This is done in order that the mine may be ventilated. The mine has two shafts at some distance from each other. At the bottom of one shaft a huge fire is kept burning. This fire makes the air very hot and light.
I t is forced up the shaft by the greater pressure of the colder and heavier air about it.
This is supplied by the other shaft. As it rushes along it mixes with the bad air and gases in the passages, but by the time it reaches the bottom of the shaft where the fire is burning there is too little firedamp in the mixture to cause it to explode. This constant circulation of air keeps the mine well ventilated. In many mines revolving fans take the place of the fires.