A whispering gallery is usually a circular, hemispherical,
elliptical or ellipsoidal enclosure, often beneath a dome or a
vault, in which whispers can be heard clearly in other parts of
the gallery. Such galleries can also be set up using two
parabolic dishes. Sometimes the phenomenon is detected in caves.
A whispering gallery is most simply constructed in the form of a
circular wall, and allows whispered communication from any part
of the internal side of the circumference to any other part. The
sound is carried by waves, known as whispering-gallery waves,
that travel around the circumference clinging to the walls, an
effect that was discovered in the whispering gallery of St
Paul's Cathedral in London. The extent to which the sound
travels at St Paul's can also be judged by clapping in the
gallery, which produces four echoes. Other historical examples
are the Gol Gumbaz mausoleum in Bijapur and the Echo Wall of the
Temple of Heaven in Beijing. A hemispherical enclosure will also
guide whispering gallery wav