forty-feet Reflecting Telescope. 403 
passed through very hot melted tallow, and kept a sufficient 
time immersed in it, that they rnay be thoroughly penetrated. 
In this state they will last a considerable time, especially when 
care is taken not to relax them often. The gallery being sus- 
pended by ropes in this state, it would be unpleasant to trust 
entirely to them. Each bracket therefore is furnished with 
four strong broad iron hooks, two of which take hold of one 
of the flats of the division (3 fig. 7. while on the opposite 
side two more take hold of the corresponding flat of $ e. When 
the gallery has been drawn up to the required altitude, the 
hooks are let down, and the ropes slackened a little, so as to 
permit it to hang in the hooks. The other two hooks on each 
side serve for an elevation between the flats half way from 
one to the other. They are upon the same centre with the 
former, and fall back as the others do when the gallery is to 
go down. 
For the safety of the tube also, there is a strong chain, 
which will sustain it, in case the ropes by which it is suspended 
should give way. This is fastened into a loop near the point 
of suspension. The other end of it is hooked upon a flat, and 
passes round one of the side beams of the ladder at a certain 
elevation above the telescope, and is sufficiently long to per- 
mit the tube to move a few inches more than is necessary. 
By this means a fall can never be considerable : if the ropes 
were to break in the worst part of a sweep of 2|- degrees 
broad, the telescope would hardly descend two feet. 
The construction of the great mirror is as in fig. 46. The 
metal itself is 49^ inches in diameter, but on the rim at a b is 
an offset of | inch broad, and 1 inch deep, which reduces the 
concave face of it to a diameter of 48 inches of polished sur- 
