10 THE PURCHASE OP COAL UNDER SPECIFICATIONS. 
rule, when dust and very fine coal are fed into the furnace they 
either check the flow of air or are taken up by the draft and after 
being only partly burned are deposited back of the bridge wall or 
pass up the stack, to the annoyance of the people in the vicinity of 
the plant. If this dust is completely burned in passing through the 
furnace there is of course no loss of fuel. It has been found that coal 
of a uniform size is most satisfactory, as it does not pack so closely 
as a mixture of sizes. 
In general it may be said that in any market the coal obtainable at 
the lowest price is the most economical, provided the furnace equip- 
ment is suitable. If the furnace is not so designed as to permit the use 
of the cheaper coal, it should be changed. 
The results of tests tend to show that, other conditions being equal, 
coals of similar composition are of value in proportion to the British 
thermal units in the coal as received — a basis on which, indeed, all 
coals may be valued approximately. It should be remembered, how- 
ever, that the value of a coal for any particular plant is influenced by 
the fact that all furnaces are not equally suitable for burning the many 
grades of coal. Aside from this factor, coals may be compared in 
terms of the British thermal units obtained for 1 cent, or on the cost 
per million heat units. 
In the purchase of coal, then, attention should be given to the 
character of the furnace equipment and the load; the character 
of coal best suited to the plant conditions; the number of heat units 
obtainable for a unit price; the cost of handling the coal and ash; 
and the possibility of burning the coal without smoke or other objec- 
tionable features. 
SPECIFICATIONS IN USE. 
As the result of a letter from President Roosevelt to the national 
advisory board on fuels and structural materials, calling attention to 
the need of a uniform and efficient basis for the purchase of the Gov- 
ernment fuel supply, the following specification was drafted by engi- 
neers in the employ of the Government and approved by this board in 
March, 1907 : 
Specifications and Proposals for Supplying Coal. 
United States 
, 190__ 
PROPOSAL. 
Sealed proposals will be received at this office until 2 o'clock p. m., , 
]!)0__, for supplying coal to the United States building 
at — as follows: 
