HOT \VATEfR. 



311 



of draught must be carefully guarded against, so much 

 only being allowed as will consume the fuel steadily, which 

 is easily learned by experience. The necessity, also, of 

 keeping the aperture in front close, so that air enters the 

 furnace only through the ash-pit, is hence evident. The 

 water (as may be observed in Fig. 31) is in close and im 

 mediate contact with the red-hot fuel on all sides, no black 

 smoking coals intervening, as in most kinds of boilers ; 

 hence the great power in proportion to size.&quot; 



The economy of fuel in these boilers is not their prin 

 cipal advantage ; but their great recommendation is a long- 

 continued and steady heat. When properly managed, they 



Fig. 32. 



may be depended on for preserving the heat for from fifteen 

 to twenty hours. They have been successfully applied to 

 all descriptions of hot-houses, but for pits they are emi 

 nently useful, from the small space they occupy ; and when 

 fired with coke, gas-cinders, or anthracite, they give off 

 very little smoke. 



It is unnecessary to describe all the numerous modi ca- 

 6 



