SOUNDING APPARATUS. 



659 



The operations of this mining under the channel of the 

 East River have to be conducted with great care. Every inch 

 of the way has to be critically explored. Seams of decom- 

 posed mica have been met, through which the water of the 

 river ran as through a sieve. In one of the shafts such a scam 

 was met, through which the water poured at the rate of six 

 hundred gallons a minute, and could be stopped only by build- 

 ing a strong shield. The floor of the shaft follows a level about 

 thirty feet below the low- water line. The raof follows of course 

 the general contour of the reef, and to determine this, sound- 

 ings of a special kind have to be taken. The bed of the stream 

 is covered, except on the highest points of the reef, with a de- 

 posit of boulders, marl and organic matter from the sewers of 

 New York, sometimes to the depth of ten or twelve feet. As 

 the exact profile of the solid rock must be known before the 

 miners can proceed, every sounding for determining this — and 

 more than 15,000 have been already made — must be carefully 

 done. The sounding apparatus consists of a float, or raft, sup- 

 porting a machine like a guillotine or pile driver, by which a 

 three-inch iron tube is driven through the overlying matter to 

 the rock bed. The contents of the tube are then pumped out 

 and an iron rod is used to determine the nature of the rock be- 

 low. If it is a boulder, a dull thud is heard, and the rod does 

 not rebound. Solid rock returns a sharp clink, and the rod 

 springs back. The bearings of the tube are then taken by in- 

 struments from the shore, and the position of the rock calcu- 

 lated by a simple process. 



Under the direction of General Newton, other submarine 

 operations are also carried on in New York ITarbor for the re- 

 moval of the rocky and dangerous obstructions known as Dia- 

 mond Reef, and Coentie's Reef, which lie in the busiest pari 

 of the harbor, directly in the track of the numerous ferryboats 

 plying between New York and Brooklyn, and are not only 

 trenblesome, but dangerous, especially at low water. To par. 



