"The cost of a steam motor per 20 stone (280 lb.) sack of flour depends
entirely on local circumstances. It depends first, on the amount of
power expended in the production of a sack of flour, that is on its mode
of manufacture, and it depends, secondly, on the cost of the necessary
amount of power, that is, on the cost of fuel burned per horse power The
average consumption of coal of first class steam engines may be taken at
2 lb. per hour per indicated horse power.

"Supposing a mill with six pairs of stones, two pairs of porcelain
roller mills, and the necessary dressing, purifying, and wheat cleaning
machinery to require a steam motor of 100 indicated horse power to drive
it, then the average consumption of fuel in this mill would be 200 lb.
of coal per hour. Such a mill working day and night will turn out about
400 sacks of flour per week of, say, 130 hours, so that 200 X 13 =
26,000 lb. of coal would be required to manufacture 400 sacks of flour.
The cost of this quantity of coal may be taken at, say, £12 (about
$58.32), and for cost of attending engine and boiler, cost of oil, etc.,
another £3 (about $14.58) per week may be added; so that, in this case,
the manufacture of 400 sacks of flour would cause an expenditure of £15
($72.90) for the steam motor. Therefore the cost of the steam motor per
20-stone sack of flour may be taken at 9d. (about 18 cents) per sack, if
an improved low grinding system is used.

"In this case it is supposed that about 55 per cent. of flour is
obtained in the first run, leaving about 30 per cent. of middlings and
about 12 per cent. of bran, which is finished in a bran duster. The
middlings are purified, ground over one pair of middling stones, then
dressed through a centrifugal and the tailings of the latter are passed
over one of the porcelain roller mills, whereas the other porcelain
roller mill treats the second quality of middlings coming from the