EXCAVATION OF SUBAQUEOUS ROCKS 35 



boat is brought back into position for blasting and the shaft is sunk 

 again. The loosened rock can be removed either through the shaft, 

 which is a comparatively slow operation, or can be removed later 

 by means of dredges. The principal advantages of such a shaft 

 consist in the facility thus offered for examining every part of the 

 bottom and leveling it by hand where necessary and in the ease with 

 which the seat of operations can be changed without loss of time. 



The boat shown in Fig. 9 with a central opening for the metallic 

 caisson was found very convenient in blasting rocks on the Rhine 

 River where the machinery had to be moved frequently to make 

 room for the passage of floats and tows made up of a number of 

 boats. Each of the shafts employed there covered a surface of 

 about 27.5 sq.yds. in which as a rule, fourteen to seventeen holes 

 were drilled. The blasting operations extended over a surface of 49 

 sq.yds., so there was one hole for about 2.7 sq.yds. of blasting 

 surface. On an average 360 ft. of hole was drilled per day by 

 one shaft. During the operations in the Rhine, near Bingen, about 

 260,000 cu.yds. of stone were blasted out. 



A similar device is also used in California in connection with 

 gold mining. There the river beds carry much fine gold, and by 

 dropping a small caisson from the center of the boat men are able 

 to go to the river bottom, excavate the gold-bearing sand and gravel 

 and by means of hoisting machinery elevate it to the separators 

 on board the scow. Such caissons are much smaller than those 

 used for rock drilling. The use of such caissons, however, has not 

 been extensive, and their value is disputed among mining engineers. 



Excavation of subaqueous rocks can be done in some cases 

 without any previous drilling, by simply placing dynamite cartridges 

 on the ledge of rock at the bottom of the river. In such cases the 

 rock is broken into slabs and the fragments are carried away by the 

 current or are raised by dredges. It is obvious that with this method 

 of blasting subaqueous rocks, a larger quantity of dynamite should 

 be used than with the usual manner of blasting by drilling holes 

 to receive the charge. Consequently it is seldom used except when 

 a small quantity of rock must be removed and the ledge is thin. 



Blasting. After the holes have been drilled by means of any one 

 of the numerous devices already described, they are charged with 

 explosives. There are several methods of charging drill holes, but 

 the one most commonly used was devised by Mr. Dunbar, the pioneer 

 of subaqueous drilling. Mr. Dunbar's apparatus consists of a cop- 



