THE importance of obtaining a record of the temperature of rock masses in deep mines can hardly be over-estimated. It has a direct bearing on future mining operations in Australasia.The facilities existing for such observations in the deep mines at Bendigo, induced the writer to recommend that a series of observations be made, not only of rock temperature but of the composition, etc., of the air in relation to the subject of ventilation. It had long occurred to him that the accepted formula used by physicists for determining the rate of increase of heat with the depth was not an absolute one, and might be modified in the light of recent deep mining in various parts of the world. In order to obtain some data which might be presented to mining engineers for their consideration, he undertook to make some systematic observations.For this purpose several of the deepest mines at Bendigo were selected, and holes were driven into the rock consisting of Silurian slates, sandstones, quartz veins and a Tertiary dyke (Limburgite). Thermometers were placed in the Lore holes, plugged up with cotton wool and clay to exclude the external air.The following instruments were used: Maximum and minimum thermometers, wet and dry bulb thermometers, aneroid barometers, and velocity anemometers.