90 A TREATISE ON DREDGES AND DREDGING 



dredges are built with high towers. The materials are then conveyed 

 to the dumping place by means of a long tube and they are discharged 

 by gravity. The height of the tower depends upon two factors, viz., 

 the distance of the dump and the elevation of the shores from deck. 

 They are built of different heights, but always between 50 and 80 

 ft. The ladder is always located at the middle of the boat in an 

 open pit. 



When the buckets have reached the top of the ladder, while 

 they revolve around the upper tumbler the materials fall into a bell 

 leading to a closed chute, which is extended into a long tube. With 

 dredges built to work in narrow rivers or canals, when both shores 

 can be easily reached by the conveying apparatus, they are usually 

 provided with two chutes and two tubes. In this manner were con- 

 structed the high-tower ladder dredges used on the Panama Canal 

 under the French and also on the Nicaragua Canal. The conveying 

 tubes are made of sheet iron 2 or 3 ft. in diameter. In dredges provided 

 with only one long discharging tube this is located on one side of the 

 boat at a right angle to the ladder. The tube is supported from the 

 tower by a series of gang wire ropes and also from a trussed A frame 

 located on one side of the boat and strongly fixed to the other side 

 by means of backstays. The conveying tube is made of varying 

 lengths, reaching sometimes 150 and even 180 ft. To reach points 

 at great distances from the dredge the conveying tube is 

 extended to land, but in such a case the land section of the tube 

 is supported by specially constructed trusses. To facilitate the flow 

 of the materials through the tube this is placed with an inclination 

 varying from 2 to 10 per cent, depending upon the quality of the 

 dredged materials. The descent of the materials through the tube 

 is facilitated also by means of water jets forced by a centrifugal 

 pump placed on deck, which continuously flush the tube. 



The high -tower ladder dredge presents several disadvantages: 

 1st, the necessity of elevating the materials to a greater height than 

 absolutely necessary, and consequently the motive power is not 

 economically utilized; 2d, the necessity of acquiring powerful 

 and expensive machines, which involve high running expenses; 

 3d, the stability of the dredge is greatly hampered <>n account of 

 having very high towers on boats usually built of small dimensions; 

 4th, the material being deposited in a fluid state and near the 

 edge of water has a tendency to run back again to the point of 

 excavation. 



