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The propagation of electromagnetic waves over the earth's surface is considered under transient conditions. The source is taken to be a vertical electric dipole whose current moment is suddenly established. The build-up of the radiated field is calculated under various assumed corditions. It is shown, even in the absence of an ionospherically reflected wave, that the influence of earth curvature has a pronounced effect on the distortion of the original pulse shape. For great distances (i.e., d>2000 km), it is found to be more convenient to regard the field as a sum of modes. Particular attention is given to the transient characteristics of the dominant mode as a function of the source waveform.