Adit (Lat. aditus, entrance), a horizontal passage made into mines for the purpose of draining them, and also for the extraction of their products at the lowest convenient level. In very mountainous regions adits often present the readiest means of access to the mineral veins known to exist in the interior of precipitous hills. Enormous sums have been expended in the silver region of Mexico in these exploring adits. One of the most famous adits in the world is that of Klausthal, in the Hartz, which is 6 1/2 miles long, and passes upward of 300 yards below the church of Klausthal. Its excavation lasted from the year 1777 till 1800, and cost about $330,000. The adit which drains the district of Gwenap, in Cornwall, is estimated with its branches to extend a distance of 30 miles; its mouth is in a valley near the sea, and from it are discharged the superficial waters of numerous mines, as also all the water pumped up in them to its level. One of the most extensive adits in the world was; commenced in the beginning of the present century by the Austrian government, and is called by the name of Joseph II. Its mouth is in the banks of the river Gran, in Hungary, and it passes by the mines of Hodritz toward those of Schemnitz, about 10 miles.

The object of its construction is partly to explore for new i veins, and in part to drain mines already in operation. A work of similar magnitude has been undertaken in the Washoe mining district of Nevada, for the purpose of developing the Comstock lode. It is known as the Sutro tun-nel, and the plan was to commence at the Carson river, 150 feet above the stream, and to excavate a space of 12 by 14 feet to a dis-tanee of 19,790 feet, when the lode would be cut at a depth of 1,898 feet below the outcrop. A cross tunnel was to be constructed along the ledge about 12,000 feet, to connect with all the mines, and four shafts were to be sunk for ven-tilation. A company for its construction received large privileges from congress in 186(5, and afterward application was made for a government subsidy. A commission was appointed to examine the project, which early in 1872 reported unfavorably, estimating the cost at $4,418,329. The work was not then far advanced, but has since been vigorously prosecuted both upon the main tunnel and the shafts.