By Mr. John Hay. 



221 



where the vacuity extending up the sides, and over 

 the arch of the furnaces, commences. These open- 

 ings are furnished with plugs, and are for the pur- 

 pose of admitting cold air, to be heated by coming 

 in contact with the warm parts of the furnace. 

 c, Warm air from the top of the furnace, passing 

 through the air flue in the lower part of the wall, and 

 escaping at d Qy Fig. 3), where there is fixed a square 

 piece of cast iron, having a socket in the centre and 

 brass plug to fit, secured by a bit of chain. These 

 plugs are withdrawn when the furnace is heated, per- 

 mitting a current of warm air to pass forward, which 

 assists in warming the outer wall. When the fire is 

 taken off, the plugs are replaced in their sockets. 

 e, a vacuity in the wall A, similar to the air flue in 

 the wall B, just described, and connected with it, to 

 prevent any overheating of this part of the wall. 

 f, a narrow vacuity extending nearly to the top, for 

 the purpose of checking the expenditure of heat upon 

 the north or back part of the wall, and to preserve 

 the heat as long as possible on that side of the flue. 

 Fig. 2. Longitudinal section ; gg, foundation ; hh, entrance to 

 the smoke flues ; i, passage for heated air from the 

 top of the furnace to warm the air flue (c, Fig. 1) in 

 the front of the wall; kk, chambers connected with 

 the smoke flue for the purpose of retaining the heat, 

 formed by laying two bricks in length ; //, divisions 

 separating the chambers, formed of bricks laid in 

 breadth. When a hot wall of this description is built 

 upon a level, openings might be left in the middle 



