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.

CENTRAL FORCES.

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 g