*A.*—If to condense a cubic inch of
water raised into steam 28.9 cubic inches of condensing
water are required, then the cold water pump ought
to be 28.9 times larger than the feed pump, supposing
that its losses were equally great. The feed
pump, however, is made sufficiently large to compensate
for leaks in the boiler and loss of steam through the
safety valve, so that it will be sufficient if the
cold water pump be 24 times larger than the feed pump.
This ratio is preserved by the following rule:—
multiply the capacity of the cylinder in cubic inches
by the total pressure of the steam per square inch,
or the pressure on the safety valve plus 15, and divide
the product by 200. The quotient is the proper
capacity of the cold water pump in cubic inches when
the engine is double acting, and the pump single acting.

340. *Q.*—By what considerations do
you determine the dimensions of the fly wheel of an
engine?

*A.*—By a reference to the power generated,
each half stroke of the engine, and the number of
half strokes that are necessary to give to the fly
wheel its standard velocity, supposing the whole power
devoted to that object. In practice the power
resident in the fly varies from 2-1/2 to 6 times that
generated each half stroke; and if the weight of the
wheel be equal to the pressure on the piston, its
velocity must be such as it would acquire by falling
through a height equal to from 2-1/2 to 6 times the
stroke, according to the purpose for which the engine
is intended. If a very equable motion is required,
a heavier or swifter fly wheel must be employed.

341. *Q.*—What is Boulton and Watt’s
rule for fly wheels?

*A.*—Their rule is one which under
any given circumstances fixes the sectional area of
the fly wheel rim, and it is as follows:—multiply
44,000 times the square of the diameter of the cylinder
in inches by the length of the stroke in feet, and
divide this product by the product of the square of
the number of revolutions of the fly wheel per minute,
multiplied by the cube of its diameter in feet.
The quotient is the area of section of the fly wheel
rim in square inches.

342. *Q.*—Can you give a rule for
telling the proper thickness of the cylinders of steam
engines?

*A.*—In low pressure engines the thickness
of metal of the cylinder, in engines of a medium size,
should be about 1/40th of the diameter of the cylinder,
which, with a pressure of steam of 20 lbs. above the
atmosphere, will occasion a strain of only 400 Lbs.
per square inch of section of the metal; the thickness
of the metal of the trunnion bearings of oscillating
engines should be 1/32d of the diameter of the cylinder,
and the breadth of the bearing should be about half
its diameter. In high pressure engines the thickness