210 BOOK VI. 



always drives forward another, they penetrate into the tunnel and change 

 the air, whereby the miners are enabled to continue their work. 



If heavy vapours need to be drawn off from the tunnels, generally three 

 double or triple bellows, without nozzles and closed in the forepart, are placed 

 upon benches. A workman compresses them by treading with his feet, just 

 as persons compress those bellows of the organs which give out varied and 

 sweet sounds in churches. These heavy vapours are thus drawn along the 

 air-pipes and through the blow-hole of the lower bellows board, and are 

 expelled through the blow-hole of the upper bellows board into the open 

 air, or into some shaft or drift. This blow-hole has a flap-valve, which the 

 noxious blast opens, as often as it passes out. Since one volume of air con- 

 stantly rushes in to take the place of another which has been drawn out by 

 the bellows, not only is the heavy air drawn out of a tunnel as great as 1,200 

 feet long, or even longer, but also the wholesome air is naturally drawn in 

 through that part of the tunnel which is open outside the conduits. In this way 

 the air is changed, and the miners are enabled to carry on the work they have 

 begun. If machines of this kind had not been invented, it would be necessary 

 for miners to drive two tunnels into a mountain, and continually, at every 

 two hundred feet at most, to sink a shaft from the upper tunnel to the 

 lower one, that the air passing into the one, and descending by the shafts 

 into the other, would be kept fresh for the miners ; this could not be done 

 without great expense. 



There are two different machines for operating, by means of horses, the 

 above described bellows. The first of these machines has on its axle a 

 wooden wheel, the rim of which is covered all the way round by steps ; a 

 horse is kept continually within bars, like those within which horses are held 

 to be shod with iron, and by treading these steps with its feet it turns the wheel, 

 together with the axle ; the cams on the axle press down the sweeps which 

 compress the bellows. The way the instrument is made which raises the 

 bellows again, and also the benches on which the bellows rest, I wiU explain 

 more clearly in Book IX. Each bellows, if it draws heavy vapours 

 out of a tunnel, blows them out of the hole in the upper board ; if they are 

 drawn out of a shaft, it blows them out through its nozzle. The wheel has 

 a round hole, which is transfixed with a pole when the machine needs to be 

 stopped. 



The second machine has two axles ; the upright one is turned by a horse, 

 and its toothed drum turns a drum made of rundles on a horizontal axle ; 

 in other respects this machine is like the last. Here, also, the nozzles of 

 the bellows placed in the conduits blow a blast into the shaft or tunnel. 



In the same way that this last machine can refresh the heavy air of a 

 shaft or tunnel, so also could the old system of ventilating by the constant 

 shaking of Unen cloths, which Pliny ^o has explained ; the air not only grows 



'"Pliny (xxxi, 28). " In deep wells, the occurrence of sulphurata or ahiminosa 

 "vapor is fatal to the diggers. The presence of this peril is shown if a hghted lamp let down 

 "into the well is extinguished. If so, other wells are sunk to the right and left, which carry 

 " off these noxious gases. Apart from these evils, the air itself becomes noxious with depth, 

 " which can be remedied by constantly shaking linen cloths, thus setting the air in motion." 



