166 



FIXED STRUCTURES FOR GROWING 



twelve feet to fifteen feet in width, and two fires, one entering at each 

 end, for a stove or forcing-house of similar dimensions ; the flues in 

 both cases being twenty inches high and twelve inches wide, outside 

 measure. Perhaps one square foot of ilue for every two feet in length 

 of iron hot- water pipes, found according to the rule given on page 174, 

 would be a near approximation to the quantity wanted, reckoning the 

 top and sides of the flue, but not the bottom. The furnace or fireplace 

 from which the flue proceeds should be one or two feet lower than the 

 level of the bottom of the flue, in order to assist in creating a draught, 

 as that depends on the length and keight of the space allowed for the 

 heated air to ascend before it is permitted to escape into the atmo- 

 sphere ; and the flue generally terminates on the top of the back wall, 

 for the same reason. The fireplace is generally formed behind the 

 back wall for the sake of concealment : but when this is not an object, 

 the best situation is at one end of the house, in a sunken area, which 

 can be covered with shutters ; because, the smoke and heat not receiv- 

 ing the check given by a turn in the flue made so near the furnace as 

 it must necessarily be when it enters from behind the house, the heat 

 Fig. 137. is more equally diffused 



along the front. A very 

 desirable arrangement for 

 flues, where it is practi- 

 cable, is to have two from 

 \ the same furnace, with the 

 power of throwing the 

 whole or any part of the 

 smoke and heated air into 

 either flue at pleasure, 

 which is easily effected 

 by a damper at the throat 

 of the flue, close to the 

 furnace, as shown in fig. 

 137, in which a is the 

 upper or extra heat flue ; 

 &, the under or reserve 

 flue : c, the damper ; <:/, the 

 Section of a furnace and flue. furnace; , the cover to 



the feeding hopper; and /is the ash-pit. One of the flues should be 

 conducted through a solid mass of brickwork or masonry, or through 

 a box or bed of sand, in order to produce a reservoir of heat ; and the 

 other flue should have thin covers and sides, and be quite detached, in 

 order to furnish an extra supply of heat, when the external air sud- 

 denly became much colder than usual, or at particular times to dispel 

 damp, &c. Both flues ought to be near the front of the house, and, in 

 most cases, the one might be over the other. Wherever flues are sunk 

 below the level of the floor, they will be found to give out their heat 

 very slowly ; or, if given out, to lose it in the adjoining ground, from 

 the want of a current of air to carry it off. But this may generally 

 be supplied by underground cross drains, as in fig. 138, in which g is 



