SOILS AND THEIR CHARACTERISTICS 9 



buckets to their utmost capacity. But after the material has been 

 raised to the surface it takes some time to unload the bucket, as 

 the clay sticks to the sides and falls off with great difficulty. The 

 ladder dredge is also adaptable to these soils, but the buckets must 

 be of special construction to permit them to scrape into the bank of 

 clay and be filled with the material, instead of sliding upon the 

 surface of the bank; the buckets are provided with a sharp edge in 

 the shape of a beak. After the buckets have dumped their contents 

 they must remain exposed as long as possible to the air in order 

 that the particles of clay sticking to the buckets may be partially 

 dried. To facilitate this exposition to the air the buckets are run 

 at very low speed and the tower supporting the revolving guiding 

 drum is located as high as possible upon the frame of the boat. 

 The hydraulic dredges are the machines most extensively employed 

 to-day for dredging through clay and loamy soils. But in order to 

 cut the material so as to be in condition to be drawn through 

 the suction pipe and pass through the centrifugal pump, the 

 lower end of the suction pipe is provided with a cutter or 

 agitator. 



Sand. Sand is one of the most common materials encountered 

 in dredging. The greatest harbor and river improvements so exten- 

 sively undertaken to-day throughout the world chiefly consist in 

 removing sandbars along the estuaries of large rivers or seashores. 

 Sand is found in different forms depending upon the mineral from 

 which it originated. Generally speaking the sand is formed by 

 the grains of materials rubbing one against another as they are 

 moved onward in brooks and rivers or pushed backward and for- 

 ward by the waves on a seabeach. Only the hardest materials 

 that resist such a destructive action can produce sand, and for 

 this reason it is usually found derived from quartz; the particles 

 of softer materials are rapidly ground into mud. Sands are classified 

 as coarse and fine, a distinction based upon the size into which 

 their particles have been reduced. Coarse sands can be easily 

 removed, even if they are mixed with gravel and shells, while it 

 is more difficult to remove the fine sands, owing to the fact that 

 the finely divided particles tend to float. An example of the different 

 efficiency of dredging through these two varieties of the same material 

 is quoted by Mr. Henry Sat re. He says that a dredge employed 

 in the improvement of the Harbor of Cette, France, used to remove 

 180 cu.m. per hour of coarse sand mixed with algae, while it could 



