310 FORCING GARDEN. 
coal which does not cake very muck has been found to ‘be 
well, adapted for the purpose, as itis soon-formed into coke:: 
te 
‘Fig. 31. 
FTES 
i@\ 
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 Gardener’s 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 isdestroyed. In this man- 
ner; one of these conical furnaces being: lighted and filled 
witb 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 maybe 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 
