ARTIFICIAL HEAT, 299 
city, and will deprecate useless expenditure, so often ex- 
hibited in this department, as injurious to the character as 
well as to the progress of horticulture. 
Artificial Heat.—Forcing-houses are heated in various 
ways’; by means of flwes 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 in 
the course of the fermentation of vegetable substances. 
Flues are generally constructed of common brick, though 
occasionally fire-brick is employed in the eck, 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 dis- 
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 quantity 
of heat, it clearly appears, from the experiments of Mr. 
Stevenson (Cal. Hort. Mem., i. 148,) that, where possible, 
the breadth should be nearly double the depth. It is ad- 
vantageous to detach flues as much as possible from the 
walls of the building which encloses them, in order that 
the heat may be communicated to the air only. Formerly 
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 above 
another, or to make it return upon itself, spaces should be 
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 occasionaily adopted, 
