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VEGETABLE GARDENING 
161. Heating frames by the use of flues. — Many grow- 
ers heat their frames by a system of flues leading from a 
furnace burning wood or coal. The plan is regarded as 
more economical than the hotbed method, and the results, 
with proper management, are equally satisfactory. The 
furnace or fire box at one end of the frame is cheaply 
constructed, and should be 3 or 4 feet long, -and about 18 
inches square, with provision for an ash pit under the 
grate bars. A main flue is built from the fire box, ex- 
tending 8 or 10 feet under the soil of the frame, where it 
FIG. 21. STEAM-HEATED FRAME 
separates into two smaller flues that continue almost to 
the other end of the frame, which may be at least 60 feet 
long, before they join again and enter a chimney. Figure 
20 shows this system of heating, but in a different form 
of construction. The flues may be made of brick, stone 
or tile, and may vary from 6 to 12 inches in size. Ten- 
inch tile are very satisfactory. Near the furnace the flue 
should be about 3 feet under the surface, rising gradually 
to 1 foot at the chimney. The air in the flue-heated 
