353 



Glass 



there should be a rise in the underlying pipe of at least 

 one foot in twenty-five. The greater the rise in this pipe, 

 the more perfect will be the draft. If the runs are not 

 too long, the underlying pipe may return beneath the 

 beds and enter a chimney directly over the back end of the 

 furnace, and such a chimney, being warmed from the fur- 

 nace, will ordinarily have an excellent draft. 



The underlying pipe should occupy a free space or 

 pit beneath the beds, and whenever it lies near to the floor 

 of the bed or is very hot, it should be covered with asbestos. 



219. Cross-section oJ a flue-heated hotbed. 



The construction of a flue-heated hotbed is thus de- 

 scribed by Ness (Tex. Exp. Sta.), and shown in Fig. 

 219 : " A furnace is constructed outside the frame and 

 sunk about two feet below the level of the surface. From 

 this furnace two lines of 4- to 6-inch vitrified sewer pipe 

 are placed obliquely through the soil to the opposite end 

 of the frame, where vent to the smoke from the furnace 



