Echoes are sounds reflected by the objects on which they strike.
742. Why do some echoes occur immediately after a sound?
Because the reflecting surface is very near; therefore the sound returns immediately.
743. Why do some echoes occur a considerable time after a sound?
Because they are at a considerable distance, and the sound takes time to travel to it, and an equal time to return.
744. Why do some echoes change the tone and quality of sound?
Because the reflecting surface, having vibratory qualities of its own, mingles its own vibrations with that of the sound.