in diameter, were
turned out in such perfection as to prompt a still bolder experiment.
The various processes needed to insure success were now ascertained and
under control; all that was necessary was to repeat them on a larger
scale. A gigantic mirror, six feet across and fifty-four in focal
length, was accordingly cast on the 13th of April, 1842; in two months
it was ground down to figure by abrasion with emery and water, and
daintily polished with rouge; and by the month of February, 1845, the
"leviathan of Parsonstown" was available for the examination of the
heavens.

The suitable mounting of this vast machine was a problem scarcely less
difficult than its construction. The shape of a speculum needs to be
maintained with an elaborate care equal to that used in imparting it. In
fact, one of the most formidable obstacles to increasing the size of
such reflecting surfaces consists in their liability to bend under their
own weight. That of the great Rosse speculum was no less than four tons.
Yet, although six inches in thickness, and composed of a material only a
degree inferior in rigidity to wrought iron, the strong pressure of a
man's hand at its back produced sufficient flexure to distort
perceptibly the image of a star reflected in it.[324] Thus the delicacy
of its form was perishable equally by the stress of its own gravity, and
by the slightest irregularity in the means taken to counteract that
stress. The problem of affording a perfectly equable support in all
possible positions was solved by resting the speculum upon twenty-seven
platforms of cast iron, felt-covered, and carefully fitted to the shape
of the areas they were to carry, which platforms were themselves borne
by a complex system of triangles and levers, ingeniously adapted to
distribute the weight with complete uniformity.[325]

A tube which resembled, when erect, one of the ancient round towers of
Ireland,[326] served as the habitation of the great mirror. It was
constructed of deal staves bound together with iron hoops, was
fifty-eight feet long (including the speculum-box), and seven in
diameter. A reasonably tall man may walk through it (as Dean Peacock
once did) with umbrella uplifted. Two piers of solid masonry, about
fifty feet high, seventy long, and twenty-three apart, flanked the huge
engine on either side. Its lower extremity rested on a universal joint
of cast iron; above, it was slung in chains, and even in a gale of wind
remained pe