MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
53 
raise and remove the cover from place to place. When a crucible is in use, 
the cover should be arranged over it in such a manner as not to touch the ves- 
sel, but rest by its edges on the earthenware plate around. 
The platina stirrers in use with this furnace have been before described 
(28. 7 5) fig. . 3 . The platina ladle consists of a small crucible of that metal 
riveted to a platina wire, and that made fast by a screw to an iron rod. (fig. 4.) 
The use and manner of working this furnace will be well understood from 
the above description, and what has before been said (26, &c.). The crucible 
should never be suddenly heated or cooled. The coke may be fed and ar- 
ranged at such of the crucible holes as are out of use at the time. From the 
very valuable effects of a trough of water under the fire bars (45) experienced 
in the larger furnace, one is constantly used in that just described. 
Finishing furnace. 
This furnace on the outside is a parallelopiped, principally of brickwork, built 
against a wall, 64 inches in length from the fire front to the beginning of the flue, 
against which it is built, 45 inches wide, and 28 inches high. (fig. 5. 6 . & 7-) It is 
the only one that has yet been built, and, for the reasons before given, shall be 
described exactly as it is. The fire-place is at one end, and the course of the 
flame and smoke is directly from that to the other end, and then immediately 
into the upright flue. The fire-place is 15 inches from back to front, 13 inches 
wide, and 11^ inches from the arched roof to the bars. Its outward side, or 
that from the wall, is 18^ inches in thickness of brickwork, which is intended 
to give stability to the structure. The mouth of the fire-place is an aperture 
8 inches by 6 inches, made in a piece of fire-stone 7 inches inwards from the 
front of the brickwork : its lower edge is level with a fire-stone sill, which, 
extending forwards from the fire-place to the outer surface of the brickwork, 
forms a shelf, on which two bricks stand, that serve in place of a door to close 
the mouth of the furnace. The ash-pit is 25 inches long, 12 inches wide 
under the fire, and 10 inches high to the bars. A trough made of rolled iron, 
riveted together, and 5\ inches high on the sides, occupies its lower part. 
This being preserved full of water, is sustained at the boiling temperature by 
the radiation of heat and the hot ashes which fall into it. 
From the back part of the fire-place, and 2 inches above the level of the fire- 
