or ECCHO, a found reflected or reverberated from a solid, concave, body, and so repeated to the ear. The word is formed from the Greek ηχος, sound, which comes from the verb ηχω, ησον.

The ancients being wholly unacquainted with the true cause of the echo, ascribed it to several causes sufficiently whimsical. The poets, who were not the worst of their philosophers, imagined it to be a person of that name metamorphosed, and that he affected to take up her abode in particular places; for they found by experience, that she was not to be met with in all. (See below, Echo in fabulous history). But the moderns, who know found 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, it is evident, may be repelled without destroying or diminishing its tremor; and consequently a found may be redoubled by the refilition of the tremulous body, or air.

But a simple reflection of the sonorous air is not enough to solve the echo: for then every plain surface of a solid hard body, as being fit to reflect a voice or found, would redouble it, which we find does not hold.

To produce an echo, therefore, it should seem that a kind of concameration or vaulting were necessary, in order to collect, and by collecting to heighten and increase, and afterwards reflect, the found; as we find is the case in reflecting the rays of light, where a concave mirror is required.

In effect, as often as a found 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 the ear be in the line of reflection: for the person who made the found to hear its echo, it is necessary he be perpendicular to the place which reflects it: and for a manifold or tautological echo, it is necessary there be a number of walls, and vaults or cavities, either placed behind or fronting each other.

A single arch or concavity, &c. can scarce ever stop and reflect all the found; but if there be a convenient disposition behind it, part of the found propagated thither, being collected and reflected as before, will present another echo: or, if there be another concavity, opposed at a due distance to the former, the found reflected from the one upon the other will be tossed back again by this latter, &c.

Many of the phenomena of echoes are well considered by the bishop of Leighs, &c. who remarks, that any found, falling either directly or obliquely on any dense body of a smooth, whether plain or arched, superficies, is reflected, or echoes, more or less. The surface, says he, must be smooth; otherwise the air, by reverberation, will be put out of its regular motion, and the found thereby broken and extinguished. He adds, that it echoes more or less, to show, that when all things are as before described, there is still an echoing, though it be not always heard, either because the direct found is too weak to beat quite back again to him that made it; or that it does return to him, but so weak, that it cannot be discerned; or that he stands in a wrong place to receive the reflected found, which passes over his head, under his feet, or on one side of him; and which therefore may be heard by a man standing in the place where the reflected found does come, provided no interposed body intercepts it, but not by him that first made it.

Echoes may be produced with different circumstances. For, 1. A plane obstacle reflects the found back in its due tone and loudness; allowance being made for the proportionable decrease of the found, according to its distance.

2. A convex obstacle reflects the found somewhat smaller and somewhat quicker, though weaker, than otherwise it would be.

3. A concave obstacle echoes back the found, bigger, 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 found, and repeat backwards; because, in such case, the word last spoken, that is, which last occurs to the obstacle, must be repeated first; which cannot be. For where in the mean time should the first words hang and be concealed; or how, after such a pause, be revived, and animated again into motion?

From the determinate concavity or archness of the reflecting bodies, it may happen that some of them shall only echo back one determinate note, and only from one place.

4. The echoing body being removed farther off, it reflects more of the found than when nearer; which is the reason why some echoes repeat but one syllable, some one word, and some many.

5. Echoing bodies may be so contrived and placed, as that reflecting the found from one to the other, either directly and mutually, or obliquely and by succession, out of one found, a multiple echo or many echoes shall arise.

Add, 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 which means, a manifold successive found will be heard; one clap of the hands, like many; one ha, like a laughter; one single word, like many of the same tone and accent; and so one viol, like many of the same kind, imitating each other.

Lastly, Echoing bodies may be so ordered, that from any one found given, they shall produce many echoes different both as to tone and intension. By which means a musical room may be so contrived, that not only one instrument playing therein shall seem many of the same fort and fize, but even a concert of different ones, only by placing certain echoing bodies so, that any note placed shall be returned by them in 3ds, 5ths, and 8ths.

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

Echoes are distinguished into divers kinds, viz. 1. Single, which return the voice but once. Whereof some are tonical, which only return a voice when modulated into some particular musical tone: Others, polysyllabical, which return many syllables, words, and sentences. Of this last kind is that fine echo in Woodstock park, which Dr Plot affures us, in the day time, will return very distinctly seventeen syllables, and in the night twenty.

2. Multiple or tautological; 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 phonoamplicum.

At the sepulchre of Metella, wife of Craffus, was an echo, which repeated what a man 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 Statius's Thebais, lib. vi. 30. which repeated the words a man uttered 17 times: it was on the banks of the Naha, between Coblenz and Bingen. Barthius affures us, he had proved what he writes; and had told 17 repetitions. And whereas, in common echoes, the repetition is not heard till some time after hearing the word spoke, or the notes sung; in this, the person who speaks or sings is scarce heard at all; but the repetition most clearly, and always in surprising varieties; the echo seeming sometimes to approach nearer, and sometimes to be further off. Sometimes the voice is heard very distinctly, and sometimes scarce at all. One hears only one voice, and another several; one hears the echo on the right, and another on the left, &c. At Milan in Italy, is an echo which reiterates the report of a pistol 56 times; and if the report is very loud, upwards of 60 reiterations may be counted. The first 20 echoes are pretty distinct; but as the noise seems to fly away, and answer at a greater distance, the reiterations are so doubled, that they can scarce be counted. See an account of a remarkable echo under the article Paisley.

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 wherein 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; as in this beautiful one from Virgil:

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

The elegance of an echo consists in giving a new sense to the last words; which reverberate, as it were, the motions of the mind, and by that means affect it with surprise and admiration.

fabulous history, a daughter of the Air and Tellus, who chiefly resided in the vicinity of the Cephisus. She was once one of Juno's attendants, 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 herself to death, having nothing but her voice left.