The presentation was made at the AGM of the Mine Ventilation Society of South Africa on 23 June 2006. The paper was published in the October/December 2005 issue of the Journal of the MVS, [Vol. 58, No. 4]. Concepts discussed in the paper were presented at the Tenth U.S./North American Mine Ventilation Symposium held in Anchorage, Alaska, in May 2004