Mine ventilation. Hollow wooden drum, top, which had two blowholes opposite each other on the circumference, C, one to suck in air and one to expel it as the drum rotated. Inside the drum, fixed to axle on which it rotated, were fans made of wood or of wood and goose quills. HHHH are rods attached to the axle and which acted as a flywheel. Hollow box, bottom, similar to the drum above, but not considered to be as efficient. From 'De re metallica', by Agricola, pseudonym of Georg Bauer (Basle, 1556). Woodcut. Photo via Newscom