in the sound which characterise it at lower
elevations. The pistol-shot resembled the
explosion of a champagne-bottle, but it was still
loud. The withdrawal of half an atmosphere
does not very materially affect a ringing bell, and
air of the density found at the top of Mont Blanc
is still capable of powerfully affecting the auditory
nerve. That highly attenuated air is able
to convey sound of great intensity is forcibly
illustrated by the explosion of meteorites at great
elevations above the earth. Here, however, the
initial disturbance must be exceedingly violent.

It is clear, then, beyond a doubt that sound
is conveyed from particle to particle through
the air. The particles which fill the cavity of
the ear are finally driven against the tympanic
membrane, which is stretched across the passage
leading to the brain. This membrane, which
closes the "drum" of the ear, is thrown into
vibration, its motion is transmitted to the ends
of the auditory nerve, and afterwards along the
nerve to the brain, where the vibrations are
received as sound. How it is that the motion of the
nervous matter can thus excite the consciousness
of sound is a mystery which we cannot fathom.

The fact that sound is caused by waves or
undulations in the air, while light and radiant
heat are the result of undulations in the
luminiferous ether, gives rise to frequent analogies
and correspondences between the phenomena
presented by each. Thus both are propagated
in time; that is, neither are instantaneous. It
takes time for the light of a star to reach us, as
it takes time for the noise of a lightning-flash to
make itself heard as a thunder-clap. The difference
of their speed does not affect their mutual
relations and resemblances, although light
travels nearly two hundred thousand miles while
sound is traversing one thousand and ninety
feet.

Light and radiant heat, like sound, are wave
motions. Like sound, they diffuse themselves
in open space, diminishing in intensity according
to the same law. Like sound also, light
and radiant heat, when sent through a tube
with a reflecting interior surface, may be
conveyed to great distances with comparatively
little loss. The celebrated French philosopher,
Biot, observed the transmission of sound through
the empty water-pipes of Paris, and found that
he could hold a conversation in a low voice
through an iron tube three thousand one
hundred and twenty feet in length. The lowest
possible whisper, indeed, could be heard at this
distance, while the firing of a pistol at one end of
the tube quenched a lighted candle at the other.

As light may be extinguished by light, so
sound may be destroyed by sound. But, to
confine ourselves to simpler phenomena, every
experiment on the reflection of light has its
analogue in the reflection of sound. We put
parabolic reflectors behind our lighthouse lamps,
to throw their rays to a greater distance. It is
recorded that a bell placed on an eminence in
Heligoland failed, on account of its distance, to
be heard in the town. A parabolic reflector
placed behind the bell so as to reflect the sound-
waves in the direction of the long sloping street,
caused the strokes of the bell to be distinctly
heard at all times.

Curved roofs, ceilings, and walls act as
mirrors upon sound. In Dr. Tyndall's laboratory,
the singing of a kettle seems, in certain
positions, to come, not from the fire on which
it is placed, but from the ceiling. The acoustic
properties of buildings, depending on their
mode of construction, vary greatly; success
seems often to have been left to chance. In
some you hear admirably in every part; of this
the late Queen's Theatre was a remarkable
instance. In others you hear imperfectly throughout,
or are stunned and bewildered with reverberated
sounds at certain points. Some public
buildings, in which intelligible speech is
impossible when empty, allow distinct utterance to
be heard when full. An assembled audience,
like the furniture of a large room, damps the
confused reflexion of sounds from voices or
musical instruments—an additional reason for
theatrical managers liking to see full houses.
Handel, therefore, was more of a stoic than a
natural philosopher when he consoled himself
for empty benches by saying, "We shall hear
de moosick all de petter."

If, instead of a confused reflexion of sounds
from short distances, as occurs in large
unfurnished rooms, there exists a sufficient interval
between a direct and a reflected sound, we hear
the latter as an echo. It is the interval, the
distinct repetition, which constitutes the veritable
echo. As the reflected sound moves with
the same velocity as the direct sound, in air at
the temperature of thirty-two degrees Fahrenheit,
the echo of a pistol-shot from the face of a
cliff distant one thousand and ninety feet is
heard two seconds after the explosion. The
singular effects of natural echoes have given
rise to sundry legends, pleasantries, and imitations.
In bygone ages, Echo was a nymph who
pined away for love until nothing remained of
her except her voice. The famous Irish echo
was so perfect, that if you asked it "How do
you do?" it replied, "Very well, I thank you."
A French actor vaunted the echo at his
country box—an echo whom he had formed by
careful training. When the rehearsals
promised a satisfactory result, he invited a party
to hear the phenomenon. Leading his friends
to the magic spot in the garden, he shouted,
"Are you ready, Echo?" to which, instead of
"Ready, Echo?" the stupid answer was "Yes;
you may begin as soon as you please."

Sound, like light, may be reflected several
times in succession; and as the reflected light
under these circumstances becomes gradually
feebler to the eye, so the successive echoes
become gradually feebler to the ear. In
mountainous districts, this repetition and decav of
sound produces effects unimaginable by dwellers
on the plain. Childe Harold's description of
the thunderstorm amongst the Alps will recur
to every reader. The writer will never forget
the long-continued reverberations of cannon
fired on a steam-boat in the Lake of Lugano.
In Switzerland generally the wonderful echoes
form part of the stock-in-trade of that attractive