As far back as 1842 our own Joseph Henry performed experiments similar to those performed by Hertz, and he inferred, prophetically, that the discharge was oscillatory. Nobody ever suggested this idea before, but Henry’s experiments permitted such an inference. Its oscillatory character was then demonstrated mathematically in 1853 by Professor William Thomson of Glasgow, and his calculation was proved to be correct by many experimental tests covering a period of over twenty-five years, and thus the electrical oscillator, similar to the one employed by Hertz, became a well-known apparatus. What, then, was the novel element in the Hertzian work? It was, broadly speaking, his demonstration that the space surrounding the oscillator (the spheres with their rods) participates in the electrical oscillations in perfect agreement with the Faraday-Maxwell theory; a participation which was foreign to all previous electrical theories. In other words, he detected in the old electrical-oscillation experiments a new action, never detected nor even dreamed of before. He discovered the electrical waves in the space outside of the oscillator. Remembering the impression which Helmholtz’s lecture on Faraday made upon my mind, I was certain at that time that nobody in Continental Europe but one of Helmholtz’s pupils like Hertz could have predicted that there was in these well-known electrical oscillations a new action, an action demanded by the Faraday-Maxwell theory. A simple analogy will, I trust, help much to illustrate the new action which Hertz expected when he started out to search for an experimental test of the modern electromagnetic theory. No scientific expedition ever started out in search of scientific treasures and returned with a richer load. Here is the analogy: If by the force of our fingers we deflect the ends of the prongs of a tuning-fork and then let go, the prongs will return to their normal position after performing a number of vibrations of gradually diminishing amplitude. The state of rest is reached when the energy of bending, produced by the work of our fingers, has been expended, partly in overcoming the internal friction in the tuning-fork, partly in overcoming the reactions of the surrounding medium, the air; this last effect results in sound-waves which are radiated off into space. The stiffness and the mass of the prongs of the fork determine the period of vibration, that is, the pitch of the fork.