ECHO

, or Eccho, a sound reflected, or reverberated from some body, and thence returned or repeated to the ear.

For an echo to be heard, the ear must be in the line of reflection; that the person who made the sound, may hear the echo, it is necessary he should be in a perpendicular to the place which reflects it; and for a | multiple or ta<*>tological echo, it is necessary there be a number of walls and vaults, rocks, and cavities, either placed behind each other, or fronting each other. Those murmurs in the air, that are occasioned by the discharge of great guns, &c, are a kind of indefinite echoes, and are produced from the vaporous particles suspended in the atmosphere, which resist the undulations of sound, and reverberate them to the ear.

There can be no echo, unless the direct and reflex sounds follow one another at a sufficient distance of time; for if the reflex sound arrive at the ear before the impression of the direct sound ceases, the sound will not be doubled, but only rendered more intense. Now it we allow that 9 or 10 syllables can be pronounced in a second, in order to preserve the sounds articulate and distinct, there should be about the 9th part of a second between the times of their appulse to the ear; or, as sound flies about 1142 feet in a second, the said difference should be 1/9 of 1142, or 127 feet; and therefore every syllable will be reflected to the ear at the distance of about 70 feet from the reflecting body; but as, in the ordinary way of speaking, 3 or 4 syllables only are uttered in a second, the speaker, that he may have the echo returned as soon as they are expressed, should stand about 500 feet from the reflecting body; and so in proportion for any other number of syllables. Mersenne allows for a monosyllable the distance of 69 feet; Morton, 90 feet; for a dissyllable 105 feet, a trisyllable 160 feet, a tetrasyllable 182 feet, and a pentasyllable 204 feet. Nat. Hist. Northampton, cap. 5, pa. 358.

From what has been said, it follows that echoes may be applied for measuring inaccessible distances. Thus, Mr. Derham, standing upon the banks of the Thames, opposite to Woolwich, observed that the echo of a single sound was reflected back from the houses in 3 seconds; consequently the sum of the direct and reflex rays must have been 1142 X 3 = 3426 feet, and the half of it, 1713 feet, the breadth of the river in that place.

It also follows that 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, or one word, and some many. Of these, some are tonical, which only return a voice when modulated into some particular musical tone; and others polysyllabical. That fine echo in Woodstock park, Dr. Plot assures us, in the day-time will return very distinctly 17 syllables, and in the night 20. Nat. Hist. Oxf. cap. 1, pa. 7.

Echoing bodies may be so contrived, and placed, as that reflecting the sound from one to the other, a multiple echo, or many echoes, shall arise.—At Rosneath, near Glasgow, in Scotland, there is an echo that repeats a tune played with a trumpet three times completely and distinctly.—At the sepulchre of Metella, wife of Crassus, there was an echo, which repeated what a man said five times.—Authors mention a tower at Cyzicus, where the echo repeated seven times.— There is an echo at Brussels, that answers 15 times.

One of the finest echoes we read of, is that mentioned by Barthius, in his notes on Statius's Thebais, lib. 6, ver. 30, which repeated the words a man uttered 17 times. This was on the banks of the Naha, between Coblentz and Bingen. And whereas, in common echoes, the repetition is not heard till some time after hearing the words spoken, or the notes sung; in this, the person who speaks, or sings, is scarce heard at all; but the repetition very clearly, and always in surprising varieties; the echo seeming sometimes to approach nearer, and sometimes farther off; sometimes the voice is heard very distinctly, and sometimes scarce at all: one person hears only one voice, and another several; one hears the echo on the right, and the other on the left, &c.

Addison, and other travellers in Italy, mention an echo at Simonetta palace, near Milan, still more extraordinary, returning the sound of a pistol 56 times. The echo is heard behind the house, which has two wings; the pistol is discharged from a window in one of these wings, the sound is returned from a dead wall in the other wing, and heard from a window in the back-front. See Addis. Travels, pa. 32; Misson, Voyag. d'Ital. tom. 2, pà. 196; Philos. Trans. N° 480, pa. 220.

Farther, a multiple echo may be made, by so placing the echoing bodies, at unequal distances, as that they may reflect all one way, and not one on the other; by which means, a manifold successive sound will be heard; one clap of the hands like many; one ha like a laughter; one single word like many of the same tond and accent; and so one musical instrument like many of the same kind, imitating each other.

Lastly, echoing bodies may be so ordered, that from any one sound 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 in it shall seem many of the same sort and size, but even a concert of different ones; this may be contrived by placing certain echoing bodies so, as that any note played, shall be returned by them in 3ds, 5ths, and 8ths.

Echo is also used for the place where the repetition of the sound is produced, or heard. This is either natural or artificial.

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

, in Architecture, is applied to certain vaults and arches, mostly of elliptical or parabolical figures; used to redouble sounds, and produce artificial echoes. —The method of making them is taught by F. Blancani, in his Echometria, at the end of his book on the Sphere.

Vitruvius tells us, that in divers parts of Greece and Italy there were brazen vessels, artfully ranged under the seats of the theatres, to render the sound of the actors' voices more clear, and make a kind of echo; by which means, every one of the prodigious multitude of persons, present at those spectacles, might hear with ease and pleasure.