Stock Photo: Mine ventilation shaft topped with a device to extract the stale air from the mine. H is a vane to make the barrel revolve so that is always facing in the optimum wind direction. From De re metallica, by Agricola, pseudonym of Georg Bauer,Basle 1556,

Stock Photo: 1746-1909 Mine ventilation shaft topped with a device to extract the stale air from the mine.  H is a vane to make the barrel revolve so that is always facing in the optimum wind direction.  From De re metallica, by Agricola, pseudonym of Georg Bauer,Basle 1556,