XLV
251. Experience teaches us that in one second we cannot
hear more than nine sounds, hence we can only distinguish sounds
from each other which succeed at intervals of 1/9 second, a second
sound reaching the ear before 1/9 second has elapsed coincides with
the first sound.
An echo can only be produced, if the reflecting object is at a
distance of at least 61 ft.
If there are many reflecting walls, a poly-syllabic echo will be
produced.
If the sound-waves strike a reflecting wall in an oblique direc
tion, the echo will be heard not at the place of its origin but some
where else.
How is it possible to hear an echo on the sea? How can a
very strong sound (f.e. the discharge of a cannon) cause an echo
in places where under ordinary circumstance an echo is never
heard?
§ 106. The use of dates for transmission of sound.
a) The speaking-trumpet. 252.
b) The ear-trumpet, an inverted speaking-trumpet.
c) The stethoscope, an instrument similar to the speaking
trumpet. One end is held against the diseased part of the body
and the ear against the other end. By means of this a physi
cian may detect anything abnormal in the chest or elsewhere.
Our outer ear represents also a sound-trumpet. In some
animals this part of the ear is long and flexible, so that the animal
can adjust it in compliance with the direction from which the
sound comes. For the other parts of the ear see: 254.
§ 107. Solid bodies do not only reflect sound-waves,
they may also be put into a state of vibration and impart these
vibrations to the air on the other side (music, conversation etc.
heard through the walls of a room). The thinner and more elastic
