For the only "machines" possible to use
in illustration of simple mechanics are the screw, the wedge, the
scale, the lever, the wheel-and-axle, and Atwood's machine. The
mathematical principles which any of these exemplify would, of course,
be incomprehensible to such a class, but the first five most of all,
and as there would naturally be some slight pretence of trying to make
the learners understand, I therefore select the last; and this
selection is justified when we remember that on the shot being heard,
Randolph leans for support on the "machine," and stands in its shadow;
but any of the others would be too small to throw any appreciable
shadow, except one--the wheel, and-axle--and that one would hardly
afford support to a tall man in the erect position. The Atwood's
machine is therefore forced on us; as to its construction, it is, as
you are aware, composed of two upright posts, with a cross-bar fitted
with pulleys and strings, and is intended to show the motion of bodies
acting under a constant force--the force of gravity, to wit. But now
consider all the really glorious uses to which those same pulleys may
be turned in lowering and lifting unobserved that "ball of cotton"
through the two apertures, while the other strings with the weights
attached are dangling before the dull eyes of the peasants.
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