In Chapter IV is discussed the question of wavepropo gation over a conducting surface, and various hypotheses are reviewed and tested in the light of the preceding chapters. In approaching a conclusion it has seemed advisable to de part a little from the purely Maxwellian idea of displace ment currents, which does not readily appeal to the imagina tion in this connection, and to substitute F araday's conception of moving tubes of induction, which embodies the same principles in more tangible form. It is hoped that this figure, so successfully used by J. J. Thomson in explain ing other electrical phenomena, may give the reader a clear understanding of what takes place when an electromagnetic wave glides over the surface of the earth. The other chapters are self-explanatory and need not be considered here.