GRAB DREDGES 171 



from place to place by tugboats. The barge can be made in the 

 shape of wooden floats similar to those used in America, or may be 

 constructed in the shape of the hull of a boat with a hold amidship 

 for carrying materials or goods. The dredging apparatus can also 

 be mounted upon a double cylindrical iron pontoon, as is done on 

 the improvements of the Nile River. When the dredge is employed 

 on work where the materials are directly deposited along the sides 

 of the excavation, and it must be moved continuously from place to 

 place without the help of tugboats, then the apparatus is mounted 

 upon a self-propelling boat. The propelling apparatus may consist 

 either of paddle wheels or screw propellers. When the dredge is 

 moved by means of paddle wheels, as a rule, only one wheel at the 

 stern of the boat is used, while the dredging machine is located 

 at the bow. In such cases only one boiler is used and it is located 

 in the hold near the wheel, the steam being carried to the hoisting 

 engine by means of pipes. A machine mounted in this manner is 

 preferred when necessary to have one of very light draft and small 

 height, so as to navigate readily in very shallow waters and 

 easily pass under bridges. The boat can be moved also by ordinary 

 screw propellers. Then both the digging and propelling apparatus 

 are mounted at the stern. The boiler is located either on the turning 

 platform of the crane and the power for the propeller is transmitted 

 by means of gearing and shafting. However, the propelling and 

 digging apparatus may be independent of one another; then the 

 machine is provided with two sets of engines and steam is supplied 

 by a stationaiy boiler. In the machines of this type the crane 

 with the digging bucket is located aft. The boat can be constructed 

 of wood or iron w r hen built to navigate in deep waters or carry 

 the materials outside the harbor. The boat can be constructed 

 with a hold for carrying the materials or with hoppers closed by 

 doors at the bottom, where the materials are temporally deposited 

 in order to be transported and dumped in deep waters. Thus the 

 machine will act like any other hopper dredge of any type, but since 

 the efficiency of the English grab dredge is very limited, the hop- 

 pers also are made of small capacity, very seldom exceeding 500 or 

 600 tons. These machines are employed in the English Colonies, 

 especially Australia. Fig. 48 represents an English grab bucket 

 dredge of the hopper type with separate boiler and propelling 

 apparatus as used in harbor works. To increase the efficiency of 

 the machine two dredging apparatus are used, mounted one at the 



