FORCING PITS OR GREEN-HOTJSES. 65 



sheltered from the northwest, the same amount of flue 

 will heat 60 feet, quite as easily as in exposed places it 

 will heat 40. The proper way of constructing the furnace 

 and flue, is of importance enough to require a description. 

 The size of the furnace doors should not be less than 1 

 foot square, the length of the furnace bars, 2 feet ; the 

 furnace should be arched over, the top of inside of the arch 

 at least 18 inches from the bars. The flue will always 

 " draw " better if slightly on the ascent throughout its en- 

 tire length ; it should be elevated, in all cases, from the 

 ground, on flags or bricks, so that its heat may be given 

 out on all sides. The inside measure of the flue should 

 not be less than 8 x 14 inches ; if tiles can be conveniently 

 procured, they are best to cover with, but if not, the top 

 of the flue may be contracted to 6 inches, and covered 

 with bricks. Care should be taken that no woodwork 

 connect with the flue at any place. I have known cases 

 where woodwork has caught fire at 70 feet from the fur- 

 nace, after the house had been in operation for three years ; 

 but an unusually strong draft intensified the heat,and the 

 charred timber ignited and totally destroyed the green- 

 house and its contents. In the plan and section on the 

 opposite page, A, is the shed, enclosing the furnaces C C ; 

 from which pass the flues, D, in the direction shown by 

 the arrows to the chimnies, L. The benches are not 

 shown here, but they are arranged as in fig. 23. 



