In a mine with shafts opening at the same level, natural ventilation once established will be effective during cold weather, as the downcast will have the temperature of the outside air, while the upcast will be filled with the warm air of the mine.
The usual form of ventilating furnace is a plain fire grate placed under an arch, and communicating with the upcast shaft by an inclined drift.
In main( ), when you place something inside the Instrument array you automatically upcast to Instrument.
In any case whether natural or artificial means be employed, a mine can only be ventilated properly when it has at least two distinct openings to the surface, one an intake or " downcast," the other a chimney serving as an " upcast."
The fire should be kept as thin and bright as possible, to reduce the amount of smoke in the upcast.