1829.] On the Manufacture of Optical Glass. 287 



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-bars, the brickwork is carried on horizontally 

 until close to the stack. The sides of this part are perpen- 

 dicular, and 12 inches apart: they are continued upwards to 

 the top of the brickwork 14 inches unbroken, except that at 

 5 inches from the bottom they are thrown back Jrd of an inch 

 so as to form a ledge there. This ledge is for the purpose of 

 receiving the edges of certain fire-tiles, which, when put in, 

 form the top of the flue and at the same time the bottom of 

 the glass chamber ; but the whole is so constructed, that the 

 tiles can be put in and taken out at pleasure without disturbing 

 the rest of the work. The side, or rather end of the chamber 

 nearest the fire, is constructed of a fire-tile, which terminates 

 and faces the brick arch over the fire-place, and extends from 

 the surface of the brickwork downwards 9 inches to the side 

 ledges before described: the further end of the chamber is 

 finished in a similar way, and beyond that the flue is carried in 

 the most convenient and direct manner, but without any unne- 

 cessary contraction, into the stack or chimney. The length of 

 this upper aperture, afterwards constituting the chamber, is 

 25 inches, its breadth 12 inches. When the bottom tiles are 

 in their places, they leave a depth of 5 inches for that part of 

 the furnace or flue beneath the chamber, which is also 38 inches 

 from the fire to the end, and, with the exception of certain 

 supports in it, is 12 inches wide. 



These supports are built in with the bottom of the flue. 

 They are essential to the permanency and regularity of the 

 bottom of the glass chamber, and require considerable nicety 

 in their arrangement. They consist of fire-bricks placed up on 

 end, so that their narrowest surfaces are towards the ends of 

 the furnace, their sides or broadest exposed surfaces parallel 

 with the sides of the furnace itself. They rise to the same 

 height above the bottom of the flue as the ledges on the sides 

 of the brickwork, or 5 inches ; and with them form the support 

 for the bottom tiles. There are three of them in the furnace, 

 placed in a line equidistant from the two sides of the flue ; and 

 being 2~ inches thick, they leave spaces for the passage of flame 

 and the reception of coke, which are 4f inches in width. The 



