Sound, therefore, when once made, is conveyed
by the agency of the air. But how is sound
originally made?

Everything which causes air to vibrate or
enter into tremulous motion, is an originator of
sound. Many instances are quite familiar; the
vibration of a metal spring, as of a tuning-fork
or a tooth in a musical box, or of a gnat's wing,
communicate their own vibrations to the air
with which they are in contact, and so cause
sound. The same is the case with a harp-string
pulled by the finger, and a pianoforte-string
struck by the hammer. Bells and glasses
produce their sounds solely on account of the
vibration excited in them by a blow. As soon
as the vibration is stopped, the sound ceases.
All doubt about the fact is removed by many of
these vibrations, as in a harp-string, being
visible to the eye; and many, which are not
visible, may be felt with the finger. The deep
bass pedal notes of the organ often cause the
whole person of the hearer to tremble.

There are several ways of rendering the
tremulous motion of sounding bodies sensible
to the eye. If the body, still giving out its
sound, be applied to the surface of water or
mercury, at the instant of contact the liquid is
violently agitated. If a little ball of cork or
ivory, suspended by a thread, is made to touch
it, the ball oscillates to and fro, as if struck by
a succession of blows. If its surface be covered
with a pinch of fine sand, the sand immediately
rebounds and dances, affording visible proof
that the sonorous body itself is not at rest.
Chladni, in 1785, was the first to study sonorous
vibrations through the movements of sand and
fine dust, and his ingenious idea led to several
curious acoustic discoveries.

The organ, just mentioned, leads us to the
case of sound produced by the vibrations of air
impelled by air and not by any solid body.
Thunder is the crash of meeting surfaces of air
that had been torn asunder by the passage of
lightning. The noise from the explosion of a
barrel of gunpowder is caused by the blow given
by air, that had been suddenly parted and opened,
suddenly rushing together again. The bullet of
a rifle produces only the whistling; the air
dislodged and closing again by igniting the charge
gives the report. Wind instruments— flutes,
fifes, flageolets, trombones, trumpets, and the
rest of them— emit their notes in consequence
of air applied outside setting the air within them
a-vibrating.

Bodies which strike the air directly also excite
in it sonorous vibrations. Thus the air cracks
under the impulse of a whip whose lash smites
it with sudden violence; it whistles when swiftly
divided by a cane or a musket-ball; it roars
when parted by very rapid revolutions of a
wheel or a weight at the end of a string, as in
the boy's toy called a "bull-roarer." Similarly,
when the air itself strikes any solid body with
sufficient force, sound is the consequence. We
hear nothing of the kind in a calm; we scarcely
hear a gentle breeze: but in a gale at sea we
hear the wind whistling among the ship's rigging
only too plainly. On land, the howling of the
hurricane, as it clashes past trees and buildings,
overpowers and sweeps away all other sound,
and cannot be heard without fear and trembling.

All noise, then, whether musical or other
sound, results from vibrations communicated to
the air, and which, travelling on till they reach
the tympanum of the human ear, so become
sensible to us. The roar of the carriages in
Fleet-street, the howlings of an excited mob, the
uproar of the storm, the murmur of the brook,
and the whispers of the forest, are all owing to
agitated air. We might indeed say that, with
out life, there is no sound; because, as we take
it, the atmosphere, the sea, the earth, this whole
worldly frame, are alive, are living things,
animated by innate forces. We may safely state
that, without motion, there is no sound.

In like manner, the sound of blows is
consequent on the aerial vibrations to which they
give rise, whether it be the anvil ringing under
the hammer, the drumstick rolling on the
tightened sheepskin, the cudgel on the donkey
that will not go, or the rod that smiteth the
back of the fool. They are all instruments of
percussion, quite as much as cymbals, triangles,
or castanets. All solid bodies whose structure
is such that a movement of vibration impressed
on some of their particles is transmissible
throughout their mass, are at the same time
capable of transmitting sound. A singular
illustrative experiment, which a philosopher will
not disdain, to repeat before children, is to apply
the ear to one end of a long beam and listen to
the taps given with the head of a pin, by way of
drumstick, at its other end. These may be heard
distinctly, whilst the same slight blows, if
applied to one side of the beam, are scarcely
audible across it. The difference arises from
the sound's following, in the first instance, the
longitudinal direction of the woody fibres, in
which the continuity of particles is more
perfect than in the transverse direction; and it is
remarkable that those particles should have
sufficient spring and elasticity to allow the sound
to travel so far with so slight a loss of strength.

Advantage has been taken of the sonority of
wood, to compose, out of bits of stick, a very
pleasing musical instrument, to which we wish to
call attention. We have discovered noise as
the offspring of thumps, but we had not yet
arrived at music. To make music, sound must
have a certain pitch, must form a note whose
corresponding sound is to be found in the
musical scale audible to the human ear. Pitch
implies a sound which we can imitate and
produce the like with our own voice, when
within its compass, or at least the octave to it,
if too high or too low.

We say audible to the human ear; for no
doubt there are sounds, inaudible to us, which
are audible to other living creatures. The
extent of the audible scale with men, varies
somewhat in individuals. The writer can hear
high notes of insects which cannot be heard by
some of his friends; whilst he cannot hear,
although he can feel, the lowest notes of