a sound reflected or reverberated from a solid concave body, and so repeated to the ear. The word is formed from the Greek ἠχος, sound, which comes from the verb ἠχω, sono.

The ancients being wholly unacquainted with the true cause of the echo, ascribed it to several which are sufficiently whimsical. But the moderns, who know sound to consist in a certain tremor or vibration in the sonorous body, communicated to the contiguous air, and by that means to the ear, give a more consistent account of echo. For a tremulous body, striking on another solid body, may evidently be repelled without destroying or diminishing its tremor; and consequently a sound may be redoubled by the resiliation of the tremulous body, or air. But a simple reflection of the sonorous air is not enough to explain the echo; for then every plain surface of a solid hard body, as being fit to reflect a voice or sound, would redouble it, which we find does not hold. In order to produce an echo, therefore, it should seem that a kind of concameration or vaulting is necessary, in order to collect, and by collecting to heighten and increase, and afterwards reflect, the sound, as we find is the case in reflecting the rays of light, where a concave mirror is required. In effect, as often as a sound strikes perpendicularly on a wall, behind which is any thing of a vault or arch, or even another parallel wall, so often will it be reverberated in the same line, or other adjacent ones. For an echo to be heard, therefore, it is necessary that the ear should be in the line of reflection; for the person who made the sound to hear its echo, it is necessary that he should be perpendicular to the place which reflects it; and for a manifold or tautological echo, it is necessary that there should be a number of walls, and vaults or cavities, either placed behind or fronting each other. A single arch or concavity can scarcely ever stop and reflect all the sound; but if there be a convenient disposition behind it, part of the sound propagated thither, being collected and reflected as before, will return another echo; or if there be another concavity, opposed at a due distance to the former, the sound reflected from the one upon the other will be tossed back again by the latter.

Echoes may be produced in circumstances wholly different. For, first, a plane obstacle reflects back the sound in its due tone and loudness, allowance being made for the proportionable decrease of the sound, according to its distance; secondly, a convex obstacle reflects the sound somewhat smaller and somewhat quicker, though weaker, than otherwise it would be; and, thirdly, a concave obstacle echoes back the sound larger, slower, and also inverted, but never according to the order of words. Nor does it seem possible to contrive any single echo that shall invert the sound, and repeat backwards; because, in such a case, the word last spoken, that is, which last occurs to the obstacle, must be repelled first, which cannot be. For where in the mean time should the first words hang and be concealed; or how, after such a pause, could they be revived and animated again into motion? From the determinate concavity or arching of the reflecting bodies, it may happen that some of them shall only echo back one determinate note, and only from one place. Fourthly, the echoing body being removed farther off, it reflects more of the sound than when nearer, which is the reason why some echoes repeat but one syllable, some one word, and some many. Fifthly, echoing bodies may be so contrived and placed, that by reflecting the sound from one to the other, either directly and mutually, or obliquely and by succession, out of one sound, a multiple echo or many echoes shall arise. Add to this, that a multiple echo may be made, by so placing the echoing bodies at unequal distances, that they may reflect all one way, and not one on the other. By this means a manifold successive sound will be heard, one clap of the hands like many, one ha continued like laughter, one single word like many of the same tone and accent, and one viol like many of the same kind imitating each other. Lastly, echoing bodies may be so arranged that from any one given sound they shall produce many echoes different both as to tone and intention. By this means a musical room may be so contrived that one instrument playing therein shall not only seem as many of the same sort and size, but even as a concert of different ones; which is effected by placing certain echoing bodies so that any note placed shall be returned by them in thirds, fifths, and eightths.

Echo is also used for the place where the repetition of the sound is produced or heard.

Echoes are distinguished into different kinds. Single echoes are those which return the voice but once. Of these some are tonical, which only return a voice when modulated into some particular musical tone; and others, polyzyllabical, which return many syllables, words, and sentences. Multiple or tautological echoes are those which return syllables and words the same oftentimes repeated.

In echoes, the place where the speaker stands is called the centrum phonicum, and the object or place that returns the voice the centrum phonocompunctum.

At the sepulchre of Metella, wife of Crassus, there was an echo, which repeated what was said five times. Authors mention a tower at Cyzicus, where the echo repeated seven times. One of the finest echoes we read of is that mentioned by Barthius, in his notes on the Thesais of Statius (lib. vi. 30), which repeated the words uttered seventeen times; it was situated on the banks of the Naha, between Coblentz and Bingen. Barthius assures us that he had proved what he writes, and had counted seventeen repetitions.

Architecture, a term applied to certain kinds of vaults and arches, most commonly of the elliptic and parabolic figures, used to redouble sounds, and produce artificial echoes.

Poetry, a kind of composition in which the last words or syllables of each verse contain some meaning, which, being repeated apart, answers to some question or other matter contained in the verse. Thus, in the line of Virgil:

Crudelis mater magis, an puer improbus ille? Improbus ille puer, crudelis tu quoque mater.

fabulous history, a daughter of the Air and Tellus, who chiefly resided in the vicinity of the Cephisus. She was once an attendant of Juno's, and became the confidant of Jupiter's amours. Her loquacity, however, displeased Jupiter, and she was deprived of the power of speech by Juno, and only permitted to answer to the questions which were put to her. Pan had formerly been one of her admirers, but he never enjoyed her favours. Echo, after she had been punished by Juno, fell in love with Narcissus; but being despised by him, pined away, having nothing left but her disembodied voice.