Air doors, in the context of mining, were necessary to ensure air was diverted to parts of a mine where it was needed. Mine workings were so complex that ventilation of the most distant parts would have been virtually impossible without them. In early days, it was commonplace for children to be employed to open these doors, in shifts of up to twelve hours, during which time they seldom saw another person. In later times, with improved ventilation, I.E. larger, more efficient fans, etc. It would have been physically impossible for a small boy to open the larger doors against the vacuum caused by the fan. This was remedied by the simple expedient of a small sliding hatch in the main door, thus breaking the vacuum, when the door would open relatively easily. This shutter could also be used to regulate the airflow to a certain extent.