llowed beams joined firmly together , so as to hold the water which flows through them , and they are covered by planks all along their course , from the mouth of the tunnel right up to the extreme end of it , to prevent earth or rock falling into them and obstructing the flow of the water . If… … A — Trough . B — Hopper .
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![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_000033125008455038-59-30.webp)
