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BOOK VI. 



the air through the blow-hole and the conduit into the shaft. Although 

 this machine has no need of men whom it is necessary to pay to work the 

 crank, still when the sky is devoid of wind, as it often is, the machine does 

 not turn, and it is therefore less suitable than the others for ventilating a shaft. 



In the kind where the fans are fixed to an axle, there is generally a 

 hollow stationary drum at one end of the axle, and on the other end is fixed 

 a drum made of rundles. This rundle drum is turned by the toothed wheel 

 of a lower axle, which is itself turned by a wheel whose buckets receive the 

 impetus of water. If the locality supplies an abundance of water this 

 machine is most useful, because to turn the crank does not need men 

 who require pay, and because it forces air without cessation through the 

 conduit into the shaft. 



A HOLLOW DRUM. B ITS BLOW-HOLE. C AXLE WITH FANS. D DRUM 



WHICH IS MADE OF RUNDLES. E LOWER AXLE. F ITS TOOTHED WHEEL. 



G WATER WHEEL. 



Of the fans which are fixed on to an axle contained in a drum or box, 

 there are three sorts. The first sort is made of thin boards of such length 

 and width as the height and width of the drum or box require ; the second 



