inherent in the machinery the engine has to drive and the equability of
motion required--the engine will work with sufficient regularity for most
ordinary purposes, but where great equability of motion is required, it
will be advisable to make the power resident in the fly-wheel equal to six
times the power generated by the engine in one half-stroke.

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.



CENTRAL FORCES.