The fundamental principles of the interferometer, particularly the liquid supersonic interferometer of Hubbard and Loomis, are incorporated in an instrument in which the liquid under examination may be put under pressures up to about 10,000 lb./sq. in. The instrument and the pressure device are built into a block of special cast steel 8×5×4½ inches. The results given are preliminary and merely show the performance and precision of the instrument. A curve is given showing the change of velocity of a high frequency acoustic wave of 195 kc from 1143 meters per second at atmospheric pressure to 1233 meters per second at a pressure of 3105 lb./sq. in. in carbon disulphide at room temperature. (The room temperature varied at different readings from 22.25°C to 25.41°C.)