his proportion.
In practice, however, they are one third less than this, which reduces the
heating surface from 9 to 6 square feet per actual horse power, and this
length even is found to be inconvenient. It is greatly preferable therefore
to increase the calorimeter, and diminish the intensity of the draught.

BOILER CHIMNEYS.

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