140 
DR. T. R. ROBINSON AND MR. T. GRUBB’S DESCRIPTION OF 
again meeting where the metal first touched the mould, which part till then was kept 
exposed by the impetus of the pour ; and he remarked that the surface-lines of imperfect 
union (“ crowsfeet ”) occurred chiefly where these waves met. To provide for a more 
regular diffusion of the melted metal over the mould, the cast-iron frame supporting it 
was made to turn at the end next the crucible, on an axis parallel to that of the cradle. 
The other end was raised or lowered by a wedge, driven by a rack and pinion under the 
floor, which again was actuated by a vertical shaft worked at a convenient distance from 
the casting. 
3. As the central core interfered with the free flow of the metal, it was raised 1^ inch 
above the surface of the mould, and in part supported there by a pin of the same dia- 
meter. 
4. Instead of kaolin, the heated bed of hoops was washed with two parts animal char- 
coal and one of plumbago in fine powder, mixed with ale. 
These preparations being made, the second speculum was cast September 22, 1866. 
The oven had been fired for eight days and nights previous, the melting-furnace was 
lighted the evening before, and the charging the crucible commenced at 2 a.m. and was 
finished at 11. Up to this time coke was mostly used, and the melting took place earlier 
than was expected. Peat now was used and the draught greatly reduced ; the pouring 
took place at 12 b 20 m . 
The mould was set by the apparatus already described at an angle of 18° with the 
horizon, so that when a third of the metal was poured it would cover only half the mould 
on one side, and on the other rise to the edge of the loam-ring at its lowest part. At 
this instant the mould was rapidly lowered to be horizontal, the pouring being continued 
without interruption, and the fluid covering the entire surface with a nearly unbroken 
wave. The time was sixteen seconds. Instead of the bar and stirrup used to draw the 
cast off the bed, a simple bight of chain was found sufficient, and it was removed into 
the oven without any difficulty. The thermoscope marked 73*. 
The third casting was made November 24, at 12 h 10 m , without any change of the 
plan which has been described, and both these disks turned out exceedingly perfect. 
In these the centre was closed by a piece of metal 1^ inch thick. This piece was 
removed by grinding with emery and a ring of soft iron (like a trepan saw) ; it was 
attached to a double handle and worked backwards and forwards by two men, its entry 
being at first guided by a board with a hole of the same size. The cutting each disk 
required three days.] 
Difficult as it is to obtain a sound disk of good speculum metal, it is even more so to 
give it the figure required to form a perfect image combined with a fine polish. The 
accuracy of the finest cutting-tool that ever was devised falls almost infinitely short of 
what is wanted here ; which can only be obtained by the mutual abrasion of two surfaces 
working in contact. Yet it is not at first evident how such abrasion can produce any- 
* The details of the pouring, and the removal of the speculum into the oven are shown in Plate IY. fig. 8. 
The dotted lines show the position of the mould at the commencement of the pouring. 
