206 GREENHOUSE CONSTRUCTION AND HEATING. 
retain its warmth, and the flue, consequently, to keep on 
. drawing, than where the stack is at a distance from the fire. 
_ In this excellent form of flue, of which the furnace is 
illustrated in Fig. 143, the lower, or “flow” portion (B B) is 
best constructed entirely of brick, as shown, though if 
more convenient the farther part, beyond the first 5ft. 
or 6ft., may consist of Gin. or Yin. pipes, as above. For the 
upper, or “return” portion (cc) the glazed socket-pipes 
already mentioned will answer 
admirably, and these may be fixed 
either directly over the lower brick 
flue (though this plan is rather in- 
convenient when the latter has to be 
BB. cleaned), or be carried along at the 
F 4 side, though of course at a slightly 
higher level. Both flues ought to 
be constructed with a gentle up- 
ward inclination throughout. 
As already stated, the chimney is 
to be placed as near as possible to 
LE 
nie the furnace, and of course the 
farther end of the return flue dis- 
BiG. 148. charges into it atm. Another and 
separate passage, or flue, c, lead- 
ing directly from the back, or upper part, of the furnace into 
the base of chimney must be arranged, and be fitted with an 
ordinary damper, as shown at B in the illustration. When 
the fire is first lit this damper is pulled out, and the fire 
then draws directly up the shaft, but as soon as there is 
a good fire, and the chimney is thoroughly warmed, the 
damper is pushed in, when the draught is forced to travel 
