A method of finding the effective velocity of propagation of radio waves in the 100-kc/s region was developed by the Admiralty Signal Establishment in 1945, using the Decca system—a hyperbolic, phase-measuring navigational aid, at that time utilizing unmodulated continuous waves on 85, 113⅓ and 127½ kc/s from a triplet of stations arranged with two base-lines about 50 km long and situated in flat, low-lying country. The only quantities measured were (a) distances over the earth's surface between transmitting aerials, (b) phase-changes of the order of 100 complete revolutions (measured correct to ± 1/50 revolution) from end to end of the hyperbolic patterns, (c) a radio frequency. Phase readings were taken in an aircraft flying at 1000 ft; the transmitting aerials were 90 ft high.Measurements were taken beyond the “near zone,” that is from 10 to 100 km from the transmitters. As no variation of the velocity with distance could be detected outside the probable experimental error, all readings were grouped together. The final average for the velocity was 2.9925 ± 0.0004 × 108m/sec, which is lower by 1.4 parts in 1000 than the velocity of short waves through air. It may be proved that the velocity derived from this method is the phase velocity. The results may be correlated with theory.