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, 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.