The Air-conditioning pump which supplied fresh air to the underground chambers and tunnel face. Air pipes were made of Klim dried milk tins fitted together. The vertical pipe (see drawing) carried fresh air from ground level sucked down by one of two bellows made from two RAF kitbags mounted on runners. The curved pipe carried air pumped by the other bellow along the tunnel to the workface. The box between the bellows contained a system of flapper valves each made of leather.