Abstract
In discussing the propagation of very low frequency radio waves to great distances, it is convenient to treat the space between the earth and the ionosphere as a wave-guide. This treatment is applied to some experimental measurements made by Dr. Weekes on signals of frequency 16 kc/s, over a range of distances from 340 km to 3640 km from the sender. It is shown that the measurements can be explained in terms of the four least attenuated wave-guide modes. If the earth's magnetic field is neglected, and the ionosphere is assumed to be a sharply bounded homogeneous, ionized medium, then it is shown that the height of the boundary is 69·1±0·5 km.

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