Ann Arbor Scientific Association. 25 



scoop. More commonly, in the depths of water to be found or 

 made in channels for vessels, machines are employed. These 

 may be classified as the scoop or dipper-dredge, the endless 

 chain and bucket dredge, and the clam-shell dredge. A descrip- 

 tion of these forms, illustrated by drawings, was given. The last 

 named is very effective. It will, for instance, remove the slabs 

 and edgings which accumulate in rivers below saw-mills. The 

 material dredged is emptied into scows, and towed to deep 

 water or a suitable dumping ground. 



The Engineer Corps designed a boat for use on the bars at 

 the mouth of the Mississippi, which stirred up the mud by pro- 

 pellers. A ten-feet channel has been deepened to fifteen feet. 

 The improvement was not permanent. Col. Eads is now trying 

 the method of jetties or piers. 



Boulders in shallow water may be removed by scows after a 

 hole is drilled and an iron bar inserted and wedged. The scows 

 are first lowered by letting in water, and then raised by bailing. 

 In tidal waters a simple raft of logs may be employed, which 

 lifts as the tide rises. 



Blasting away ledges may be done in shallow water by 

 drilling from a moored boat, and inserting a tin tube containing 

 the charge, which may be fired by a water-proof fuse or by a 

 battery. The space to be worked upon may be laid bare by a 

 cofferdam. A steam drill is sometimes employed in ten or 

 twelve feet of water, by placing the drill on the top of a strongly 

 braced tripod to keep the steam cylinder from being chilled by 

 contact with the water, and using a sufficiently long drill rod. 

 Generally, in water of ten feet and over, the aid of divers is 

 called in. 



The dress of the diver, with his means of protection against 

 cold, and the manner of supplying him with air, were then de- 

 scribed in detail. The different sorts of blasts were described as 

 surface, face and hole blasts. Gunpowder, dualin and other ex- 

 ploders were described, and an account given of the work on 

 Blossom Rock in San Francisco Harbor, and on Hell Gate, New 

 York. 



