ADIT. An adit is the name given to a horizontal working made from the surface to strike a seam of coal or a mineral vein. Other names for an adit are "day drift," "day hole" surf (Forest of Dean), "sough" (lead mines of Derbyshire), "water-gates" (north of England), "sorre" (Yorkshire), "aridod" and "audit" as well as adit. Mining by means of adits is very ancient, and was practised in hilly country where the seams were cut by val leys. It was perhaps the earliest method of mining. The adits were driven at such a level as to drain off the water from the workings. The figure shows a seam coal outcropping on the side of a hill with ventilating shaft and adit level.
Sometimes the coal was carried up the shaft on the backs of men or women climbing up ladders, or by means of a jack-roll worked by manual labour. The coal was worked by means of narrow passages driven in the coal, small pillars being left for the support of the roof. The extent to which working was possible was limited by three factors : lack of ventilation, crush of pillars and water difficulties. Mechanical ventilation was not practised. A fire was placed in the shaft to heat the air and so increase the air current. The pillars were made very small with a view to getting as much coal as possible, and as the area worked extended, a point was reached at which the support offered by the pillars of coal became insufficient and a crush of the superincumbent strata supervened, inducing a "creep" or heaving up of the floor, if the floor were a soft fireclay. If the floor was hard, a "thrust" took place and the pillars were crushed. The methods of rais ing the water from the dip workings to the water level or adit being by means of pumps worked by manual labour, it was not possible to proceed far to the dip from beyond the point where the adit struck the seam of coal, or as it was termed beyond "the line of free drainage." Where the ground was flat, working by adits was not possible, so that it was usual, in very early times, to get the coal by means of bell pits (q.v.) sunk near the outcrop. (R. R.)