PNEUMATIC DREDGES 149 



Jets of compressed air, arranged around the orifice and directed 

 against the earth, disintegrate the latter and increase the proportion 

 of the material carried along by the velocity of the water; a pro- 

 portion which in ordinary depths of 20 or 25 ft. reaches, as regards 

 sand, 25 per cent of the volume of water. 



The suction pipe 10 in. in diameter was actuated by a compressor 

 of the capacity of 211 cu.ft. of free air per minute. The forcing 

 apparatus consisted of a cylindrical reservoir 10 ft. in diameter and 

 22 ft. long with convex ends having a capacity of 176 cu.ft. This 

 received the mixture of water and material, the air escaped through 

 an opening surmounted by an open dome, upon the side of which 

 there was a wastepipe. When the reservoir was full and the water 

 was making its escape through the wastepipe, a single external 

 lever, operated by the chief dredgeman, closed valves that in turn 

 closed internally the orifice of the dredging pipe, and opened the 

 air port, and at the same time reversed, through three-way cocks, 

 a current of compressed air, which was then forced through distinct 

 pipes into the reservoir and led to injection tubes, properly spaced 

 in the lower part of the reservoir. The effect of the jets of compressed 

 air, formed under the mass of earth and water, was to lift the material 

 while mixing it with water and throwing it, as if by successive shovel- 

 fuls, toward the orifice situated at the lowest point of the excavation. 

 The air traversed the mass of water and material and flowed to 

 the upper part of the reservoir, where was gauged the pressure, cor- 

 responding to the distance and height to which the material was 

 forced. 



The total time taken to force to a distance of 1000 ft. was six 

 minutes, two of which were consumed in the passage through the 

 conduit. The tubing or pipe was filled by the escape, at the end of 

 the conduit, of a wheat-sheaf jet of water and air projected through 

 an explosion to 48 ft. from the orifice, the conduit remaining empty 

 and being cleaned out by this final action of the air. At the same 

 time the automatic valve that closed the upper orifice of the reservoir 

 was opened by its own weight. The lever that worked the cock was 

 then reversed and the air sent to the dredging pipe and the pipe was 

 again filled. 



Thus the dredging and forcing occurred successively at periods 

 of from 5 to 6 minutes, the boat remaining immovable during the 

 forcing period. 



The compressor used was provided with double horizontal 



