As a means of transmitting power from the surface to the underground workings, and more particularly for the transmission of power to the actual working face, compressed air at present holds the premier position. For the driving of rock drills it may be said to be without any serious competitor, although, with the development of the electric drill, it is likely enough that electricity will become a more formidable antagonist in the course of time. For this work, indeed, it has been up to now the only suitable system available, and its incidental advantages in promoting ventilation at the working faces have to some extent compensated for the poor economy of the transmission. Great mechanical economy is not to be expected when the air is used without expansion, as in the ordinary rock drill, and in an ordinary way considerably less than one-third the power generated by the steam at the surface is available at the face. Compressed air, however, is used for a variety of other purposes, such as driving winches underground and direct-acting pumps, where its incidental advantages are relatively of less importance, and where its mechanical economy is a greater factor in determining whether it should be used in place of steam or electricity or other driving power which may take its place. Unfortunately, from a mechanical point of view, the use of compressed air underground is subject to two great disadvantages. It generally has to drive a motor in which the compressed air is used either non-expansively or with very little expansion, and the use of a re-heater near the motor, which is such an important factor in an economical system of air transmission, is generally attended with formidable difficulties underground. These unfavourable conditions both very seriously affect the efficiency of the system. Some tests made by Mr. Searing on a compressed air pumping plant belonging to the Colorado Fuel and Iron Co., and recorded in the “Trans. of the American Inst. of M.E.,” vol. xxvi., p. 1,085, showed an efficiency of only 9 per cent. as measured between the actual work done in pumping and the work done in the steam cylinder. The compressed air plant in this case was not of the best, and the air was transmitted 4,000 ft.; but it is probable that in most cases where compressed air is used in mines for pumping the efficiency is less than 30 per cent. Unfortunately few actual measurements of such efficiencies seem to be available.