16 BULLETTX 747, U. S. DEBASEMENT OF AGEICULTTTEE. 
The functions of the fire box are to provide- means for burning the 
fixed carbon in the fuel, for the distillation of the gases, and for the 
thorough mixing of air at a high temperature with the unburned 
gases. 
COMBUSTION CHAMBER. 
The combustion chamber is really an extension of the fire box in 
which the burning of the volatile gases takes place and where the 
heat thus produced is absorbed by the water through the heating sur- 
faces of the boiler. "With horizontal return-tubular-boiler settings 
the term " combustion chamber " is applied to the space between the 
bridge wall and the ends of the boiler tubes. The horizontal dimen- 
sions of the combustion chamber are fixed to a great extent by the 
type of boiler setting. The depth seems to have little effect on the 
efficiency of the setting : consequently, the depth of the combustion 
chamber may be as great as the form of setting will permit. The 
common practice of sloping the* floor of the combustion chamber 
downward from the bridge wall to the clean-out door seems to have 
little value other than facilitating the removal of ashes. 
ASH PIT. 
The ash pit beneath the grates is to catch the ashes and refuse from 
the fire above and to provide an air reservoir for supplying air to the 
burning fuel. The depth of the ash pit should be sufficient to provide 
plenty of air for the burning of the fuel and to hold a considerable 
accumulation of ashes and clinkers without choking the air supply and 
burning the grate bars. The* bottom of the ash pit should be made 
wedge-shaped, of cement, to facilitate cleaning and to allow water to 
be placed under the gTates to prevent them from being burned. It 
should not be necessary to keep water in the ash pit except when the 
boiler is being forced. 
ASH-PIT AND FLUE DOORS. 
The ash-pit door, usually called the " ash-pit damper." is an open- 
ing to the ash pit for the removal of ashes, and it also serves to con- 
trol the air supply through the fuel bed. It is very important to 
have this door tight fitting in order to control effectually the air 
supply through the grates. 
The flue doors provide an opening to the smoke box at the front 
ends of the flues. These doors, unless they fit tightly, are a source of 
air leaks. 
CHIMNEY. 
The function of a chimney is to provide a draft to effect combus- 
tion of fuel on the grate and to carry off the resulting obnoxious 
gases. The chimney should be built carefully to prevent cracks 
