216 On the Construction of Flues of Hot-Houses. 



set up, A.B. is supported a few inches from the ground by 

 short pillars at each corner of the pieces forming it ; or it 

 may be laid on stones or bricks ; the casting the pieces with 

 pillars C.C. adds but little to the expense. Each piece form- 

 ing the flue should not exceed eighteen inches square ; or if 

 they are required to form a large flue, they should be made 

 oblong, so as not to have a greater superficies than a square 

 of eighteen inches ; for the larger the pieces are made, the 

 more liable they will be to injury from various causes. When 

 it is desired to turn the flue into different directions, the cor- 

 ners are made of three pieces, and the form of them is ob- 

 vious, without being expressed in a drawing. 



The bottom may be left bare, as it is not exposed to injury, 

 and that the air passing along it may be heated as much as 

 possible. But the upper part, while it must be covered in 

 order to make it air-tight, and to protect the iron, must have 

 some substance applied to it, from which heat may quickly 

 radiate. From a mixture consisting of one part of clay and 

 three parts of sand, we obtain this object. After the iron is 



