REMOVAL OF BARS AND OTHER OBSTRUCTIONS. 45 



These dredges are at present made of sizes from one-half to eight yards in capacity, 

 and to operate in depths of water up to 40 feet. They require a considerable amount 

 of space in which to work, owing to their method of operation, and in this respect they 

 are inferior in certain situations to the clam-shell dredge. They will, however, excavate 

 almost any material except solid rock. 



Clam-shell Dredge. This style is similar in general arrangements to the dipper 

 dredge, but has a clam-shell bucket instead of a dipper. This consists of a bucket 

 shaped like a semi-cylinder, hinged so it will open into two parts. It is suspended 

 from the end of the boom, being kept from twisting by two long poles which work up 

 and down through eyes on the boom. The operation is done by two chains, one of 

 which lowers the bucket rapidly in an open position so it will penetrate the river-bed, 

 while the other is arranged to pull the halves together, thus scraping up the load, and 

 then hoists the bucket to the surface. The load is dumped by slacking this chain and 

 holding the other, when the bucket opens again as in its first position. Frequently 

 a hemispherical shape, divided into four parts, is used instead of a semi-cylindrical one 

 divided into two; the bucket is then called an "orange-peel" or a grapple. This style 

 is more serviceable for general use than the other. 



Formerly a type was made with only one chain, which pulled the parts together, 

 latches being employed to release the load. It was found, however, that when_ the 

 blades caught on any immovable substance, such as a buried log or a projecting rock, 

 it was almost impossible to get them loose, as the chain only acted for closing, and for 

 this reason the type has been abandoned. 



The clam-shell dredge will only operate in loose material, such as gravel, blasted 

 rock, etc. For hard or tenacious clay it is of little value. It has the advantage, how- 

 ever, of being able to work in confined positions, such as in sinking cylindrical caissons, 

 and also of being suited to great depths, as there is little straining on the spuds, such 

 as occurs with the dipper dredge. 



Combination Dredge. For miscellaneous work the dipper and clam type are 

 sometimes combined, the boom and machinery being so arranged that either style of 

 bucket can be used. This is sometimes very convenient for river work, especially in 

 dredging in lock-chambers. 



Elevator Dredge. The type of dredge generally employed in England and other 

 parts of Europe is that known as the elevator or bucket dredge. It is used in Canada 

 also, although it has not met with much favor in the United States. With it tolerably 

 hard materials may be excavated to a considerable depth. 



It consists of two parallel endless chains, carrying a number of buckets, which 

 are successively presented in a horizontal position to the soil to be excavated by means 

 of an arm swung at its' upper end from a frame, while its lower end rests on the river- 

 bed. The chains pass around drums at the two extremities of the arm and are driven 

 by an engine. The arm may be adjusted to suit the depth of water, etc., and may be 

 vertical or inclined. As the material is brought up it is discharged into a hopper or 



