. 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
boilers. Two independent series of pumps, with separate connections,
feed the hydraulic lifting apparatus, thus permitting repairs to be
made when necessary, without interrupting the work. A special machine
with three cylinders drives the pumps of the condenser. An accumulator
regulates the hydraulic pressure and serves to raise or lower the
spuds.

At the end of the Suez Canal next to the Red Sea, the bottom consists
of various conglomerates containing gypsum, sandstone and sometimes
shells. It was upon a bed of this nature that the machine was first
put to work. The mean depth of water, originally 8.25 meters (26 ft. 3
in.), was for a long time sufficient for the traffic of the canal; but
as the variations in level of the Red Sea are from 1.8 to 3 meters (5
ft. 11 in. to 9 ft. 10 in.), the depth at the moment of low water is
scarcely adequate for the constantly increasing draught of water of
the steamers. Attempts were made to attack the rocky surface of the
bottom with powerful dredges, but this method was expensive because it
necessitated constant repairs to the dredges.

[Illustration: FIG. 3.--DREDGE MOVEMENT.]

These last, although of good construction, seldom raised more than 153
cubic meters (200 cubic yards) in from eight to fifteen da