novelty and interest stops you, and prevents your
going on. Like the trees, seen for the first time by
the travellers from St. Kilda, their beautiful leaves
and branches pull you back to contemplate them,
when you would otherwise be advancing along
your road. To his other accomplishments Dr.
Tyndall adds the great advantages of foreign
travel. As in Heat, he clears up a doubt or
establishes a fact by experience obtained in distant
lands. Thus, falling snow has often been referred
to, as offering a great hindrance to the passage
of sound; but it appears to be less obstructive
than is usually supposed. Sound seems to
make its way freely between the falling flakes.
On the 29th of December, 1859, Dr. Tyndall
traced a line across the Mer de Glace of Chamouni,
at an elevation of nearly seven thousand
feet above the sea. The glacier there is
half a mile wide, and during the setting out of
the line snow fell heavily. He has never seen
the atmosphere in England so thickly laden.
Still he was able to see through the storm quite
across the glacier, and also to make his voice
heard. When close to the opposite side, one
of the assistants chanced to impede his view.
The professor called out to him to stand aside,
and he did so immediately. At the end of the
line the men shouted, "We have finished," and
their voices were distinctly heard through the
half-mile of falling snow.

In the lecture-room Dr. Tyndall is as bold
and adventurous as he has proved himself upon
the mountain. The motion of sound, we are
informed, like all other motion, is enfeebled by
its transference from a light body to a heavy
one—and this is illustrated by the action of
hydrogen gas upon the voice. The voice is
formed by urging air from the lungs through an
organ celled the larynx. In its passage it is
thrown into vibration by the vocal chords,
which thus generate sound. "But when I fill
my lungs with hydrogen," says the professor,
"and endeavour to speak, the vocal chords
impart their motion to the hydrogen, which
transfers it to the outer air. By this transference
from a light gas to a heavy one, the sound is
weakened in a remarkable degree. The
consequence is very curious." You have already
formed a notion of the strength and quality of
my voice. I now empty my lungs of air, and
inflate them with hydrogen from this gasholder.
I try to speak vigorously; but my voice has
lost wonderfully in power, and changed
wonderfully in quality. You hear it, hollow, harsh,
and unearthly: I cannot otherwise describe it."

MOTION appears to be the basis of all sensation,
and consequently of all consciousness of
life. What the nerves convey to the brain, we
have the strongest reason for believing, is in all
cases motion. Motion communicated to the
ear by any cause, and imparted to the auditory
nerve, or the nerve of hearing, is translated by
the brain into the sensation of sound. According
to this idea, all that goes on outside of
ourselves is reducible to pure mechanics; if we
hear one sound louder than another, it is because
our nerves are hit harder in the one case
than in the other.

The motion transmitted by the nerves to the
brain is not meant the motion of each nerve as
a whole, but the vibration or tremor of its
molecules, or smallest particles. What we call
silence is, therefore, the absence of all vibratory
motion in the air, and, consequently, of any
corresponding pulse in our auditory nerve. The
rapidity with which an impression is transmitted
through the nerves, as first determined by
Helmholz, and confirmed by Du Bois Raymond,
is ninety-three feet in a second. A giant, therefore,
say one hundred feet high, would not feel
a thorn in his foot until one second after it had
pricked him. Were you to put salt on the tail
of a sea-serpent eighteen hundred and sixty
yards long, it would not be aware of your
familiarity until a whole minute afterwards.

In air at the temperature of freezing water,
the vibratory pulse which constitutes sound
travels at the rate of one thousand and ninety
feet a second. Again, and as in the case of the
nerves, the motion of the pulse of air must not
be confounded with the motion of the particles
of air which at any moment constitute the pulse.
For while the wave moves forward through
considerable distances, each particular particle of
air makes only a small excursion to and fro.

That sound is really the consequence of waves
in the air, or, in other words, that air is necessary
to the propagation of sound, is proved by
causing a bell to ring in a vacuum, that is,
under the exhausted receiver of an air-pump.
When the air is gone, the sound of the bell
ceases to be heard. Dr. Tyndall renders the
experiment still more striking by first exhausting
the receiver of its atmospheric air as far as
possible, and then allowing hydrogen gas, which
is fourteen times lighter than air, to enter the
vessel. The sound of the bell is not sensibly
augmented by the presence of this attenuated
gas, even when the receiver is full of it. By
again working the pump, the atmosphere
surrounding the bell is rendered still more
attenuated, and a vacuum much more perfect
than the previous one is obtained. This is of
great importance, for it is the getting rid of
the last traces of air which chiefly cause the
experiment to be so extremely effective. However
hard the hammer may pound the bell, no sound
will now be heard. An ear placed close to the
exhausted receiver is unable to perceive the
faintest tinkle. Note that the bell must be
suspended by strings; for if it were allowed to
rest upon the plate of the air-pump, the vibrations
would communicate themselves to the
plate, and be transmitted to the air outside.

On permitting air gradually to re-enter the
jar, a feeble sound is immediately heard, which
grows louder as the air becomes more dense, until
the ringing of the bell is again distinctly heard.

At great elevations in the atmosphere (where
the air is much rarer) sound is sensibly
diminished in loudness. De Saussure thought
the explosion of a pistol at the summit of Mont
Blanc to be about equal to that of a common
cracker below. Dr. Tyndall has several times
repeated the experiment. What struck him
was the absence of that density and sharpness