FURNACE AND ITS APPENDAGES. 



be supplied, not in large quantities, and at distant intervals, but 

 in small quantities and more frequently. On the other hand, the 

 more common practice is to allow the fuel on the grate to be in a 

 great degree burned away, and then to heap on a large quantity 

 of fresh fuel, covering over with it the burning fuel from the 



Fig. 10. 



back to the front of the grate. "When this is done, the heat of the 

 ignited coal acting upon the fresh fuel introduced, expels the gases 

 combined with it, and, mixed with these, a quantity of carbon, in 

 a state of minute division, forming an opaque black smoke. This 

 is carried through the flues and drawn up the chimney. The 

 consequence is, that not only a quantity of solid fuel is sent out 

 of the chimney unconsumed, but the hydrogen and other gases 

 also escape unburned, and a proportional waste of the combustible 

 is produced ; besides which, the nuisance of an atmosphere filled 

 with smoke ensues. Such effects are visible to all who observe 

 the chimneys of steam vessels, while the engine is in operation. 

 When the furnaces are thus filled with fresh fuel, a large volume 

 of dense black smoke is observed to issue from the chimney. This 

 gradually subsides as the fuel on the grate is ignited, and does 

 not reappear until a fresh feed is introduced. 



16. The former method of feeding, by which the furnace would 

 be made to consume its own smoke, and the combustion of the fuel 

 be rendered complete, is not however free from counteracting 

 effects. In ordinary furnaces the feed can only be introduced by 

 opening the fire-doors, and during the time the fire-doors are 

 opened a volume of cold air rushes in, which passing through the 

 furnace is carried through the flues to the chimney. Such is the 

 effect of this in lowering the temperature of the flues, that in 

 many cases the loss of heat occasioned is greater than any economy 



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