This photograph is dated 17 March 1888. It shows evidence of the tin mining industry which operated on Dartmoor until around 1939.
Blowing houses (sometimes called blowing mills) contained small blast furnaces into which a forced draught of air was blown using waterwheel-powered bellows. Cassiterite (tin oxide) ore was smelted in the blast furnace and cast into ingots using a mould stone.
Burnard's comment on the photograph is 'Remains of furnace. Stone marked is "slagged"'. The stone is marked A in photograph. Slag was the black glassy substance that is a waste product from the blast furnace after smelting tin ore.