In the early days of coal mining where the coal usually -outcropped near the surface, the old miners followed the seams as they moved into the hillsides, but the problem was that in time the air would become foul and unbreathable so work could not proceed further. It was found that by sinking a vertical shaft into the mine and relying on the principle of cold air being heavier than warm air, displacement of the foul air took place. There were several reasons why mine air was warmer than ambient air, the main one being heat generated by the miners and perhaps horses working in the mine, also that from candles for lighting, Etc. This principle was taken advantage of later by means of the furnace, which leads us into induced ventilation, culminating in the use of fans.