The timbering of a mine also affords very convenient lurking-places to the numerous rats which are met with underground, where they contrive to live upon the crumbs or offal of the miners’ meals, or upon candle-ends and remains of wicks. Not seldom the timbering of a gallery, weakened by rot, gives way under the pressure of the roof, which falls in with a tremendous crash, and sometimes buries the unfortunate miner under its ruins. Another disadvantage attending timbering is its liability to catch fire, and thus, wherever the cost is not found too great, the chief galleries of a mine are now usually constructed of stone. Sometimes the two sides of a gallery are lined with vertical walls, and its roof is supported by an ogival vault or an arch. If the sides of the mine are solid, a simple arch is sufficient to sustain the roof; and at other times the whole surface of a gallery is formed of a single elliptic vault, the great axis of which is vertical, and the bottom is surmounted by a wooden plank, under which the waters run off. [Illustration: DRAINAGE OF A MINE BY ADIT LEVELS. _a._ Shaft. _b._ Shallow adit. _c._ Deep adit. _d._ Mineral lode. ] The miner is generally in a state of perpetual warfare with the water, which threatens to inundate the scene of his labours; and as in a leaky ship the pumps must be kept continually working to prevent the vessel from sinking, so here also perpetual efforts are necessary to keep off the encroachments of this never-tiring foe. When a mine is situated above the level of a valley or of the neighbouring sea, its drainage may be effected in a comparatively easy manner by means of sloping galleries, dry-levels or adits, which in many cases serve also for the transport of the ore or coal. In some mines these drainage levels are executed on a truly gigantic scale. Thus the Great Cornish Adit, which extends through the large mining district of Gwennap, begins in the valley above Carnon, and receives the branch adits of fifty mines in the parish of Gwennap, forming excavations and ramifications which have an aggregate extent of between thirty and forty miles, and which are in some places 400 feet below the surface of the ground. The longest branch is from Cardrew mine, and is five and a half miles in length. This great adit opens into the sea at Restronget Creek, and empties its waters into Falmouth Harbour.