sted of a
boat with a broad rake fitted to the bow, capable of adjustment to
different depths. At the sides of the boat were hinged two wings of
the same depth as the rake and in a line with it. When the rake was
dropped to the bottom of the river and the wings extended to the side,
they formed a sort of temporary dam, and the water began to rise
gradually. As soon as a sufficient head was raised, varying from 6 to
12 in., the whole machine was driven forward by the pressure, and the
rake carried the mud with it. Progress at the rate of about 3 m. an
hour was made in this manner, and to prevent the accumulation of the
dredgings, operations were begun at the mouth of the river and carried
on backwards. The apparatus was very effective and the river was
cleansed thoroughly, but the distance travelled by the dredger must
have been great.

In 1876 J. J. Rietschoten designed a "propeller dredger" for removing
the shoals of the river Maas. It consisted of an old gunboat fitted
with a pair of trussed beams, one at each side, each of which carried
a steel shaft and was capable of being lowered or raised by means of a
crab. An ordinary propeller 3 ft. 6 in. in diameter was fixed to the
lower end of the shaft, and driven by bevel gear from a cross shaft
which derived its motion by belting from the fly-wheel of a 12 h.p.
portable engine. The propellers were lowered until they nearly reached
the shoals, and were then worked at 150 revolutions per minute. This
operation scoured away the shoal effectively, for in about 40 minutes
it had been lowered about 3 ft. for a space of 150 yds. long by 8 yds.
wide.

A. Lavalley in 1877 designed an arrangement for the harbour of Dunkirk
to overcome the difficulty of working an ordinary bucket-ladder
dredger when there is even a small swell. A pump injects water into
the sand down a pipe terminating in three nozzles to stir up the sand,
and another centrifugal pump draws up the mixed sand and water and
discharges it into a hopper, the pumps and all machinery being on
board the hopper. To allow for the rising and falling of the
vessel--either by the action of the tide or by the swell--the ends of
the pipes are made flexible. The hopper has a capacity of 190 cub.
yds., and is propelled and the pumps worked by an engine of 150 i.h.p.
From 50 to 80 cub. yds. per hour can be raised by this dredger.

The "Aquamotrice," design