44 A TREATISE ON DREDGES AND DREDGING 



this part of the project was modified, as it was believed that the 

 required depth could be obtained more economically by dredging 

 a portion of the debris. 



After the completion of the galleries, the roof and the pillars 

 were drilled to a depth of 33 ft. below mean low water, with holes 

 enough to contain 0.79 of a pound of No. 1 dynamite for every 

 cu.yd. of rock and every 7000 pounds of water overhead, amounting 

 to 1 . 04 pounds per cu.yd. of rock broken. The holes drilled upward 

 were at angles of 60 and 45 degrees, the former, along the center of 

 the gallery (see Fig. 11), were 8 ft. deep, the latter 10 ft., so as to 

 reach as far over the pillars as possible. These lengths, however, 

 were often reduced by the drill cutting into seams open to the river. 



FIG. 11. Honeycombing of Flood Rock, showing the Direction of Drill Holes in 



Galleries. 



The holes were of such a diameter as to receive a rigid 2J-inch cart- 

 ridge throughout their entire length, and altogether 113,102 ft. of 

 such holes were required. 



An elaborate set of experiments was planned and carried out 

 in order to obtain more simple methods of firing the mine and to 

 extend the list of available explosives, which was then practically 

 limited to dynamite or other nitroglycerine compounds. These 

 experiments resulted in proving that all the electrical connections 

 between the drill holes and the battery could be dispensed with, as 

 the explosion of a few pounds* charge of dynamite would fire with 

 absolute certainty under water another charge of dynamite parked 

 in a thin elastic envelope at a distance of 27 ft. They also proved 

 the efficacy of firing long narrow charges of rackrock, an explosive 

 so inert and so safe to handle that a pistol bullet may be shot into 



