46 THE IMPROVEMENT OF RIVERS. 



chute, sufficient water accompanying it to wash it to such a point as desired, or to load 

 it into scows. Some of these dredges are arranged for self -propulsion, and the material 

 is then dumped directly into a hopper in the boat itself. When this is full, dredging 

 operations cease and the boat is run to the dumping-ground and the material deposited. 

 This process has the advantage of causing no lost time, the same crew doing all the work, 

 but it is evident that it is not as speedy as when dredging goes on constantly, with 

 a separate crew attending to the deposit of the material excavated. In working, the 

 dredge is moved slowly over the part of bottom being worked upon in order that each 

 bucket may be filled. 



As a general thing the bucket-ladder is located along the center line of the hull, 

 but in some it is situated along the side. The type is also built with two sets of 

 buckets or two elevator systems. 



Two dredges of the elevator type were used on the lower Mississippi in 1888, and 

 did very efficient work. One of them is reported to have dredged 4000 cubic yards 

 in a lo-hour day. 



Rock Dredge. There was used in France as far back as 1852 a dredge for excavat- 

 ing rock. A somewhat similar apparatus was also used on the Suez Canal, and later 

 at the rapids of the Mississippi, at -Rock Island, and at the Iron Gates of the Danube. 

 This machine has a series of chisels or pointed rams, about 8 inches square and 16 feet 

 in length, and weighing from 4 to 10 tons, which are run in leads and let fall from a 

 height on to the rock. Behind the chisels in some of these machines is arranged an 

 endless chain with buckets for removing the rock when broken. In others the ram 

 is separated from the dredge proper, the one cutting and breaking the rock into pieces 

 while the other removes it. 



Hydraulic Dredge. A great amount of the dredging in the rivers of America consists 

 of sand and silt, which it is possible to remove by means of pumping. This is done by 

 means of dredges fitted with centrifugal pumps having suitable suction- and discharge- 

 pipes, the largest ones of this type being in use on the Mississippi River. Their duty 

 consists in opening up channels in low water through sand-bars, the sand being 

 pumped up and discharged through a long length of pipe to where it will not wash into the 

 cut again. The work has, of course, to be repeated every season, and sometimes more 

 than once during the same season. Experience, however, shows that, at least in some 

 of the chutes, the material filled in is looser than the older river-bed, and that in the 

 succeeding season the old channel may be reopened by the river because of the greater 

 ease with which the new material can be cut out. 



Mississippi River Dredging. The method adopted, after considerable experimental 

 work, for improving low-water navigation on the Mississippi River was by means of 

 hydraulic dredges of large capacity, which would open a channel through a bar, within 

 a short time, sufficiently wide and deep to accommodate navigation, and to a considerable 

 extent, to direct the flowing of the current along the line of least resistance. The width 

 of this channel is usually 250 feet and its depth 9 feet. The Commission having the 



