put the speculum into the tube, there is provided a 
small narrow carriage, going upon two rollers. It has upright 
sides, between which the speculum, when suspended vertically 
by a crane in the laboratory, is made to pass in at one end, 
and being let down, is bolted in. The carriage is then drawn 
out, rolling upon planks, till it comes near the back of the te- 
lescope. The tube must be put back as far as the bar-machine 
will permit it to go. Two beams connected together so as to 
form a parallelogram of 8 feet 6 inches long, and 2 feet broad, 
are sloped away on one end, while the other contains two 
hooks, by which it may be hooked into two holes at the end 
of the foundation timber, fig. 3. in the middle between the 
rolling beams r s. This affords a passage of an easy ascent to 
