In order to supply fresh air to the well diggers during construction, a dividing wall, usually of wood, was built into the well shaft, any gaps being stuffed with straw and pitch to make it as airtight as possible. Over the projecting "chimney" so formed, a fireplace was built that sucked air through the well shaft below. Fresh air (and hence oxygen) circulated through the artificially built U-shaped pipe so created, its two halves being separated by the dividing wall.