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.

STRENGTHS OF LAND ENGINES.

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 of the cylinder should be about 1/16th its diameter, which, with
a pressure of steam of 80 lbs. upon the square inch, will occasion a strain
of 640 lbs. upon the square inch of section of the metal; and the thickness
of the metal of the trunnion bearings of high pressure oscillating engines
should be 1/13th of the diameter of the cylinder. The strength, however, is
not the sole consideration in proportioning cylinders, for they must be
made of a certain thickness, however small the pressure is within them,
that they may not be too fragile, and will stand boring. While, also, an
engine of 40 inches diameter would be about one inch thick, the thickness
would not be quite two inches in an 80 inch cylinder. In fact there will be
a small constant added to the thickness for all diameters, which will be
relatively larger the smaller the cylinders become. In the cylinders of
Penn's 12 horse power engines, the diameter of cylinder being 21-1/2
inches, the thickness