ECONOMICAL. USE OP FUEL IN CEEAMEEIES. 19 
tiou of power ; consequently the economical firing of the boiler pre- 
sents problems different from those in plants that operate under 
practically constant load throughout the day. In many large power 
plants instruments are used to measure the temperature and com- 
position of the flue gases, which greatly aids the fireman in main- 
taining the fire bed in the best condition. In the smaller plants, 
where the load is intermittent and the boilers are fired for only a 
few hours during the day, it is impracticable to use flue-gas instru- 
ments ; consequently the fireman is forced to depend entirely on his 
judgment as to the condition of the fire. 
The chemical action that goes on in the furnace during the com- 
bustion of the fuel is an exceedingly complicated one and is not very 
well understood. While important, it is not absolutely necessary, in 
order to get good results, for the fireman to understand in general 
the chemical changes that take place during the combustion of the 
fuel. Hand firing of a boiler furnace is a combination of science and 
skill which is generally acquired through long experience and by 
careful observation of the conditions of the. fire. 
To get the best results definite proportions of fuel and air must 
be used, and the air must be evenly distributed throughout the fuel 
bed so that all parts will get the proportion of air necessary to effect 
complete combustion. Too much air absorbs heat from the fire and 
furnace walls, and. the heat is carried up the chimney, while too 
little air causes the fire to smoulder,, with the result that the com- 
bustible gases from the fuel are carried off without being consumed. 
The fireman must learn from the appearance of the fire whether 
the required proportion of air is being admitted, and if it is properly 
distributed. He must be able also to recognize any defects in the 
fuel bed, clue to clinkers, ashes, holes, too thick or too thin fires, etc, 
With an even fuel bed clinkers or ashes on the grate are indicated 
by dark spots on the fuel bed or by shadows cast on the floor of the 
ash pit. In order better to observe the condition of the fuel bed 
through the casting of shadows in the ash pit the floor of the pit 
should be kept clean. Holes or thin spots may be noted by the white 
color of the burning fuel, indicating that too much air is being ad- 
mitted at that point. The color of the fire bed indicates whether it 
is too thin or too thick. If too thin, the fire shows a uniform!} 7 white 
color, but if too thick it is more of a red color. An uneven fire bed 
shows dark and bright spots. 
Different, kinds of coal require different treatment in firing to 
obtain the best results. For instance, with soft coal the best results 
are obtained when the fires are kept level and relatively thin, from 
G to 8 inches, under average conditions of draft. Coal should be 
added often and in small quantities, and carefully spread evenly 
over the thin spots in the fuel bed. The fuel bed as a whole should 
