that six weeks elapsed before the mirror had reached the temperature
of the external air. The necessity for extreme precaution in the
operation of annealing will be manifest if we reflect on one of the
accidents which happened. On a certain occasion, after the cooling
of a great casting had been completed, it was found, on withdrawing
the speculum, that it was cracked into two pieces. This mishap was
eventually traced to the fact that one of the walls of the oven had
only a single brick in its thickness, and that therefore the heat had
escaped more easily through that side than through the other sides
which were built of double thickness. The speculum had,
consequently, not cooled uniformly, and hence the fracture had
resulted. Undeterred, however, by this failure, as well as by not a
few other difficulties, into a description of which we cannot now
enter, Lord Rosse steadily adhered to his self-imposed task, and at
last succeeded in casting two perfect discs on which to commence the
tedious processes of grinding and polishing. The magnitude of the
operations involved may perhaps be appreciated if I mention that the
value of the mere copper and tin entering into the composition of
each of the mirrors was about 500 pounds.

In no part of his undertaking was Lord Rosse's mechanical ingenuity
more taxed than in the devising of the mechanism for carrying out the
delicate operations of grinding and polishing the mirrors, whose
casting we have just mentioned. In the ordinary operations of the
telescope-maker, such processes had hitherto been generally effected
by hand, but, of course, such methods became impossible when dealing
with mirrors which were as large as a good-sized dinner table, and
whose weight was measured by tons. The rough grinding was effected
by means of a tool of cast iron about the same size as the mirror,
which was moved by suitable machinery both backwards and forwards,