The Great County Adit is a system of underground tunnels that drained tin and copper mines between Redruth and Bissoe in west Cornwall, England. It was begun to drain the Poldice Mine by William Lemon, a major adventurer[a]shareholder in the mines. The Poldice Mine which was mined for tin and copper was started before 1512 and by the 1730s its two water-powered pumping engines could not keep the workings dry. Lemon decided that an adit from Wheal Nangile near Twelveheads to the the Poldice sett[b]the area from which the company could extract minerals would dewater the mines by gravity into the Carnon River. Work on the adit, which was driven using hand tools and gunpowder, was supervised by the mine captain[c]manager, John Williams. By 1756 it was 1.5 miles long and had reached the eastern edge of the Poldice sett boundary. The adit was continually extended with branches to other mines. Poldice at 100 fathoms (200 yards) was the deepest and largest mine in the area and water flowing into it increased as more mines were opened. In the mid 1770s the enlarged network of adits took the name the Great County Adit.
The completed adit drained an area of about 12 square miles containing 100 mines in five parishes. More than 40 miles in length and the longest branch was 3.4 miles. The adit drained mines to a depth of 40-60 fathoms (80-120 yards). In 1817-1819, more than 11 million gallons of water flowed from the adit every day. At its peak in 1839, when more steam engines were pumping water into the adit than were used in all of continental Europe and America combined, 14.5 million gallons a day flowed into the Carnon River.
Such was the adit’s importance, that in May 1843, the Mining Journal described it as “the most extensive, valuable, and systematic undertaking of the kind in Cornwall — perhaps in England, and we believe, but few in the world exceed it in importance”. It was maintained by a management committee of mineral lords and mine captains until 1886. Maintenance took place during the summer when flows were lower.
Over time, the adit had cost about £250,000 but it saved up to £20,000 annually on pumping costs. More than thirty steam engines pumped water into the adit in the 1860s, but as the mining industry declined, by 1875 many mines drained by the adit had ceased production.
In winter 1876 poor weather caused flooding in the adit and torrents of water, mining debris and silt were expelled into the Carnon River and deposited downstream at Devoran. Just ten mines along the adit were working after 1870, and by 1900 only Wheal Peevor, Killifreth Mine and Wheal Busy were still open. Activity continued intermittently in the 20th century at a few mines but none are now working. The adit is untended and by 1980 its outflow had dropped to half a million million gallons per day.
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|b.||^||the area from which the company could extract minerals|