GENERAL MANAGEMENT OF GREENHOUSES. 
133 
In order to justify the foregoing statement, it will be necessary to refer to former 
usages, and point out whence the present advantages arise. The common furnace 
has usually consisted of a large grate, which formed the floor and support of the fire, 
and it was generally surmounted by an arch or boiler, or any body to be heated. 
It frequently, but not always, had two doors, one to admit the fuel, the other to 
facilitate the stoking, and to draw out the ashes ; but this excessive extent of grate 
admits too much air, by which the fire is kept below the temperature required to 
consume the smoke ; and, omitting all consideration of the clouds of smoke vomited 
during the first kindling and raising the fire, a similar emission of clouds occurs 
whenever a fresh supply of fuel is required, until the parts of the coal which sublime 
in dense smoke and soot are dispersed in nuisance and waste. The great cause of 
this is, that the moment the door of the furnace is thrown open, a vast quantity of 
cold air sweeps under the boiler or through the flues, and carries away the heat; then 
the cold, raw, or perhaps wet coal, is thrown into the centre of the fire, which reduces 
the temperature ; destroying, in one moment, all the effect required, liberating the 
product of the coal in an imperfect state at a low temperature, and thus producing 
volumes of thick smoke, which, when once formed, it becomes impossible to burn, 
except at a temperature that will melt iron, which is stated to be 3000 degrees, and 
this, of course, would require an amazing quantity of fuel. In Witty’s Patent Gas 
Furnace, a certain proportion, say about a third only, of the floor consists of a grate, 
a, between which and the feeder, b, an inclined plane is placed, c, at an angle of 
thirty or forty degrees ; instead of the common door or doors, a sufficient orifice is 
well fitted by an iron box, d, the part of which, nearest the fire, is left open in the 
form of a hopper or feeder, to receive the charge of fuel. A slate is closed upon 
the fuel, and it is pushed forward on the inclined plane by a smaller box, or square 
piston ( e ), fitted within the hinder part of the large box, moved by a screw, by 
which it is brought back to its first position, when a fresh supply is required. An 
arch of brick-work (f), covered by the best non-conductors of heat, such as powdered 
charcoal, or saw-dust, &c., surmounts the plane, and leads to the flue, to the bottom of 
the boiler, or to the body to be heated. A breast- work of brick behind (g), projecting 
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