THE GREAT MELBOURNE TELESCOPE. 
139 
was not in sufficiently close contact with the bed of hoops, and a film of fluid metal 
found its way here and there, and sunk partially into the small hollows where the ends 
of the hoop iron abutted on the ring which confined it. The remedy was obvious — to 
lute the joint with a little wet loam, which dries instantly ; and it proved quite success- 
ful in subsequent casting. The iron stoppers of the oven, already described, were set 
in their places, hot bricks were placed outside them, and the whole luted up ; all other 
apertures, including the entries to the chimneys, were carefully stopped. In doing this 
a thermo-couple of platinum and iron wire was inserted, so that its joint was nearly over 
the centre of the speculum ; and its elements were connected by copper wires with a gal- 
vanometer in Mr. Howard Grubb’s office, where it kept watch over the cooling. Next 
morning the galvanometer deflected 69°, which in twenty-four days decreased to 0°, 
showing that the cooling was complete. See Appendix No. I. 
The whole was effected without any accident, though the workmen in some parts of 
the room suffered a good deal from the heat. Gas had not yet been introduced, and 
the candles melted so quickly that they were of little use. 
On removing the casting from the oven it proved to be sound, but its surface was 
“ in winding ” about \ inch ; beside these were some of the imperfections which Lord 
Rosse has called “ crowsfeet.” Had these been at the highest part of the surface, they 
would have disappeared in the grinding ; but this was not the case, and Mr. Grubb pre- 
ferred recasting it. 
[Advantage was taken of breaking up the cast to try its strength. It was placed 
horizontally on four short blocks of wood under an iron ram weighing 70 lbs., which 
could fall on it from a height of 4 feet. A strong wooden bar, about 30 inches long, was 
laid across the speculum and received the blows of the ram without effect ; but when a 
metal bar was substituted for the wooden one, the disk broke into four very equal 
portions. These, on being brought together, showed no sign of unequal tension, which, 
along with the considerable force required for fracture, shows how perfect the annealing 
had been. 
The breaking up of this casting revealed an unexpected defect — a cavity about 9 inches 
long, its cross section being a flattened oval large enough to admit three fingers ; it lay 
in the middle thickness of the metal, and was lined with oxide studded with brilliant 
metallic crystals. Probably it was caused by air entangled in the rapid pouring, and 
had the speculum been polished, its figure might have changed with any considerable 
variation of temperature. 
Before a second trial the following changes were made : — 
1. To guard against flexure of the bed of hoops, it was strengthened by additional 
underlying bars, and also placed for the casting on a strong cast-iron frame*. 
2. Mr. Grubb observed in the first pouring that the fluid metal was separated by the 
central core into two waves meeting at the opposite side of the mould, returning and 
* The importance of this is obvious ; for as the bed of hoops becomes red-hot when the fluid metal is poured 
on it, it is liable to “ sag ” under the great pressure, and thus warp the surface of the casting. 
