ECHO, from the Greek word s:zo;, a sound. In the article Aeousr les, we have already treated the subject of echoes, as connected with the general doctrines of sound, having reserved for the present article, brief no tices of some of the most celebrated echoes.
Dr Plot, in his Natural History of Oxfordshire, men tions an echo in Woodstock Park, in ()x tot &hire, which repeats 17 syllables in the day-time, and 20 in the night. This effect he ascribes to the superior density of the air during the night, which diminishes the velocity or the sound.
Harris describes an echo on the north side of Shipley church, in Sussex, as repeating 21 syllables distinctly, under favourable circumstances.
Dr Birch informs us, that there was an echo at Rose neath, in Argyllshire, in Scotland, which distinctly re peated, three times, a tune played Oh a trumpet. When a person, placed at a proper distance, plays eight or ten notes, they arc correctly repeated, but a third lower after a short silence, another repetition is heard, in a still lower tone; and, after another short interval. there is a third repetition, in a still lower tone.
Addison describes an echo at the palace of Simonetta, near Milan, as returning the sound of a pistol 56 times.
The palace has two wings ; and, when a pistol is fired from a window in one of the wings, the sound is reflect ed from a dead wall in the other wing, and is heard from a window in the back front. The following account of it, however, from Keyslcr, is more minute and interest ing.
" At the Marquis Simonetta's villa is a very extraordi nary echo; it is occasioned by the reflection of the voice between the opposite parallel wings of the building, which are fifty-eight common paces from each other, and without any windows or doors, by which the sound might be dissipated or lost. The repetition of the sound dwells chiefly on the last syllable, which might have been altered by allowing a greater distance between the two wings ; but possibly it was apprehended, that the num bet of the repetitions would be diminished by that means. Two or more bodies placed opposite each other, at dif ferent distances, are requisite to form a multiplied echo ; or the wall at which the speaker stands, must have ano ther wall opposite to it, so as to form two parallel planes, which will alternately reflect to each other the sound communicated to them, with as little dissipation as pos sible. This last circumstance is found in the two paral
lel wings of this seat, which, forming right angles with the main body of the building, have a very surprising effect. A man's voice is repeated above forty times, and the report of a pistol above sixty, by this echo : but the repetition is so quick, that it is difficult to tell them, or even to mark them down, unless it be early in the morning, or in a calm still evening: when the air is ra ther too moist or too dry, the effect is found not to an swer so ve 1 I." Southwell mentions a building, similar to the palace of Simonetta, which had projecting wings, and produced 60 repetitions of every sound.
The Abbe Guynet describes an echo on the road from Rochepot to Chalons, which repeats, in the day-time, 14 syllables well articulated, and, during the night, 16 syllables.
About three leagues from Verdun, there is a singular echo, occasioned by two towers projecting from the body of a house, and distant 26 toises, or about 50 metres. When a person stands in the line between the two towers, and pronounces a word in a pretty high tone, he will hear it repeated 12 or 13 times at equal intervals, and always more feebly. If he places himself at a certain distance out of this line, the echo is no longer heard. One of these towers has a low apartment vaulted with hewn stone, while the other has its vestibule vaulted. See Hist. Acad. Par. 1710.
In the Memoirs of the French Academy of Sciences for 1792, there is described a curious echo, at Genefay, in the neighbourhood of Rouen. A person who sings, does not hear the repetition of the echo, but merely his own voice ; whilst those who,listen hear only the repe tition of the echo, though with singular variations. Some times the echo seems to approach, and at other times to recede. One person hears a single voice, and another several voices : one person hears the echo on the right, and another on the left; the echo always varying with the position of the person who hears it.