h. Vary the shape and
size of the cavity until the sound of the fork suddenly increases in
volume, showing that the right adjustment for resonance has been made.
_This intensification of the sound is due to the vibration of the air
in the mouth cavity, together with the sympathetic vibration of the
surrounding walls._

Experiment VII

As an illustration of sympathetic resonance without contact, sing
forcibly a tone that is within easy range, and at the same time
silently hold down the corresponding key of the piano. On ceasing to
sing you will hear the tone sounding in the piano. This may be further
illustrated by playing on the open string of one violin while another,
tuned to the same pitch, rests untouched near by. Through _sympathetic
resonance_ the corresponding string of the second violin will vibrate
and sound its note. The louder the first violin is played the louder
will be the sympathetic tone of the second.

The deep pedal-tones of a church organ often induce sympathetic
resonance that may be felt beneath the feet of the listener. One
writer, a singer, speaks of living in the same house with two
deaf-mutes. He lodged on the first floor, they on the third. One day,
meeting at luncheon, one of the deaf-mutes told the singer that he had
begun practice earlier that morning than usual. Surprised, the writer
asked how he knew. The deaf-mute replied that they always knew when he
was singing because they felt the floor of their room vibrate.

If tone vibrations can be transmitted so readily throughout a house,
it is not difficult to understand how easily the vibrations of bone
and tissue can be transmitted until the whole framework of the body
responds in perceptible vibration.

It is said that Pascal at the age of twelve wrote a dissertation on
acoustics suggested by his childish discovery that when a metal dish
was struck by a knife the resulting sound could be stopped by touching
the vibrating dish with a finger.

With this in mind it is not difficult to understand how compression of
the human instrument by the pressure of tight clothing without, or by
false muscular tension within, must interfere with its free vibration
and so rob the produced tone of just so much of perfection.

From these experiments we can understand that, while the tones of the
voice are initiated by or at the vocal cords, the volume and character
of the tones are dependent upon _resonance_,--the vibration of the air
in the various