Convection: Low Wax Knott miners' bothy

 We'd read the story about the murdered miner so we went in search of the hut ... and found it.

At first it could have been a sheep fold, but it has a fireplace, a chimney and roof slates are scattered around inside. 

Living in a house with central heating as a child, I never understood the ventilating effect of a chimney. As the fire burns, the air expands, becomes less dense and floats upwards, up the chimney. The departure of air reduces the pressure below because there are fewer particle collisions. This means that the air outside which is at atmospheric pressure can force its way in to equalise the pressures. The advantage is that this brings in more oxygen for the fire. The bothy is small; it would have been very cramped but the fire would have made it hot inside. Hot but draughty with the air being pulled in through the doorway and the cracks by convection.