22. *Q.*—–Can you give a practical
rule for determining the proper quantity of cast iron
for the rim of a fly-wheel in ordinary land engines?

*A.*—One rule frequently adopted is
as follows:—Multiply the mean diameter
of the rim by the number of its revolutions per minute,
and square the product for a divisor; divide the number
of actual horse power of the engine by the number
of strokes the piston makes per minute, multiply the
quotient by the constant number 2,760,000, and divide
the product by the divisor found as above; the quotient
is the requisite quantity of cast iron in cubic feet
to form the fly-wheel rim.

23. *Q.*—What is Boulton and Watt’s
rule for finding the dimensions of the fly-wheel?

*A.*—Boulton and Watt’s rule
for finding the dimensions of the fly-wheel is as
follows:—Multiply 44,000 times the length
of the stroke in feet by the square of the diameter
of the cylinder in inches, and divide the product
by the square of the number of revolutions per minute
multiplied by the cube of the diameter of the fly-wheel
in feet. The resulting number will be the sectional
area of the rim of the fly-wheel in square inches.

24. *Q.*—What do you understand by
centrifugal and centripetal forces?

*A.*—By centrifugal force, I understand
the force with which a revolving body tends to fly
from the centre; and by centripetal force, I understand
any force which draws it to the centre, or counteracts
the centrifugal tendency. In the conical pendulum,
or steam engine governor, which consists of two metal
balls suspended on rods hung from the end of a vertical
revolving shaft, the centrifugal force is manifested
by the divergence of the balls, when the shaft is
put into revolution; and the centripetal force, which
in this instance is gravity, predominates so soon as
the velocity is arrested; for the arms then collapse
and hang by the side of the shaft.

25. *Q.*—What measures are there of
the centrifugal force of bodies revolving in a circle?

*A.*—The centrifugal force of bodies
revolving in a circle increases as the diameter of
the circle, if the number of revolutions remain the
same. If there be two fly-wheels of the same
weight, and making the same number of revolutions
per minute, but the diameter of one be double that
of the other, the larger will have double the amount
of centrifugal force. The centrifugal force of
the *same wheel*, however, increases as the square
of the velocity; so that if the velocity of a fly-wheel
be doubled, it will have four times the amount of
centrifugal force.

26. *Q.*—Can you give a rule for determining
the centrifugal force of a body of a given weight
moving with a given velocity in a circle of a given
diameter?