SOUNDS AND ECHOES.
Sound is propagated successively from the sounding body to the places which are nearest to it, then to those more distant, &c. Every observer knows that when a gun is fired at a considerable distance from him, he perceives the flash a certain time before he hears the report; and the same thing is true with respect to the stroke of a hammer, or of a hatchet, the fall of a stone, or, in short, any visible action which produces a sound or sounds. In general, sound travels through the air at the rate of eleven hundred and forty-two feet in a second, or about thirteen miles in a minute. This is the case with all kinds of sounds; the softest whisper flying as fast as the loudest thunder. Sound, like light, after it has been reflected from several places, may be collected into one point as a focus, where it will be more audible than in any other part; and on this principle whispering galleries are constructed. The particulars relative to the celebrated whispering gallery in the dome of St. Paul’s church, London, will be comprehended in the description of that noble edifice.
An echo is the reflection of sound striking against a surface adapted to the purpose, as the side of a house, a brick wall, hill, &c., and returning back again to the ear, at distinct intervals of time. If a person stand about sixty-five or seventy feet from such a surface[a surface], and perpendicularly to it, and speak, the sound will strike against the wall, and be reflected back, so that he will hear it as it goes to the wall, and again on its return. If a bell situated in the same way be struck, and an observer stand between the bell and the reflecting surface, he will hear the sound going to the wall, and also on its return. Lastly, if the sound strike the wall obliquely, it will go off obliquely, so that a person who stands in a direct line between the bell and the wall will not hear the echo. According to the greater or less distance from the speaker, a reflecting object will return the echo of several, or of fewer syllables; for all the syllables must be uttered before the echo of the first syllable reaches the ear, to prevent the confusion which would otherwise ensue. In a moderate way of speaking, about three and a half syllables are pronounced in one second, or seven syllables in two seconds: therefore, when an echo repeats seven syllables, the reflecting object is eleven hundred and forty-two feet distant; for sound travels at the rate of eleven hundred and forty-two feet per second, and the distance from the speaker to the reflecting object, and again from the latter to the former, is twice eleven hundred and forty-two feet. When the echo returns fourteen syllables, the reflecting object must be twenty-two hundred and eighty-four feet distant, and so on.
The most remarkable echo recorded, is at the palace of a nobleman, within two miles of Milan, in Italy. The building is of some length in front, and has two wings jutting forward; so that it wants only one side of an oblong figure. About one hundred paces before the mansion, a small brook glides gently; and over this brook is a bridge forming a communication between the mansion and the garden. A pistol having been fired at this spot, fifty-six reiterations of the report were heard. The first twenty were distinct; but in proportion as the sound died away, and was answered at a greater distance, the repetitions were so doubled that they could scarcely be counted, the principal sound appearing to be saluted in its passage by reports on either side at the same time. A pistol of a larger caliber having been afterward discharged, and consequently with a louder report, sixty distinct reiterations were counted. From this example it follows, that the further the reflecting surface is, the greater number of syllables the echo will repeat; but that the sound will be enfeebled nearly in the same proportion, until at length the syllables can not be distinctly heard. On the other hand, when the reflecting object is too near, the repetition of the sound reaches the ear, whilst the perception of the original sound still continues, in which case an indistinct resounding is heard, as may be observed in empty rooms, passages, &c. In such places, several reflections from the walls to the hearer, as also from one wall to the other, and then to the hearer, clash with each other, and increase the indistinctness of the sound.