208

BOOK VI.

or by a horse or by water-power; if by a man, the lower board of a large bellows isfixed to the timbers above the conduit which projects out of the shaft, and soplaced that when the blast is blown through the conduit, its nozzle isset in the conduit. When it is desired to suck out heavy or pestilentialvapours, the blow-hole of the bellows is fitted all round the mouth of theconduit. Fixed to the upper bellows board is a lever which coupleswith another running downward from a little axle, into which it ismortised so that it may remain immovable ; the iron journals of this littleaxle revolve in openings of upright posts ; and so when the workman pullsdown the lever the upper board of the bellows is raised, and at the same timethe flap of the blow-hole is dragged open by the force of the wind. If thenozzle of the bellows is enclosed in the conduit it draws pure air into itself,but if its blow-hole is fitted all round the mouth of the conduit it exhauststhe heavy and pestilential vapours out of the conduit and thus from theshaft, even if it is one hundred and twenty feet deep. A stone placed on theupper board of the bellows depresses it and then the flap of the blow-hole is

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A —Smaller part of shaft. B —Square conduit.

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C—Bellows.

D—Larger part