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10 in the electric beam. Listen, and you hear a low, humming sound. This is caused by the movements of the fork sending a number of pulsations through the air, which strike against and agitate the drum-head of the ear, and from it the movements are communicated to the

nervous structures in the deeper ear. If I touch the limb of the fork with my finger, the fuzziness vanishes, the image now appearing sharply on the screen, and the sound is no longer heard. I shall next cause this smaller fork to sound by drawing a fiddle bow across one of its prongs. You hear it sounding. I