Made for economy and efficiency, of the new air-compressing Plant, designed and built by the Austral Otis Engineering Company, South Melbourne, for the Long Tunnel Gold Mine, it was my intention merely to give a short description of the plant and method of testing, illustrated by typical diagrams taken from the steam cylinders and the compressing cylinders under various conditions, and a tabulated statement of the mean results obtained: This of itself, though sufficient to show an engineer the high efficiency and economy that can be obtained by properly designed compressing plant, as compared with the crude appliance's usually employed for the purpose on our mines, and the great savings to be made in the fuel bill, does not go far enough, because, though a great saving is shown in the cost of production of the compressed air, compared 'with the usual type of compressors employed, larger savings still can be effected by a proper use of the air after compression, a matter that is little understood or considered in the usual mine equipment.