310 FORCING GARDEN. 



coal which does not cake very much has been found to be 

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



Fig. 31. 



The following is the rationale of the process of the heat* 

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

 of the Gardeners Magazine for 1840 : 



" As fuel cannot be consumed without air, if a furnace 

 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 re- 

 ceives 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 man- 

 ner, one of these conical furnaces being lighted and filled 

 with fuel, that portion in the upper part of the furnace, 

 which cannot burn, absorbs 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 by the chimney the whole being taken up by the 

 surrounding water. The economy, therefore, of fuel in 

 such an apparatus is very great. It is evident that excess 



