Year: 1878




The great audibility of sounds during
the night is a phenomenon of consider,
able interest, and one which had been
observed even by the ancients. In
crowded cities, or in their vicinity, the
effect was generally ascribed to the rest
of animated beings, while in localities
where such an explanation was inapph-
cable, 1t was supposed to arise from a IA
yorable direction of the prevailing wind.
Baron humboldt was particularly struck
with the phenomenon when he first
heard the rushing of the great cataracts
of the OrOnocO in the plain which sur
rounds the mission of the Apures.
These sounds he regarded as three times
louder during the night than during the
day. Some authors ascribe this fact to
the cessation of the humming Of insects,
the singing of birds, and the action of
the wind on the leaves of the trees; but
NJ. Humboldt Justly maintains that this
cannot be the cause of it in the OronOcO,
where the buzz of insects is much louder
in the night than in the day, and where
the breeze never rises till after sunset.
Hence he was led to ascribe the phe-
nOmenOn to the perfect transparency
and uniform density of the air, which
can exist only atnight, after the heat of
The ground has been uniformly diffused
through the atmosphere.


When the rays of the sun have been
beating on the ground during the day,
currents of hot air of different tempera-
lures, and consequently of different dem
sites, are constantly ascending from the
ground, and mixing with the cold air
above. The air thus ceases to be a hOm-
OgeneOus medium, and every person
must have observed the effect Of it upoE
objects seen through it, which are very
indistinctly visible, and have a tremU-
lous motion, as if they were dancing in
the air" The very same effect is per-
ceived when we look at objects through
spirits and water that are not perfectly
mixed or when we view distant objects
over a red-hOt poker or over flame.


As sound moves with different veloci-
ties through media of different densi-
ties, the wave which produces the sound
will be partly reflected In passing from
one medium to the other, and the dlrec-
ton of the transmitted wave changed;
and hence in passing through such me-
dia, different portions Of the wave will
reach the ear at different times, and thus
destroy the sharpness and distinctness
IF the sound. This may be proved by
many olllh44lS 4cOtos Il VIC Bu1 o IOAA Ii
Il receiver containing a mixture of hy-
drogen gas and atmospheric air, the
sound of the bell can scarcely be heard.


During a shower Of rain or OF snow,
noises are greatly deadened, and when
sound is transmitted along an iron wire
or in an iron pipe Of sufficient length,
we actually hear two sounds-one trans
mitted more rapidly through the sOlids
and the other more slowly through the
air. The same property is well illustra-
ted by an elegant and easily-repeated
experiment of Chladni's. When spark
ling champagne is poured into tab
glass till it is half full, the glass loses its
power of ringing by a stroke upon its
edge, and emits only a disagreeable and
puffy sound. This effect will continue
while the wine is flied with bubbles Of .
air, or as long as the efeTvescence lasts;
but when the effervescence begins to
subside, the sound becomes clearer and
clearer, and the glass rings as usual
when the air-bubbles have vanished If
we reproduce the efferyescence by stir
ring the champagne with a piece Of
bread, the glass will again cease to ring.
The same experiment Will succeed with
other efervescing fluids.