406 Dr. Herschei/s Description of a 
At the same time the platform must be gradually removed, 
that the tube may be brought back by the bar-motion, when- 
ever the mirror is high enough to pass over the back of it. 
Then letting down gently the round motion handle, and guid- 
ing the mirror properly, it is to be placed upon a small hollow 
square, with a sloping back, which is planted under its support. 
The height of the square frame is such as will bring the centre 
of the mirror into the centre of the tube ; and the sloping 
back receives it in going down, and throws it from the back of 
the tube, just as much as is required to make the adjustment 
at the top act properly. 
When the mirror is in its place, two loops which are pre- 
pared are to be screwed fast to it. They contain the collets 
that receive the adjusting screws from the back, through the 
strong upper bar eg, fig. 34. and as soon as these are fastened 
the pulleys may be unhooked, and all the apparatus that has 
been used removed. The six planks are then to be laid upon 
the same rolling bars at n 0, where a passage across the work 
is wanted, and where they may remain till zenith sweeps re- 
quire them to be moved. 
The method of preserving the speculum from damp is by 
having a flat cover of tin soldered upon a rim of iron, about 
i-| inches broad, and thick, the diameter of which is equal to 
the iron ring which holds the speculum. Upon the flat part 
of the rim is cemented, all around, some close-grained cloth of 
an equal breadth with the rim. The cover has two handles 
near the upper end, and under them two flaps .that project 
about an inch and are six inches broad. When the cover is 
hung or laid upon the speculum, so that the two flaps are close 
to the ring which incloses it, the rim of the cover, as far as it 
