Watermills are those kind of mills, the motion of which is derived
from the flow of a stream of water against the lower part of a large
wheel, provided with paddle-boards similarly to the paddle-wheels
of steam-vessels; or else by the weight of a stream of water
falling against the upper part of the wheel from a spout or trough;
the former of these is called the under-shot, and the latter the
over-shot mill. The former is used where there is a large body of
water flowing at a sufficiently rapid rate, and the latter kind where
there is but a small supply, the whole of which is often used for
driving the mill; but other circumstances, of position, &c., may
determine which shall be used. The large wheel being thus driven
round, any kind of machinery may of course be attached, according to
the nature of the work to be done. Like windmills these watermills
are for the greater part superseded by steam power; the locality,
&c., must determine which can be used with most advantage.