This photograph is dated 28 January 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.
This one is just below Yealm Steps waterfall on the east bank of the river. The boy is Lawrence Burnard.
Image - Glass Plate