accomplished by a fly-wheel, which is in fact nothing more than a
wheel having a very heavy rim, so that the greater part of its
weight is near the circumference. It requires great power applied
for some time to put this into rapid motion; but when moving with
considerable velocity, the effects are exceedingly powerful, if
its force be concentrated upon a small object. In some of the
iron works where the power of the steam-engine is a little too
small for the rollers which it drives, it is usual to set the
engine at work a short time before the red-hot iron is ready to
be removed from the furnace to the rollers, and to allow it to
work with great rapidity until the fly has acquired a velocity
rather alarming to those unused to such establishments. On
passing the softened mass of iron through the first groove, the
engine receives a great and very perceptible check; and its speed
is diminished at the next and at each succeeding passage, until
the iron bar is reduced to such a size that the ordinary power of
the engine is sufficient to roll it.

21. The powerful effect of a large flywheel when its force
can be concentrated on a point, was curiously illustrated at one
of the largest of our manufactories. The proprietor was shewing
to a friend the method of punching holes in iron plates for the
boilers of steam-engines. He held in his hand a piece of
sheet-iron three-eighths of an inch thick, which he placed under
the punch. Observing, after several holes had been made, that the
punch made its perforations more and more slowly, he called to
the engine-man to know what made the engine work so sluggishly,
when it was found that the flywheel and punching apparatus had
been detached from the steam-engine just at the commencement of
his experiment.