152 INTERFERENCE OF SOUND. [VIL § 76.
only because we are accustomed to tbink of Sound as
something with an external objective existence ; not
as consisting merely in a state of motion of certain
air-particles, and therefore liable, on the application
of an opposite system of equal forces, to be absolutely
annihilated.
A single tuning-fork presents an example of this
very important phenomenon. Each prong sets up
vibrations corresponding to a simple tone, and the
two notes so produced are of the same pitch and
intensity. If the fork, after being struck, is held
between the finger and thumb, and made to re¬
volve slowly about its own axis, four positions of
the fork with reference to the ear will be found
where the tone completely goes out. These posi¬
tions are mid-way between the four in which the
faces of the prongs are held flat before the ear.
As the fork revolves from one of these positions
of loud tone to that at right-angles to it, the
sound gradually wanes, is extinguished in passing
the Interference-position, reappears very feebly im¬
mediately afterwards, and then continues to gain
strength until its quarter of a revolution has been
completed.
7 6. The case of coexistent unisons has now been
adequately examined : we proceed to enquire what
happens when two simple tones differing slightly in