with a fall of about 5.5 meters (18.04 ft.) there was broken up on an
average more than 0.113 cubic meter (0.148 cubic yard) of hard rock
per blow. The first blow, delivered 90 centimeters (2 ft. 11½ in.)
from the wall face, produced an almost imperceptible rent, a second or
a third blow applied at the same place extended this opening often to
a length of 1.50 meters (4 ft. 11 in.) and to a depth of from 90 to
120 centimeters (2 ft. 11 in. to 3 ft. 11 in.) The next blow opened
the fissure and detached the block of rock.

The application of the same system under water upon an unknown surface
would obviously modify the conditions of the experiment. Nevertheless,
the results obtained with the "Derocheuse," the first dredging machine
constructed upon this principle, have realized the hopes of the
inventor.

This dredging machine was launched on the Clyde and reached Port Said
in twenty days. It measures 55 meters (180 ft. 5 in.) in length, 12.20
meters (40 ft. 1 in.) in breadth, and 3.65 meters (12 ft.) in depth.
Its mean draught of water is 2.75 meters (9 ft. 2½ in.) It is divided
into eighteen watertight compartments. Five steel-pointed battering
rams, each of four tons weight, are arranged in line upon each side of
the chain of buckets of the dredging machine. See Figs. 1 and 2. The
battering rams, suspended by chains, are raised by hydraulic power to
a height varying from 1.50 to 6 meters (4 ft. 11 in. to 19 ft. 8 in.),
and are then let fall upon the rock. The mechanism of the battering
rams is carried by a metallic cage which can be moved forward or
backward by the aid of steam as the needs of the work require. A
series of five battering rams gives from 200 to 300 blows per hour.

[Illustration: FIG. 1.--LONGITUDINAL SECTION.]