168 



FIXED STRUCTURES FOR GROWING 



Fig. 139. 



narrow flue than a broad one. Hence also narrow deep flues are found 

 to " draw" better than broad shallow ones. The ordinary dimensions 

 of narrow flues are eight inches in width and fifteen inches in depth ; 

 and they are formed by tiles one foot square for the bottom, and ten 

 inches square for the covers, and three paving-bricks, which are only 

 two inches thick, on edge, for each of the sides, as 

 in fig. 139. The joints of the sides and covers are 

 formed by lime putty, and the bottom tiles are set 

 on bricks on edge. In fig. 139, a is the brick on 

 edge, which supports the one-foot tile 5, which 

 forms the bottom of the flue ; c is the smoke cham- 

 ber, 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. 



The furnace, when built in the usual manner, 

 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 bar 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 

 Section of a common ash-pit door for the same purpose. Where cinders, 

 brick flue, with a co ], e or anthracite coal only are burnt, no horizontal 

 zinc cistern over , J . . -, /. i 



iL opening to the grate containing the fuel is ne- 



cessary. 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 heat arid width of the opening, and the bars of the 

 grate can be freed from ashes with a hooked poker applied from the 

 ash-pit. By this kind of construction less heat is lost than by any 

 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 produced 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, except through the grating below it ; because 

 air admitted over the fuel can serve no purpose but 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-con- 

 suming furnace, described in l 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 



