286 On the Manufacture of Optical Glass. [1S2& 



bent round the opposite edges of the latter. The bent extremity 

 of an iron rod passed under the loop thus formed over the middle 

 of the bottom, serves to raise and remove any 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 vessel, 

 but rest by its ecjges on the earthenware plate around. 



The platinum stirrers in use with this furnace have been before 

 described (28. 7.5), fig. 3. The platinum ladle consists of a small 

 crucible of that metal riveted to a platinum 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 arranged at such 

 of the crucible holes as are out of use at the time. Because of 

 the very valuable effects of a trough of water under the fire-bars 

 (45) experienced in the larger furnace, one is constantly used 

 with that just described. 



Finishing furnace. This furnace on the outside is a paral- 

 lelepiped, principally of brickwork, built against a wall ; it is 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 

 (figs. 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 imme- 

 diately 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 

 18J 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 thq 

 bars. A trough made of rolled iron, riveted together, and 5~ 

 inches high on the sides, occupies its lower part. This being 



