Stock Photo: Revolving wooden wind vanes fitted to the top of mine ventilation shafts. When they revolved they acted as extractor fans sucking stale air from the mine. From De re metallica, by Agricola, pseudonym of Georg Bauer (Basle, 1556). Woodcut

Stock Photo: 1746-1907 Revolving wooden wind vanes fitted to the top of mine ventilation shafts.  When they revolved they acted as extractor fans sucking stale air from the mine.   From De re metallica, by Agricola, pseudonym of Georg Bauer (Basle, 1556).  Woodcut