A similar great drain is Nent Force Level, in the north of England, which drains the numerous mines in Alston Moor. It consists of a stupendous aqueduct nine feet broad, and in some places from sixteen to twenty feet high. For more than three miles it passes under the course of the river Nent, to Nentsbury engine-shaft, and is navigated underground by long narrow boats. At the distance of a mile in the interior, daylight is seen at its mouth like a star, and this star is continually enlarging upon you until you find yourself in open daylight. The ramifications of the Great Adit Levels of the mines of Freiberg in Saxony have a total length of seventy-two miles; but the most stupendous works of this description are those in the district of Clausthal and Zellerfeld, in the Hartz, where, as the mining operations have been carried on deeper and deeper, adits have been successively driven below adits. Four of these levels date from times previous to the seventeenth century; but, as they were found insufficient, the famous Georg Stollen was added to their number in 1777. This gigantic tunnel, which, piercing the hard rock, required twenty-three years for its completion, is above five miles long, and passes 900 feet below the church of Clausthal. It serves the double purpose of a draining gallery and of a navigable canal. The water is always kept at a height of from fifty to sixty inches; and the boats, which carry about five tons, are propelled by means of a chain attached to the vault, along which the boatmen drag or push them forward. In this economical manner about 20,000 tons of ore are annually brought to daylight. The boats are made and repaired in a subterranean wharf, which, though far from being one of the largest, may probably boast of being the deepest. Until 1851 the Georg Stollen answered all the purposes for which it was constructed, but at the end of that period the increased depth and extension of the mines rendered necessary the addition of a new great adit level, which has been named the ‘Ernst August Stollen,’ in honour of the late king of Hanover. In spite of its vast dimensions, this magnificent work, which is six and three-quarter miles long, about ten feet high, and six and a half broad, required only thirteen years for its completion, and may justly be considered as one of the triumphs of modern engineering. The excavation was begun simultaneously at ten different points, and such was the admirable precision of the plans that all the junctions of the different sections of the gallery fitted accurately into each other.