THE SCIENCE OF MUSICAL SOUNDS
of rods, longitudinal vibrations of rods, plates, organ pipes,
membranes, and strings. Tuning forks, when properly con¬
structed and used, proved to be the most suitable source of
high tones. Koenig made his first experiments in 1874 when
he was forty-one years old and at that time was able to hear
tones up to F9# — 23,000, which he considered the highest
directly audible simple tone ; he constructed a set of forks up
to F9 == 21,845, which he exhibited at the Centennial Expo-
sition in Philadelphia in 1876. (These forks and much other
Fig. 38. Steel bars for testing the highest audible frequency of vibration.
interesting acoustic apparatus exhibited by Koenig are now
in the laboratory of Toronto University.) In his fifty-
seventh year the limit of audibility for Koenig was E9 =
20,480, and in his sixty-seventh year it was D9# = 18,432.
A set of Koenig forks for tones of high pitch is shown in Fig.
39 ; Koenig has made a complete series of such forks extend¬
ing more than two octaves above the limit of audibility to a
frequency of 90,000 complete vibrations (180,000 motions
to and fro) per second. Sounds which are inaudible are
made evident by cork-dust figures in a tube, Fig. 39; the
stationary air waves produced by the vibration of the fork
at the end of the tube cause the cork dust to accumulate in
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