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This paper comprises a review of the present state of knowledge of the speed of transmission of radio waves under the practical conditions of certain applications in which such knowledge is important. It is shown first that, for radio waves in a vacuum, their speed of transmission is equal to the velocity of light (299,775 km/s), to within the limits of experimental error. When waves of frequencies in the neighborhood of 100 kc/s are propagated at a height of a fraction of a wavelength above the earth's surface, their speed is reduced by an amount dependent upon the electrical conductivity of the earth. For overland transmission, the speed is about 299,250 km/s. For higher frequencies propagated at a height of several wavelengths, the speed of the waves is determined by the refractive index of the air, rather than by the properties of the ground. Since the refractive index decreases with the height of transmission, so does the speed of the waves increase toward the velocity of light. For example, centimeter waves propagated at heights of a few hundred feet have been observed to travel at a speed of about 299,690 km/s. When the waves are transmitted between ground and aircraft flying at a height of 30,000 feet (9,800 meters) this speed is increased to about 299,750 km/s.