A wooden duct ran the entire length of the tunnel and protruded into the outside air. The fire heated stale air inside of the tunnel, drawing it up the exhaust shaft and out of the mine by the chimney effect. The resulting vacuum then sucked fresh air in from the mine entrance via the wooden duct, which carried it down the length of the tunnel to the place in which the miners were working.[5] That avoided the need for additional ventilation shafts, which could have been observed by the enemy, and it also easily disguised the diggers' progress.