Agricola (Latin pen name of the German schoolteacher George Bauer) was a humanist and investor in German mines. His great work, De Re Metallica, carefully depicted and described all facets of mining from ores to mining machinery. Here depicted are three ventilation machines. At the top, a man and a horse turn the toothed wheel P, which in turn lifts the tops of a pair of bellows through the cams. In the middle, a horse, feeding from the basket, turns a wheel (H) with his forelegs to pump the bellows behind the structure G. At the bottom, a man treads on pedals to force air from the bellows to the crouching miner. A later wit quipped that Agricola's book had drawn more money from readers than had ever been found by mining.