46 A TREATISE ON DREDGES AND DREDGING 



in a third. Between this third cup and the remaining pole of the 

 battery stood the circuit closer, composed of an iron cup containing; 

 mercury in which sat a thin glass tumbler partially filled with mercury. 

 The leading wires were connected with the mercury in the iron cup 

 while the returning wires were connected with the mercury of the 

 tumbler. To close the circuit it was only necessary to break the 

 glass tumbler. An iron rod J in. in diameter and 4 ft. long terminated 

 with a disk, was used for this purpose. 



When all the work was completed the mine was flooded and 

 the blast was fired at 11.13 A.M., October 11, 1885. The explosion 

 occurred over the whole area of the reef. There was no loud report 

 and no dangerous shock through the earth, though a slight vibration 

 was observed as far away as Cambridge, Mass. A few panes of 

 glass were broken in the neighborhood, a number of loose ceilings 

 came down and several bricks were shaken from a chimney of a 

 house near the water's edge in Astoria. This was all the damage 

 done. Soon after the explosion a diver was put down and found 

 the rock was cracked and shattered; the surface blocks very large, 

 but in good shape to be dredged with reasonable amount of surface 

 blasting. Immediately two grapple dredges were set to work and 

 they removed 120 tons per day. 



The total cost of the work was estimated at $2.99 per cu.yd. 

 of rock broken. 



Lift Holes. The method of removing the subaqueous rock by 

 a large single blast resulting from the simultaneous firing of long 

 lift holes driven from a pit inclosed by a coffer-dam was recently 

 employed at Henderson's Point at the Portsmouth Navy Yard in 

 New Hampshire, an account of which was published in the Kimincrr- 

 ing News, August 3, 1905; from which the following description is 

 taken: 



Henderson's Point was a ledge of trap rock 400 ft. wide at the 

 base and projecting 300 ft. into Portsmouth Harbor in such a manner 

 as to make it extremely difficult for large war vessels to reach the 

 new drydock. The area of rock to be excavated below low water 

 was about three acres, and the contractors decided to build a coffer- 

 dam, horseshoe shape, and excavate all the rock possible within this 

 coffer-dam by ordinary methods. By doing this a rim of rock was 

 left under the coffer-dam and extending out beyond it into the ha H >< >r. 

 This rim of rock contained approximately 35,000 cu.yds. which VM* 

 broken up by a single large blast. Figs. 12 and 13. 



