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Echo, a phenomenon which is observed when a train of sound-waves strikes a large and fairly smooth body, such as a hillside or the side of a building.  The phenomenon is identical in principle with that which occurs when water-waves strike a solid pier and are reflected, or with the reflection which occurs at a fret when waves travel along a guitar-string.  Newton employed the echo in a cloister at Trinity College, Cambridge, to measure the speed of sound.  Standing at one end of the cloister, he started a group of waves by stamping his foot.  These waves were reflected at the far end of the cloister.  The interval between the stamping of the foot and the hearing of the echo measures the time necessary for a sound-wave to make one round-trip of the cloister.  From this interval and the length of the cloister the speed of sound is directly computed.  The most interesting examples of echoes are, possibly, those which occur in the whispering-galleries, such as that in the dome of the Capital at Washington or in the dome of St. Paul’s in London.  For further details see Tyndall on Sound.