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Mine ventilation. Engraving of water-powered air ventilation in a 16th century mine. The water wheel (at left) powered a closed drum (at centre) within which there was a four-bladed fan (inset, at top left); during use the fan was fitted with feathers. The invention of ventilators like this allowed the development of deep mines. Image taken from De re metallica (1556) by Agricola.
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