BOOK VI.

209

closed. The bellows, by the first method, blows fresh air into the conduitthrough its nozzle, and by the second method blows out through the nozzlethe heavy and pestilential vapours which have been collected. In thislatter case fresh air enters through the larger part of the shaft, and the minersgetting the benefit of it can sustain their toil. A certain smaller part of theshaft which forms a kind of estuary, requires to be partitioned off from theother larger part by uninterrupted lagging, which reaches from the top of theshaft to the bottom ; through this part the long but narrow conduit reachesdown nearly to the bottom of the shaft.

When no shaft has been sunk to such depth as to meet a tunnel drivenfar into a mountain, these machines should be built in such a manner thatthe workman can move them about. Close by the drains of the tunnelthrough which the water flows away, wooden pipes should be placed andjoined tightly together in such a manner that they can hold the air ; theseshould reach from the mouth of the tunnel to its furthest end. At the mouthof the tunnel the bellows should be so placed that through its nozzle it canblow its accumulated blasts into the pipes or the conduit ; since one blast

Wl

mm

MS

rcr-m

A—Tunnel. B—Pipe. C—Nozzle of double bellows.