shing of the great cataracts of the Orinoco in the plain which
surrounds the mission of the Apures. These sounds he regarded as
three times louder during the night than during the day. Some authors
ascribed 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 M. Humboldt justly maintains that this cannot be the cause
of it on the Orinoco, 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 phenomenon to the perfect
transparency and uniform density of the air, which can exist only at
night 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 temperatures,
and consequently of different densities, are constantly ascending from
the ground and mixing with the cold air above. The air thus ceases
to be a homogeneous medium, and every person must have observed the
effects of it upon objects seen through it which are very indistinctly
visible, and have a tremulous motion, as if they were "dancing in
the air." The very same effect is perceived 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 a flame. In all
these cases the light suffers refraction in passing from a medium of
one density into a medium of a different density, and the refracted
rays are constantly changing their direction as the different currents
rise in succession. Analogous effects are produced when sound passes
through a mixed medium, whether it consists of two different mediums
or of one medium where portions of it have different densities. As
sound moves with different velocities through media of different
densities, the wave which produces the sound will be partly reflected
in passing from one medium to the other, and the direction of the
transmitted wave changed; and hence in passing through such media
different portions of the wave will reach the ear at different times,
and thus destroy the sharpness and distinctness of the sound. This
may be proved by many striking facts. If we put a bell in a receiver
containing a mixture of hydrogen gas and atmospheric air, the sound of
the bell can scarcely be heard. During a shower of rain or of snow,
noises are grea