Mines in high ground are sometimes drained to a certain depth by an adit or gallery dug from the bottom of a shaft in a sloping direction to a neighbouring valley. One of these adits extends through the large mining district of Gwennap, in Cornwall; it begins in a valley near the sea, and very little above its level, and goes through all the neighbouring mines, which it drains to that depth, and with all its ramifications is 30 miles long. Nent Force Level, in the north of England, forms a similar drain to the mines in Alston Moor: it is a stupendous aqueduct 9 feet broad, and in some places from 16 to 20 feet high; it passes for more than 3 miles under the course of the river Nent to Nentsbury engine-shaft, and is navigated underground by long narrow boats. Daylight at its mouth is seen like a star at the distance of a mile in the interior. Most of the adits admit of the passage of men and horses, with rails at the sides for wagons. The ventilation of mines is accomplished by burning fires in some of the shafts, which are in communication with the others, so that currents of air flow up one and down the others. In some cases fresh air is carried into the mines by streams that are made to flow down some of the shafts. Were this not done, the heat, which increases with the depth, would be insupportable; ventilation diminishes the danger from the fire-damp, for, even where Sir Humphry Davy’s safety-lamp is used, accidents happen from the carelessness of the miners.[98] The access to deep mines, as in Cornwall, is by a series of perpendicular or slightly inclined ladders, sometimes uninterrupted, but generally broken at intervals by resting-places. It is computed that one-third of a miner’s physical strength was exhausted in ascending and descending a deep mine: they are now drawn up by the steam-engine.