300 FORCING GARDEN. 
but their use has not hitherto become general. The ar. 
rangement of the flues must depend upon the nature of the 
house; it may, however, be remarked generally, that, as 
heated air has a tendency to ascend, they should be placed 
as near as can conveuiently be done to the front of the 
house, where, of course, the sloping roof is lowest. It is 
likewise-important that the flue should be introduced, and 
exert its greatest influence, at that point of the structure 
which is most exposed to any refrigerating cause. 
The furnace is most properly situate behind the house, 
and is generally covered by a shed. For the most part it 
is constructed so that the upper part of its arch shall be on 
a level with the top of the flue; but where a considerable 
heat is required, as in pine-apple stoves, it is found pre- 
ferable to sink the furnace, in order to produce a neck or 
rise of about a foot and a half in heighth, which moderates 
the intensity of the heat on its first entrance, and, by in- 
creasing the draught, causes the fire to burn freely. The 
size of the furnace must be regulated by the kind of fuel 
employed. Where coke or charcoal is used, it may be 
about eighteen inches square; but where small coal, turf, 
or peat is to be burned, it should be two feet, or even two 
and a half square, by two feet in height. A large furnace 
insures the long continuance of the fire, a fact which in 
practice has received too little attention. To resist the 
effects of heat, the interior should be lined with fire-brick. 
The roof should be strongly arched. The door may be 
about a foot square, and when it is double, as it ought al- 
ways to be, the outer half should be a little larger than 
the inner. The grate is of the same breadth as the door, 
and may extend about two-thirds of the length of the fur- 
nace, The ash-pit is, equally wide, and from fifteen to 
eightcen inches deep; it is furnished with a ventilator in 
