30 A TREATISE ON DREDGES AND DREDGING 



strike from 110 to 130 blows per minute. It took from 10 to 15 

 minutes to sink holes 8 cm. in diameter to the depth of .05 to .1 m. 

 With these machines from 8 to 10 holes from 1 to 1 . 80 m. deep were 

 made in a day. 



In this country, in the year 1866-67 Mr. Dunbar used a method 

 of his own for blasting a rock bed from the channel and harbor 

 of Erie, Pa., on Lake Erie. It was described by Mr. Dean in the 

 Proceedings of the Am. Soc. of C. E.; the ledge of rock excavated 

 was only 6 ft. deep. It was drilled by hand by means of jumpers 

 or crowbars, the men standing on the ice in winter and on a specially 

 constructed raft in other seasons. The thickness of the ice guided 

 the drills to strike their blows in the same spot. The raft was rigged 

 with a device so as to make the drills repeat their blows in the same 

 place. 



This rough device led to the design of the modern drilling machine. 

 In the year 1872 Mr. Dunbar undertook the excavation of a sub- 

 merged rock at Port Colborne, Lake Erie, and the drilling was done 

 by hand. The rock, however, was so hard that three men could 

 hardly drill 1 ft. per day. Accordingly Mr. Dunbar constructed a 

 drilling machine, using the hull of an old dredge to carry the machin- 

 ery. The scow was provided with vertical spuds at each comer to 

 hold it firmly in position. A track was laid on the edge of the deck 

 on which the drills were moved. On this track were mounted two 5 

 in. steam-cylinder percussion drills, arranged to slide up and down 

 in a vertical frame overhanging the side of the scow. The drills. 

 were raised or lowered by means of a windlass operated by hand. 

 and were moved along the track by means of crowbars. The scow 

 was 50 ft. long, and by starting one drill at the end, and the other 

 at the middle of the track, each drilling five holes at intervals of 

 5 ft., it was possible to drill ten holes from one position of the scow. 

 Holes 9 ft. deep could be drilled without changing the drill bits for 

 longer ones. As soon as a hole was drilled to the requisite depth, 

 it was charged and blasted without moving the scow. When one 

 row of holes had thus been disposed of, the scow was moved hack 

 from the face of the excavation a distance of 6 ft. and the operation 

 was repeated. 



This method of drilling holes was afterward greatly improved. 

 A hydraulic ram was introduced in order to move the drills both 

 vertically and horizontally. The great inconvenience arising from 

 the debris which filled the bottom of the hole, thus forming a cushion 



