ECHO, from ἦχος a sound, or rather from ἠχώ an echo. Sounds are reflected from the surfaces of hard bodies, such as hills, rocks, walls, &c., in such a manner that the angle of reflection is equal to the angle of incidence; and, in certain circumstances, these reflexions produce very remarkable echoes. The form of the reflecting surface, plane, or convex, or concave, &c., modifies the reflection. Sounds are also reflected by the surface of water, and sometimes even by clouds. In an elliptical room, if the sound proceed from one of the foci of the ellipse, a person placed in the opposite focus will hear the sound much more distinctly than if placed in any other part of the room. The phenomena of whispering domes and whispering galleries depend on similar principles,—for example, in the case of the dome of St Paul's Cathedral, London. Some echoes repeat only sounds of a certain pitch. Others are said to repeat sounds, but not at the same pitch as the original sounds. Such is the echo at Roseneath in Scotland, which is said to repeat a musical sound three times, but each time lower and lower by the interval of a tone. There seems to be some error in this observation at Roseneath. An echo at Genetay, six miles from Rouen, is said to repeat words several times in varied tones. Other echoes, called polysyllabical, repeat many syllables or words. Multiple or tautological echoes repeat the same word several times; as at the castle of Simonetta, about two miles from Milan, where the echo repeats a word about twenty times over. Another, near Coblenz, is said to repeat a word seventeen times. Gassendi mentions an echo near the tomb of Cecilia Metella at Rome, which repeated the first verse of the Aeneid eight times. Dr Plot, in his Natural History of Oxfordshire, men-
tions a remarkable echo at Woodstock Park, near Oxford, which repeats seventeen syllables in the day time, and twenty at night. Another echo, on the north side of Shipley Church, in Sussex, is said by Harris (Lex. Tech.) to repeat distinctly, in favourable circumstances, twenty-one syllables.