ARTIFICIAL HEAT. 299 



city, and will deprecate useless expenditure, so often ex- 

 hibited in this department, as injurious to the character ss 

 well as to the progress of horticulture. 



Artificial Heat. Forcing-houses are heated in various 

 ways ; by means of flues conveying smoke and heated air ; 

 by pipes conducting steam or hot water ; by so construct- 

 ing the glazed house as to increase the calorific action of 

 the sun's rays * and sometimes by the heat generated i;i 

 the course of the fermentation of vegetable substances. 



Flues are generally constructed of common brick, though 

 occasionally fire-brick is employed in the neck, or that part 

 of the flue immediately adjoining the furnace. * The bricks 

 in the side walls are placed on their edges, and the top 

 covering is of tile an inch and a half in thickness. In di;-- 

 tricts where sandstone flag abounds, the covers are often 

 formed of that material. Horticultural writers have recom- 

 mended that flues should be about eighteen inches deep, and 

 of nearly equal breadth ; but to obtain the greatest quantif y 

 of heat, it clearly appears, from the experiments of Mr. 

 Stevenson (Col. Hort. Mem., i. 143,) that, where possible, 

 the breadth should be nearly double the depth. It is ad- 

 vantageous to detach flues as much as possible from tl.o 

 walls of the building which encloses them, in order tint 

 the heat may be communicated to the air only. Former];/ 

 they were often built, sometimes one above another, with 

 only one side exposed, a practice which, as it occasioned 

 great waste of heat from conduction, has been generally 

 abandoned. When it is necessary to lead one flue abovo 

 another, or to make it return upon itself, spaces should b3 

 left between them, to allow the free passage of caloric from 

 every side. 



With a view to economy of fuel, can-flues and cast-iron 

 cylinders have been proposed, and occasionally adopted, 



