zle ; then the wind , penetrating through the hole in the middle bellows - board , which is called the air - hole , lifts up the upper part of the bellows , upon whose upper board is a piece of lead , heavy enough to press down that part of the bellows again , and this being pressed down blows a… … A — Iron hoop . B — Double bellows . C — Its nozzle . D — Lever . I have spoken of the furnaces and the iron hoop ; I will now speak of the muffles and the crucibles . The muffle is made of clay , in the shape of an inverted gutter tile ; it covers the scorifiers , lest coal dust fall into them and…
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![An illustration from M. Vitruvii De architectura libri decem, summa diligentia recogniti, atque excusi, cum nonnullis figuris sub hoc signo * positis, nunquae antea impraessis, additis Iulii Frontini De aqueductibus libris, ppter [i.e. propter] materiae affinitatem (1523)](https://img.insideoutlibrary.com/img/200/erara-6155905-335-1.webp)
![An illustration from Georgii Agricolae De re metallica : libri XII. Quibus officia, instrumenta, machinae, ac omnia deni[que] ad metallicam spectantia, non modo luculentissimè describuntur, sed & per effigies, suis locis infertas, adiunctis latinis, germanicis [que] appellationibus ita ob oculos ponuntur, ut clarius tradi non possint. ; Eiusdem De animantibus subterraneis liber, (1556)](https://img.insideoutlibrary.com/img/200/ia-gri_000033125008455038-382-256.webp)
