ed to the end of a sequently that the drill was brought to a sufficiently The electrical features flexible cable. practical state to compete with other drills operated One of the most noteworthy features of the drill is of the system are so ex- by compressed air or steam. Within the past few the… … silica. The drills in this mine work to particular ad- revolutions per minute. Every revolution of the arma- Fig. 1—THE MARVIN ELECTRIC ROCK DRILL. vantage, owing to the very large face walls upon which ture produces one complete stroke of the drill. The they operate, and which permit of eight-foot…
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