of this tremendous mirror, six feet across, and about four or five
inches thick. The dimensions were far in excess of those which had
been contemplated in any previous attempt of the same kind. Herschel
had no doubt fashioned one mirror of four feet in diameter, and many
others of smaller dimensions, but the processes which he employed had
never been fully published, and it was obvious that, with a large
increase in dimensions, great additional difficulties had to be
encountered. Difficulties began at the very commencement of the
process, and were experienced in one form or another at every
subsequent stage. In the first place, the mere casting of a great
disc of this mixture of tin and copper, weighing something like three
or four tons, involved very troublesome problems. No doubt a casting
of this size, if the material had been, for example, iron, would have
offered no difficulties beyond those with which every practical
founder is well acquainted, and which he has to encounter daily in
the course of his ordinary work. But speculum metal is a material of
a very intractable description. 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,