minute? The
speed of the piston will be 7 (the length of a double stroke) x 85 =
595 feet per minute. Now 42 x 42 = 1,764 x 595 = 1,049,580 / 6,000 =
175 horses power.

DUTY OF ENGINES AND BOILERS.

226. _Q._--What is meant by the duty of a engine?

_A._--The work done in relation to the fuel consumed.

227. _Q._--And how is the duty ascertained?

_A._--In ordinary mill or marine engines it can only be ascertained by the
indicator, as the load upon such engines is variable, and cannot readily be
determined; but in the case of engines pumping water, where the load is
constant, the number of strokes performed by the engine will represent the
work done, and the amount of work done by a given quantity of coal

represents the duty. In Cornwall the duty of an engine is expressed by the
number of millions of pounds raised one foot high by a bushel, or 94 lbs.
of Welsh coal. A bushel of Newcastle coal will only weigh 84 Lbs.; and in
comparing the duty of a Cornish engine with the performance of an engine in
some locality where a different kind of coal is used, it is necessary to
pay regard to such variations.

228. _Q._--Can you tell the duty of an engine when you know its consumption
of coal per horse power per hour?

_A._--Yes, if the power given be the actual, and not the nominal, power.
Divide 166.32 by the number of pounds of coal consumed per actual horse
power per hour; the quotient is the duty in millions of pounds. If you
already have the duty in millions of pounds, and wish to know the
equivalent consumption in pounds per actual horse power per hour, divide
166.32 by the duty in millions of pounds; the quotient is the consumption
per actual horse power per hour. The duty of a locomotive engine is
expressed by the weight of coke it consumes in transporting a ton through
the distance of one mile upon a railway; but this is a very imperfect
method of representing the duty, as the tractive efficacy of a pound of
coke becomes less as the speed of the locomotive becomes greater; and the
law of variation is not accurately known.

229. _Q._--What amount of power is generated in good engines of the
ordinary kind by a given weight of coal?

_A._--The duty of different kinds of engines varies very much, and there
are also great differences in the performance of different engines of the
same class. In ordinary rotative condensing engines of good construction,
10 lbs. of coal per nominal horse power pe