326 



FORCING GARDEN. 



coke, gas-cinders, and anthracite ; but common coal 

 which does not cake very much has been found to be 



Fiii. 31. 



well adapted for the purpose, as it is soon formed into 

 coke. 



The following is the rationale of the process of the 

 heating of this boiler, as given by Mr. Rogers in the 

 volume of the Gardener'' s Magazine for 1840 : — 



" As fuel cannot be consumed without air, if a fur- 

 nace be constructed of considerable depth, and filled 

 with fuel, and air be admitted only at the bottom, that 

 fuel alone is consumed which lies immediately on the 

 bars, and first receives the draught of air. The fuel 

 above, provided it transmits the air, becomes red-hot, 

 or nearly so, but does not consume until that below it 

 is destroyed. In this manner, one of these conical fur- 

 naces being lighted and filled with fuel, that portion in 

 the uppei^part of the furnace, which cannot burn, ab- 

 sorbs the heat of the burning fuel below, and radiates 

 or transmits it to the water on every side. So perfect 

 is this absorption of heat that for several hours after 

 the furnace has been filled up with cinders, though 

 there may be a fierce fire below, little or no heat escapes 



