From experiments made in the quarry of Craigmiller, near Edinburgh, with a weight of two tons shod with a steel point, it was found that 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. 111/2 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. 21/2 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.]

[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.