Adits are long, slightly sloping tunnels created to drain water from mines by gravity; lowering the natural water table and shortening the distance water had to be brought up from below. Adits driven from the lowest suitable points were widely used in Cornish mines from c. 1700 onwards.
By 1839 the Great County Adit discharged around 66 million litres per day and had more steam engines pumping into its course than were used by the whole of continental Europe and America combined.
By the second half of the 18th century, most established mines in Cornwall and west Devon possessed adit systems.
The Great County Adit
Also known as the Great Adit or the County Adit – this was a venture headed by the Lemon and Williams families that drained the largest concentration of copper mines in the world. It was commenced in 1748 and its branching network of drainage tunnels eventually drained over 100 mines to an average depth of 80-100m and reached a length of over 65km. It drained into the Carnon River, near Point Mills which feeds into Restronguet Creek, a tributary of Carrick Roads upstream from Falmouth.
In 1839 it discharged around 66 million litres per day and had more steam enginespumping into its course than were used by the whole of continental Europe and America combined. A high level of dissolved metal salts in this discharge gave rise to copper precipitation works and iron ochre works in the Bissoe Valley.
Even when steam engines were introduced, adits remained an essential part of a mine’s drainage system with the water needing only to be pumped to adit level before being discharged. In some areas considerable ore-ground could be kept clear purely through the use of adits, and on the coast near St Agnes and Perranporth up to 100m of vertical ore-ground could be drained using this method.
In the deeply incised Tamar Valley Mining District the depth was even greater and an adit driven in the mid 19th century at Gunnislake Clitters mine, on the western bank of the river, met the ore-ground at a depth of 160m.
At Wheal Rose near St Agnes in 1725, the Newcomen pumping engine was so costly in coal consumption that the adventurers decided to drive a 2.4km adit to alleviate the cost of continuing to fuel it. Expensive steam pumping engines would only be adopted at mines when excavations extended below the lowest adit level.