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287. *Q.*—By what process do you ascertain
the dimensions of the chimney of a land boiler?

*A.*—By a reference to the volume
of air it is necessary in a given time to supply to
the burning fuel, and to the velocity of motion produced
by the rarefaction in the chimney; for the area of
the chimney requires to be such, that with the velocity
due to that rarefaction, the quantity of air requisite
for the combustion of the fuel shall pass through the
furnace in the specified time. Thus if 200 cubic
feet of air of the atmospheric density are required
for the combustion of a pound of coal,—though
250 lbs. is nearer the quantity generally required,—and
10 lbs. of coal per horse power per hour are consumed
by an engine, then 2000 cubic feet of air must be
supplied to the furnace per horse power per hour, and
the area of the chimney must be such as to deliver
this quantity at the increased bulk due to the high
temperature of the chimney when moving with the velocity
the rarefaction within the chimney occasions, and which,
in small chimneys, is usually such as to support a
column of half an inch of water. The velocity
with which a denser fluid flows into a rarer one is
equal to the velocity a heavy body acquires in falling
through a height equal to the difference of altitude
of two columns of the heavier fluid of such heights
as will produce the respective pressures; and, therefore,
when the difference of pressure or amount of rarefaction
in the chimney is known, it is easy to tell the velocity
of motion which ought to be produced by it. In
practice, however, these theoretical results are not
to be trusted, until they have received such modifications
as will make them representative of the practice of
the most experienced constructors.

288. *Q.*—What then is the rule followed
by the most experienced constructors?

*A.*—Boulton and Watt’s rule
for the dimensions of the chimney of a land engine
is as follows:—multiply the number of pounds
of coal consumed under the boiler per hour by 12,
and divide the product by the square root of the height
of the chimney in feet; the quotient is the area of
the chimney in square inches in the smallest part.
A factory chimney suitable for a 20 horse boiler is