Should it happen, as it sometimes does, that after having gone through those operations, the quarrier fails in getting the cracked mass thrown down to the bottom of the quarry, he varies his mode of attack, and proceeds to bore a row of _plug-holes_ on the face of the rock in such a line as to cut off a part from one end of the shaken mass; and for that purpose he is often obliged to hang a scaffold over the face of the rock on which to stand while boring the holes. Those plug-holes should be slightly inclined, so that, when the wedges, called _plugs_[21] and _feathers_, are driven into them, they may rend the rock in such a direction that the piece intended to be cut off may be a little narrower on the inner than the outer face, so that, thus resting on an incline it may be more easily taken out. The plug-holes should be cut at one foot asunder, and bored with a jumper 1¹⁄₂ inch diameter to the depth of 9 inches; and if the plug-holes be deep, and some difficulty in driving be expected, the plugs should be carefully greased or oiled previously to being driven. Having cut off a block as above described, an attempt may be made, if the mass be great, to throw it down by means of _bulling_; but if it be of lesser dimensions, and there be reason to expect that it may be removed in the ordinary way, the power of the crane may be applied to draw it down. For that purpose, the quarrier employs an instrument called a _Dog_, which is a strong short hook, armed like a pick on the point with steel, and having a ring in the end of it for the hook of the crane-chain to pass through. Having cut a small hole with a pick, on the upper part of the block which is meant to be removed, the _steeled_ point of the dog is inserted into it, in such a manner that the weight of the crane-chain may retain it steadily in its place. Five or six men then heave on the crane a strain just as much as they suppose it may bear, without danger of carrying away any of its fixtures; and as many men as can find room are, at the same time, employed at the top of the rock, working with crow-bars behind the block, so as to shake it and loosen its hold. The two parties continue their work reciprocally, leading and following,--the men at the crane, still keeping up the strain, and taking care not to heave so much as to break any of the chains, while those on the top continue to shake the block by means of the crow-bars, or throw in stones into the opening, which is always getting wider between the block and the cliff, so as to prevent the loosened mass from falling back into its old place. When the block has been drawn as far forward as to appear just ready to fall over the cliff, one of the most expert men at the crane stands carefully watching the movement of the block; and whenever the stone begins to fall, he instantly throws the crane _out of the gear_, so as to prevent the wheelwork being pulled to pieces by the tumbling mass getting entangled in the chains,