146 A TREATISE ON DREDGES AND DREDGING 



be cleansed, and the " hedgehog " is moved backward and forward 

 by blocks and chains. Such a machine, made of oak and well bolted 

 together, is most effective; as the cylinder revolves on its axis, 

 its sixty-four spades are all brought into work, thus stirring up a vast 

 quantity of mud, which the stream through the sluice aids in carry- 

 ing away. The timber frame is scantling 6 in. by 4 in., cross-stays 

 from one side to the other being placed 3 ft. apart; and between 

 them are diagonal braces, which are made fast to the frame and 

 stays by iron bolts. The stocks on the cylindrical drum or iron 

 wheel are 6 ft. long and made of oak 6 by 4 in. ; into which pass the 

 iron spades. 



