VALUE OF COAL AS A FUEL. 7 
slate and other impurities, or jigging or washing, a great deal de- 
pends on the care used in these processes. The mining companies are 
responsible in a large measure for variations in the grade of pre- 
pared coal. The purchase of coal under a contract on the basis of 
quality stimulates the operator to make a better preparation of the 
coal before it is shipped to market. An example of fluctuations in 
quality is furnished by the tables on pages 24-26, which show varia- 
tions in the ash and British thermal units in coal delivered to some 
of the Government Departments at Washington. 
It evidently will not be satisfactory to either the buyer or the seller 
to establish a standard for the coal unless this liability to variation 
is recognized and provision made for settlement when the coal is bet- 
ter or poorer than the standard. Experience with any method of 
buying coal shows that it will seldom be rejected when of poor qual- 
ity, because of the difficulty, delay, and cost of removing it from the 
bins. The buyer is often confronted with the alternative of burn- 
ing the coal delivered or going without fuel until more can be pro- 
cured. Uuless the coal is very bad it is usually expedient to use it 
and pay a smaller price. This is also more favorable to the contrac- 
tor, as to remove the coal would be costly and it would not be satis- 
factory as fuel to any other customer. 
VALUE OF COAL AS A FUEL. 
The purpose of burning coal under boilers is to abstract the heat 
for use in developing power, in drying various materials, or in 
warming buildings. The most valuable coal, therefore, is that which 
gives up the most heat to the boiler for a given weight burned. 
Coal is now burned for power purposes in gas producers and boiler 
furnaces. For coals and lignites high in moisture or high in ash, the 
gas producer, used in connection with a gas engine, is best adapted 
to develop power. But for the generation of steam, which can be 
used for heating as well as for power purposes, a more convenient 
method is to burn the coal in a specially constructed furnace under 
a boiler. 
The aim in the purchase of coal for any power plant should be 
to obtain a fuel which will produce a horsepower for the least cost, 
all things being considered, such as the equipment, the price of coal, 
and the cost of labor and repairs. Experiments have been made 
which seem to indicate that almost any fuel may be burned with 
reasonable efficiency in a properly designed apparatus. The recog- 
nized requirements are as follows: 
A supply of fuel fed to the furnace as uniformly and continuously 
as possible. 
