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Measurements of the signal strength from the very-low-frequency Post Office sender, GBR (16 kc/s), have been made at distances up to 850 km from the sender, using a receiver installed in an aircraft. The measurements have been repeated using various aerials in the aircraft, including a rotating loop; the results of these measurements are in quite good agreement. Up to 300 km from the sender, the resulting interference pattern between the ground wave and the wave reflected from the ionosphere is in good agreement with that plotted by Budden, Ratcliffe and Wilkes,1 who showed that this part of the pattern could be interpreted by assuming an ionospheric wave reflected, with an effective reflection coefficient of about 0.15, from a horizontal layer at a height of 67 km, with a phase change of Â¿ at reflection, or at a height of 75 km with no phase change. The extended curve shows that this picture is inadequate, and a detailed study of the pattern beyond 350 km indicates that the reflection conditions change considerably in the neighbourhood of 400 km. At 500 km the effective reflection coefficient appears to be about 0.35, and the effective reflection height, or the phase change at reflection, has altered. The experiment with the rotating loop shows that the polarization, which Budden, Ratcliffe and Wilkes had shown to be nearly circular up to 300 km, is elliptical at 350 km and approximately linear at 500 km; the change in reflection conditions at about 400 km thus includes a change in the polarization. The data are not sufficiently consistent to allow a detailed determination of the changes of reflection coefficient with distance in the transition region between 300 and 500 km.