[Supported by a published paper entitled "Method of estimating the slip in air compressors by altering the clearance volume."] In comparison with the experimental research which has been devoted to the internal combustion engine there is a lack of thorough research into the problems inherent in air compressors, with the result that the problems met with in the study of air compression cover a wide field. The main purport of this thesis is to investigate the beneficial and adverse effects of large and small clearance spaces in air compressor cylinders and to show that altered clearance offers a means of arriving at an approximation to a compressor's air slip. Since the sizes and types of compressor valves fitted more or less determine the volume of the clearance space and since small valves have high, and large valves low air speeds through them, effort usually seeks to achieve a suitable compromise, securing at once a fairly small clearance and a reasonably slow passage of air. The essence is that it is hoped that the tests carried out with altered clearance will prove that the effect of clearance below a certain pressure may be neglected, but that above this pressure clearance has an increasingly adverse influence. The fact that the theoretical volumetric efficiency is a linear function of the clearance at any delivery pressure suggests a means, if it can be shown that the actual actual volumetric efficiency is also of straight line form, of estimating at zero clearance the volumetric efficiency and hence the air slip. (This is discussed in the enclosed publication - "Engineering," 7th August 1936, p.138-139.) Information has been gathered and recorded regard - :ing types of valves, air speeds through them, typical clearance percentages allowed in practice, mean values of the compression exponent and the effect of cooling. A section has been devoted to reviewing a few of the most common methods of measuring air quantities, particular comparison being drawn between the British and .merican commercial practices. Several methods are in existence for measuring air, and, although reasonable accuracy can be attained by most of them when correctly applied in a scientific laboratory, it is only within recent years that a standard apparatus has been adopted for commercial tests. Another section is given to a description of the Compressor and Air Measurement Plant installed in Edinburgh University, for the design of which the author was partly responsible,.