The wheel, which is represented in the centre of the engraving,
is exactly similar to a common water wheel; the treadboards upon
its circumference are, however, of considerable length, so as to
allow sufficient standing room for a row of from ten to twenty
persons upon the wheel. Their weight, the first moving power of
the machine, produces the greatest effect when applied upon the
circumference of the wheel at or near the level of its axle; to
secure therefore this mechanical advantage, a screen of boards is
fixed up in an inclined position above the wood, in order to
prevent the prisoners from climbing or stepping up higher than
the level required. A hand rail is fixed upon this screen, by
holding which they retain their upright position upon the
revolving wheel, the nearest side of which is exposed to view in
the plate, in order to represent its cylindrical form much more
distinctly than could otherwise have been done. In the original,
however, both sides are closely boarded up, so that the prisoners
have no access to the interior of the wheel, and all risk of
injury whatever is prevented.
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