THE GBEAT MELBOURNE TELESCOPE. 
137 
oven G. This oven was constructed with great care ; it is circular, to ensure uniformity 
in the cooling of the enclosed speculum, and is strongly bound with a massive band of 
iron to prevent cracking. Upon a thick foundation of rubble work were laid several 
courses of brick with a layer of sheet iron interposed to prevent moisture rising from the 
ground, and on this were constructed the fire-place and flues for heating the bed of the 
oven. This bed is formed, first, of several courses of fire-bricks, on which are laid fire- 
tiles 12 inches square previously ground separately to the required curve, and then set so 
as to form a continuous bed of the radius required for the under side of the speculum. 
The circular wall of the oven is 27 inches thick, and the details of its construction are 
fully shown in Plates III. & IV. figs. 2 to 8. The system of flues below the bed, of which 
one or more can be readily stopped if required, gives great facility for heating it equably; 
and the wall and arched roof of the oven (which was covered with ten tons of sand) are 
heated by a mixed fire of coke and turf, burning in the oven itself, the combustion being 
regulated by its independent air-hole, damper, and chimney. 
For the quick opening and closing of the mouth of the oven, a series of cast-iron 
stoppers were provided, coated internally with loam, and with removable handles outside. 
These, when in situ , complete the circle of the wall, and give great facilities for opening 
and closing the oven when the speculum is introduced. 
A single crucible of cast iron holds enough of metal for a casting ; it was cast with 
the precautions indicated by Lord Rosse; its internal dimensions are 25 inches dia- 
meter, 32 inches deep ; the bottom is rounded, and its thickness is from 2 inches 
there to 1| at top. It easily holds 30 cwt. of the alloy, 27 being required for a 4-feet 
speculum. 
The operations of casting and annealing were conducted essentially as described by 
Lord Posse, with such modifications as the case required, or were indicated by experience. 
In constructing that part of the mould which forms the edge of the speculum, a different 
plan was followed from the beginning, for two reasons. First, Lord Rosse had expe- 
rienced a difficulty in “ timing ” the operation, because the wet sand-ring which was the 
edge of the mould was quickly dried when placed in contact with the hot bed of hoops, 
and consequently was liable to crumble down. Second, the plan for lateral support of 
the speculum proposed by Mr. Grubb (G. M. T. Correspondence, p. 25) makes it desirable 
that the speculum should be cast with a central band about ^ inch thick, and one- third 
of the speculum thickness in breadth ; and to have effected this with a mould of damp 
sand would have been nearly impracticable. The edge of the mould was therefore 
formed as follows. A strong sheet-iron hoop, 8 inches deep, and of the same diameter as 
the bed of hoops, strengthened internally with rings of angle-iron, and its lower edge 
turned true, was lined with loam moulded to the required shape. This being fully dried, 
the ring was attached by clamps to the bed of hoops previously heated, where it remained 
while other matters connected with the casting were preparing. It may be remarked 
that this ring was not withdrawn from the speculum when it was removed to the oven ; 
this was convenient for the manipulation, and Mr. Grubb thought it would probably 
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