[Illustration: FIG. 2.--PLAN]

A dredging machine combined with the apparatus just described, raises
the fragments of rock as they are detached from the bottom. A guide
wheel is provided, which supports the chain carrying the buckets, and
thus diminishes the stress upon the axles and bearings. With this
guide wheel or auxiliary drum there is no difficulty in dredging to a
depth of 12 meters (39 ft. 4 in.), while without this accessory it is
difficult to attain a depth of 9 meters (29 ft. 6 in.)

A compound engine, with four cylinders of 200 indicated horse power,
drives, by means of friction gear, the chain, which carries the
buckets. If the buckets happen to strike against the rock, the
friction gear yields until the excess of resistance has disappeared.

Fig. 3 indicates the manner in which the dredge is operated during the
work. It turns alternately about two spuds which are thrust
successively into the bottom and about which the dredge describes a
series of arcs in a zigzag fashion. These spuds are worked by
hydraulic power.

A three ton hand crane is placed upon the bridge for use in making
repairs to the chain which carries the buckets. A six ton steam crane
is placed upon the top of the cage which supports the hydraulic
apparatus for raising the battering rams, thus permitting them to be
easily lifted and replaced.

The dredging machine is also furnished with two screws driven by an
engine of 300 indicated horse power, as well as with two independent