LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
513 
then broken, and it was found that the speculum metal had penetrated into the 
iron in various directions, and a small portion of speculum metal, when analysed, 
yielded iron. The reverberatory furnace was therefore resorted to, but I found that, 
owing - to the rapid oxidation of the tin, the quality of the alloy was continually 
changing, and the result therefore was uncertain. In the mean time I found that 
the affinity of speculum metal for cast iron was very slight, although for wrought iron 
it was considerable ; that the iron detected in the speculum metal was owing to the 
corrosion of the wrought-iron screw, with which the hole had been stopped, and that 
the failure of the crucible had arisen, not from the unfitness of the material, but from 
defective workmanship. After some experiments in my own laboratory, I found that 
crucibles cast in the usual way, with the mouth down, were generally defective, the 
speculum metal passing through minute pores not visible externally, as quicksilver 
through the sap vessels of wood. When the crucible, however, was cast with the 
mouth up, there was no such defect; we therefore have in cast iron a material ca- 
pable of being formed into crucibles of unlimited dimensions. I cast one which held 
fifteen hundred weight, in which the speculum metal was made, but two crucibles of 
half the size were manageable with less machinery, and therefore were used in cast- 
ing the speculum ; they were raised from the furnaces by proper tackling, and placed 
in iron swing frames, so contrived that each crucible of metal could be thrown almost 
instantaneously into the mould. The fuel I make use of is wood or peat ; the latter 
when of proper quality is preferable, as it yields a steadier fire, without the intense 
heat of coke, which would without great care endanger the crucibles. The mould was 
made in the same manner as in casting the plates, differing from it merely in shape 
and dimensions ; it was what founders technically call an open one. The disc of 
hoop-iron was made circular three feet six inches diameter, three inches and a half 
thick, and turned upon a lathe convex to a radius of fifty-four feet. The speculum 
was cast with a groove round the edge, so that it might be securely embraced by a 
circular clamp tightened upon it with screw-bolts, to which the proper tackling could 
be hooked whenever it was necessary to move it. 
When the metal had become solid, but was still red-hot, a strong hoop, somewhat 
larger than the diameter of the speculum, was placed upon it ; to this hoop a chain 
from a windlass, passing through the annealing oven, had been previously attached, 
and by the action of the windlass the speculum was drawn into the hot oven, and 
every opening closed ; in about a fortnight it was cool, and was found to be free 
from blemish. 
It is of course impossible to ascertain a priori , whether it would be practicable to 
obtain still larger discs of fine speculum metal by this process, and polish them 
without accident ; possibly it might, as the principal cause of fracture, unequal con- 
traction, no longer exists. The question, therefore, will arise, whether in endeavour- 
ing to obtain still larger specula, to approach nearer that limit beyond which it is 
not permitted to us to pass, the better course would be to attempt, with the risk of 
3 u 
MDCCCXL. 
