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.
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