So instead I decided to randomly select this, as a change from all those 19th century mines. This was probably once a Roman furnace, for smelting iron ore. It's just outside a large first-century-AD Roman fort, which was discovered a few years ago by some very surprised archaeologists who were looking for evidence of medieval silver mining.
The big question here is whether the Romans knew there was a rich silver lode just down the hill or not.
So this is probably a military furnace, created by troops commanded by Vespasian during his campaign in the South West of Britain back when he was a surprisingly -young Legate in his 30s. About 25 years later, he would become Emperor.