23, with a curved shield or gate in front, which can be moved
around the periphery of the wheel for the purpose of regulating its
speed or stopping its motion by cutting off the stream from the buckets.
The float, rising and falling with the stream, is held in position by a
braced frame swinging on anchorages within the mill on shore, and
parallel with a swiveled shaft.
Tide wheels and tidal current wheels have been in use for more than 800
years, and were largely in use in Europe and the United States during
the first half of the present century. No less than three were running
in the immediate vicinity of New York, in 1840, for milling purposes.
Their day seems to be past, except in some special localities. We will
also pass them, and illustrate some of the
SELF-ACTING WATER-RAISING DEVICES.
The tympanum derives its name from its similarity to a drum as made by
the Romans, but its origin was Egyptian. It is a current wheel with
frame like Fig. 23, to the outside of which a set of chambers or tubes
are fixed, radiating spirally, so as to lead the water to the shaft as
the wheel revolves, as shown in Fig.
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