There is, of course, no practical
difficulty in melting the copper, nor in adding the proper proportion
of tin when the copper has been melted. There may be no great
difficulty in arranging an organization by which several crucibles,
filled with the molten material, shall be poured simultaneously so as
to obtain the requisite mass of metal, but from this point the
difficulties begin. For speculum metal when cold is excessively
brittle, and were the casting permitted to cool like an ordinary
copper or iron casting, the mirror would inevitably fly into pieces.
Lord Rosse, therefore, found it necessary to anneal the casting with
extreme care by allowing it to cool very slowly. This was
accomplished by drawing the disc of metal as soon as it had entered
into the solid state, though still glowing red, into an annealing
oven. There the temperature was allowed to subside so gradually,
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.
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