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.