Miscellanies. 209 



tical good sense. He has been working silently on, until the attention 

 of the world is arrested by the magnitude of the result in the produc- 

 tion of metallic reflectors of a size heretofore unknown. Until he de- 

 monstrated the contrary, it was thought impossible to cast a speculum 

 of six feet in diameter. As a preliminary to his present gigantic work, 

 there has been exhibited during the last ten or twelve years on the earl 

 of Rosse's lawn, a reflecting telescope made by himself, with a mir- 

 ror of three feet diameter, and a focal length of twenty seven feet. It 

 is suspended by a frame-work similar, at least, if we may judge from 

 the figure, to that of Dr. Herschel's great telescope. 



Being in equilibrio, it is managed with the greatest facility. The 

 casting, grinding, and polishing of these specula, and the machinery of 

 the tubes and their suspension, were all accomplished under his lord- 

 ship's eye and by his own direction. According to lord Rosse's ex- 

 perience, the only metals that can be employed in forming specula, 

 are copper and tin, and the proportions should be, tin 58-9, and copper 

 126*4, or very nearly 3 of tin and 7 of copper. 



Of these metals, for his large speculum, he melted three tons in three 

 cast iron crucibles — crucibles of this metal cast with their mouths up- 

 ward having been found the best. Each crucible containing a ton of 

 metal was placed in a distinct furnace, and for nineteen hours subjected 

 to an intense heat. The crucibles were lifted by an immense crane 

 from their furnaces, and at nine in the evening of April 18, 1842, with- 

 out accident "or delay, they simultaneously poured forth their glowing 

 contents, a burning mass of fluid matter, hissing, heaving, pitching 

 itself about for a minute, and then calmly settling forever into a monu- 

 ment of man's industry and skill. "When the metal had settled, it was 

 drawn by a capstan into a heated oven and built in, where it remained 

 for sixteen weeks annealing. The great difficulty experienced in pro- 

 ducing large reflectors is, that in cooling, the metal generally cracks ; 

 and when this does not occur, the number of holes often found in the 

 solid mass renders it of no use. Lord Rosse has the merit of having 

 overcome both of these difficulties in a manner hereafter to be de- 

 scribed in a distinct work. By an ingenious combination of motions, 

 lord Rosse effected the difficult object of producing a parabolic figure 

 on a large scale. It required six weeks to grind the speculum to a fair 

 surface. The grinding tools being covered with pitch and sprinkled 

 over with crocus, answered for polishing — nothing else being necessary 

 — some precautions being observed to prevent an unequal action. 



The tube in which the speculum is to be mounted, is fifty two feet 

 long and seven feet in diameter, sufficiently large to receive a platoon 

 of soldiers. This tube is of wood hooped with iron ; it was constructed 

 in a long gallery over a range of outhouses, which were thrown down 



Vol. xlvi, No. 1.— Oct.-Dec. 1843. 27 



