Roney manages to shake Quatermass out of his trance, and remembering the legends of demons and their aversion to iron and water, Roney proposes that a sufficient mass of iron connected to wet earth may be sufficient to short circuit the apparition.
Roney manages to shake Quatermass out of his trance. Roney was remembering the legends of demons to iron and water. Roney was remembering the legends of their aversion to iron and water. A sufficient mass of iron may be sufficient to short circuit the apparition. This is what Roney proposes. This iron is connected to wet earth.