AOT) A SELECTION FEOM THE OBSEEVATIONS MADE WITH THEM. 
683 
which of course was introduced gradually. In about twenty-six hours from the time the 
fires were lighted the metal was ready for pouring. Peat of good quality is about equal 
to wood in heating-power, when consumed in furnaces where there can be no accumu- 
lation of charcoaL The mould was constructed on the principles explained in my 
former paper ; but, the scale being now so much enlarged, little matters of detail, which 
might have been before overlooked with impunity, were found to be of vital importance. 
The bed of hoop-iron was 6 feet 6 inches diameter, and 4 inches thick. We had not at 
the time a lathe sufficiently large to turn it ; and therefore it was turned horizontally, on 
the machine which was to grind and polish the future speculum. To remove little 
in’egularities arising from the imperfection of the turning-apparatus, the bed of hoops 
was ground for two or three days with a disc about 6 feet diameter, composed of 
fragments of sandstone cemented together within an iron ring. The annealing-oven was 
built on four arches communicating with two low chimneys. The floor being laid upon 
the arches, could easily be heated to redness. The interior of the oven was 8 feet by 10. 
For want of room, the brickwork at the ends was but 2 feet thick, the sides nearly 4. 
The thrust of the arch was, in the usual way, sustained by bolts. The crucibles were 
raised from the furnaces by a crane and tongs just as at the Mint, and placed in rings 
s-winging on trunnions a little above the centre of gravity of the mass. The metal being 
of a proper temperature, levers were fixed upon the trunnions, and at a signal the crucibles 
were simultaneously inverted as rapidly as possible. The operation of pouring was 
accomplished in about three seconds. If the metal was not poured rapidly, the con- 
ducting-power of the iron surface is so great that partial solidification would take place, 
and the casting would be imperfect. In about twenty minutes, the metal was solid 
throughout ; the frame containing the sand forming the sides of the mould was then 
removed, and the speculum, being grasped by an iron ring, was drawn into its place by a 
capstan. The temperature of the oven was red, just perceptible in the dark, about 900°. 
All the apertures w’ere then closed ; and in about six weeks the speculum was cool. 
When removed from the oven the speculum was found perfect ; but the radius of curva- 
ture was much longer than it should have been, which rendered the grinding a very 
tedious operation. The cause, however, was obvious ; the floor of the oven had been laid 
carefully flat to prevent warping ; no other precaution had been taken ; indeed, no other 
had been necessary with the 3-feet specula. 
The speculum was removed from the oven to the bed of supporting levers in the 
following manner : — A pit was dug about 4 feet deep, near the oven, commanded by a 
crane. The speculum, weighing about four tons, was drawn out of the oven in the same 
way that it had been drawn into it. Planks were provided for the speculum to slide 
upon to the edge of the pit, into which it was lowered gently, the ring still grasping it. 
The speculum was now resting principally upon its edge, the face supported by the side 
of the pit. By means of wooden handspikes, and with little effort, the speculum was 
made to rest entirely on its edge, bearing upon the soft earth. Two bars, 7 feet long 
each, and 2 inches square, one of them cranked in the centre, were placed against the 
