115 



about half a ton each, and manufactured with the precautions 

 pointed out by Lord Rosse (Phil. Trans. 1840). Notwith- 

 standing' their great strength, they yield so much that it is 

 obviously hazardous either to use them frequently or to in- 

 crease their dimensions. Three were employed at once, each 

 containing about one and a half tons of the alloy : they were 

 placed in furnaces whose mouths were level with the ground, 

 eight feet deep and four in diameter, disposed round a large 

 stack or chimney, into which their flues vent. The fuel is 

 turf, peculiarly fitted for this work, as giving a much more 

 manageable heat than coke ; about 2200 cubic feet of it are 

 consumed in a casting. The furnaces were filled with fuel, 

 and ignited at the top, on the preceding evening, that the 

 crucibles might be gradually heated ; and in about ten hours 

 they were ready to receive the metal. This was unintention- 

 ally made of a lower standard than that of the three feet, in 

 consequence of the atomic number for tin being taken as 

 given in Turner's Chemistry, 57.9 instead of 58.9, causing a 

 deficiency of about half per cent, too trifling to impair mate- 

 rially its reflective power, though it will certainly make it 

 more liable to tarnish. That its uniformity might be insured, 

 each ingot of it was broken into three pieces, as nearly equal 

 as possible, and stored in three casks, each of which contri- 

 buted equally to form the successive charges of the crucibles 

 in an order regularly varying. They were charged at inter- 

 vals of two hours, and the whole was fused in twelve : they 

 were then withdrawn from the furnaces by a powerful crane, 

 and transported to the iron cradles of pouring frames, arranged 

 90° asunder round the mould. 



The essential part of the mould is its base, composed of 

 hoop iron six inches broad, packed on edge in a strong frame 

 seven feet diameter, and supported by strong transverse bars 

 below. The upper surface was turned to a convex segment 

 of a sphere, 108 feet radius, on the polishing machine, over 

 which a self-acting slide rest was fixed, whose frame was of 



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