200 FIXED STRUCTURES FOR GROWING 



c is the smoke chamber, and d the zinc cistern, over the ten-inch tile cover. 

 The inside plastering should be of the best mortar, mixed 

 with lime, but without sand, as being less liable to crack. 

 494. The furnace, when built in the usual manner, 



/ [^ j\ should have double iron doors to prevent the escape of 



heat; and the fuel- chamber should be about double 

 the area of that portion of it which is occupied by the 

 bars or grate, in order that the fuel not immediately 

 over the grate may burn slowly. A damper in some 

 accessible part of the flue, and as close to the furnace as 

 is practicable, affords a convenient means of regulating 

 the draught ; and there ought always to be a register 

 valve in the ash-pit door for the same purpose. Where 

 cinders, coke, or anthracite coal only are burnt, no hori- 

 zontal opening to the grate containing the fuel is neces- 

 sary. It may be put in by an opening at the top, as in 

 fig. 137, which will contain a supply for any length of 

 time, according to the height and width of the opening' 

 and the bars of the grate can be freed from ashes by a 



Fig. 139. Section of a com- hooked poker applied from the ash-pit. By this kind of 



line cistern*, llrit^ construction less lieat is l st than V anv other. Indeed, 

 this kind of fireplace, with a reserve flue, will be found 

 by far the most economical mode of heating hothouses ; but it will not answer 

 where the practice is to depend on the sudden action of the flue, which is pro- 

 duced by stirring up the fuel : in lieu of this, the damper must be drawn so 

 as to admit the heated current into the extra heat flue. Whatever may be 

 the construction of the furnace, no air ought ever to be admitted to the fire, 

 excepting through the grating below it ; because air admitted over the fuel 

 can serve no purpose excepting that of cooling the flue ; unless in very rare 

 instances, where it might assist in consuming the smoke. Where this 

 object is a desideratum, Witty's smoke-consuming furnace, described in 

 Gard. Mag. vol. vii. p. 483, which roasts or cokes the coal, before it is put on 

 the fire, may be had recourse to. This and various other details, however, 

 must be left to the bricklayer or mason employed. All flues ought to have 

 flag-stones of the width and height of the interior of the flue, or iron doors 

 built into them at the extremities of each straight -lined portion, which may 

 readily be taken out or opened in order to free the flue from soot ; an opera- 

 tion which will require to be performed at least once a year in all houses, 

 and in stoves twice a year, or oftener, according to the kind of fuel used. 



495. As substitutes for smoke-flues, earthenware pipes, or can- flues, as they 

 are called, have long been in use in Holland and France ; and as the fuel 

 used in these countries is almost always wood, which produces little soot in 

 comparison with coal, they are found to answer as perfectly as brick flues. 

 When they are only occasionally employed, the entire surface of the pipes 

 is exposed ; but when they are used constantly, as in houses for tropical 

 plants, they are embedded in a casing of dry sand, which forms a reservoir 

 of heat capable of being increased to any extent, even to that of the entire 

 floor of the house, over which a flooring for plants may be placed. Pipes 

 of this kind might also be conducted through a bed of small stones, so as 

 to form a very effective mass of heated material as a reservoir, while a portion 

 of naked pipe might serve for raising the temperature on occasions of extra- 



