This paper comprises a discussion of standards for comparing conditions in mines, of methods for measuring the flow of air, of a comparison of fan performances, and the application of these matters to mines in Western Australia.UNDERGROUND CONDITIONSIn approaching the question of mine ventilation one naturally searches for some standard by which to measure the conditions in a mine.There are several clauses in the Mines Regulation Act defining the conditions which are permitted. The quantity of CO2 is limited to 025 by volume, but the estimation of CO2 involves a chemical determination and is not suited for general use in a mine.The maximum temperature is fixed at 87 0 F. (dry bulb), and at 80 (wet bulb). The reading of temperature is easily done in the working places, and this is a very valuable and practical test of the conditions in a mine.The wet bulb temperature is probably the most important factor in determining working conditions. It is obvious that human life cannot be sustained for any appreciable period if the wet bulb temperature is above blood heat (about 97F.). In places where the wet bulb temperature is 90F. there is danger of heat stroke.In places where the air in a mine is still, the wet bulb temperature rises to a value close to the rock temperature.Adequate ventilation cools the workings by transference of heat from the rock to the air stream, and the temperature...