198 



FIXED STRUCTURES FOR GROWING 



determined. One fire with a flue in front, and a return in the back, 

 is generally found sufficient for a greenhouse of thirty feet or forty feet 

 in length, and from twelve feet to fifteen feet in width, and two fires, 

 one entering at each end, for a stove or forcing-house of similar dimen- 

 sions ; the flues in both cases being twenty inches high, and twelve inches 

 wide, outside measure. Perhaps one square foot of flue for every two 

 feet in length of iron hot- water pipes, found according to the rule given in 

 Art. 500, 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 height of the space allowed for the heated air to 

 ascend before it is permitted to escape into the atmosphere ; and the flue gene- 

 rally terminates on the top of the back wall, for the same reason. The fire- 

 place 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 receiving 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 is more equally diff'used along the front. A very desirable arrange- 

 ment for flues, where it is practicable, 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 efi^ected 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 ; 6, the 

 under or reserve flue ; c, 

 the damper ; the fur- 

 nace ; e, 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 

 Fig. 137. Section of a furnace and double flup. quite detached, in Order 



to furnish an extra supply of heat, when the external air suddenly 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 giv-e 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 oft\ But this may generally be supplied by underground cross drains, as 

 in fig. 138, in which g is the floor of the house ; A, the reservoir flue, three 

 feet broad, which is sunk so that its top is on a level with the floor ; an air- 

 drain from the back of the house ; /<:, an upper flue for additional heat ; /, 



