Rosse Telescope
This wonderful telescope was made by the renowned astronomer Lord Rosse (born 1800, died 1867), who devoted a great portion of his life to the improvement of reflecting telescopes, and succeeded in mounting one of 3-feet aperture at his home, Birr Castle, Ireland, in the year 1839. In 1842 the now celebrated six-foot reflector was successfully cast and polished, being finally mounted in 1845. The immense tube which contains it is 54 feet long and 7 feet diameter. The speculum metal employed for casting this reflector consisted of 4 equivalents of copper to 1 of tin, which is equal in weight to the following proportions: Copper 252.8, tin 117.8. This alloy is exceedingly hard and brittle, will take a beautiful white polish, and does not readily tarnish; but, owing to its extreme brittleness, there was much difficulty experienced in obtaining a speculum casting of this magnitude absolutely free from shrinkage cracks, gas-holes, and a decided tendency to warp out of shape. To obviate these difficulties, the cooling of the mass must be controlled and the gas eliminated; all of which, we are told, was successfully accomplished by Lord Rosse after a somewhat novel fashion. He formed the face side of his mould with hoop-irons, side by side, and edge up. When this iron bed had been thus made, the outside edge was formed with sand, and the casting poured as an open mould. The closely packed hoop-iron bed contained comparatively no gas-producing substances, as sand does, and whatever gas might exude from the metal thereon would be instantly pressed through the countless interstices by the superincumbent pressure of the metal above. See SPECULUM METAL; TIN.
Rosthorn's Austrian Metal for Cannon.--See GUN-METAL.