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The diurnal variation of the time of arrival of a 16-kilocycle signal traversing a transatlantic path has been found to be about 40 microseconds. This variation is presumably caused by a day-to-night change in the equivalent height of reflection of 10 to 12 kilometers, and appears to be very predictable. The great phase stability of very low frequency transmission permits intercontinental frequency comparison to a precision of at least 1 part in 1010. Variations of the frequency of the arriving signal are apparently always less than Â±3 parts in 109; a figure to be compared with estimates of the order of Â±2 parts in 107 for high-frequency transmission. With such stability of propagation, extremely narrow receiving bandwidths are attainable. These bandwidths, in turn, make possible highly reliable networking of frequencies for communication station allocation and for navigational purposes.