When we touch a bell while ringing, why does the sound stop?
Because we stop its vibrations when we touch it.
Why does a crack in a bell check its ringing?
Because the crack stops the vibrations in the metal.
What other substances ring when struck?
All vessels made of glass, earth, stone, iron, etc., ring when struck.
How may we detect a crack in any such vessel?
By striking the vessel; if it is cracked there will be no ringing sound.
Trumpets and Speaking-Tubes.
How do the waves of sound move?