T importance of the investigation of aerodynamic phenomena at High and at Supersonic Velocities is evident. However, it is not yet imperative enough to justify the building of tunnels with large cross-sections or continuous action, because of the high cost of construction and operation. Instead, most investigations in this field are carried out by means of wind tunnels of small cross-section, depending upon an accumulator of compressed air or a vacuum tank. A relatively small compressor or vacuum pump is sufficient for intermittent operation of the wind tunnel. Such arrangements, however, while answering their purpose to a certain extent, are far from the ideal solution. The intention of this paper is not to give such a solution, but to make a contribution to this difficult and complicated problem by approaching it from another standpoint.* All experiments with models are carried out on the basis of Dynamic Similarity and, in finding the effects of compressibility, it is necessary to take into account the Mach Number V/C, where V is the velocity of the fluid in which the model is placed and C is the velocity of sound in the same fluid. This condition requires that the speed of flow be practically the same in the model test as it is under actual conditions. However, if gases or vapors other than air are used it would be possible to change the value of C and thus operate at smaller velocities while maintaining the same Mach Number. Of course, such experimentation would require an airtight tunnel, and the operation of this tunnel, similar to that of a Variable Density Tunnel, would require remote control for all apparatus inside the tunnel, and in the case of certain compounds, might require temperature conditioning. In addition to this there must be a special arrangement allowing replacement of models in the tunnel in a short time without admitting air or allowing the gas to escape, and another arrangement for the collection and storing of the gas and for its purification (from air, water vapors, etc.). All of these extra difficulties are compensated for by a saving in power, providing heavy enough gases are used. Several factors must be taken into consideration in selecting the substance to be used as a flowing medium.