Echo, in Acoustics, is a reflection of sound. Sound is a wave-motion through some sort of medium capable of transmitting it; when the wave reaches a surface that either partially or wholly resists farther motion onwards, it is partially or wholly reflected back through the original medium. The wave of sound may affect the ear of an observer both before and after reflection, and so produce the effect of two distinct sounds. If the source of sound be close to him, the interval that elapses before the echo reaches him will represent the time taken for the wave to travel to the reflecting surface and back again. Taking the speed of sound as 1,100 feet per second, a cliff 55 feet away from the source of the wave-motion would reflect it back to the same place in 1/10th of a second, because in that time 110 feet could be traversed by the wave. Such an interval as this is just distinguishable by an ordinary ear; but for the echo to be more distinctly isolated greater distances than 55 feet are necessary. For the echo to be as loud as possible the reflecting surface must be so shaped that the wave is concentrated towards the observer; but whereas this makes the echo louder in that immediate region, it renders the sound less audible elsewhere. Cliffs that are hollowed out with deep recesses illustrate this well, as also do the "whispering galleries" in various buildings. An irregular reflecting surface may send back many reflected waves, and so produce several echoes. This is shown by the echoes of thunder among the clouds, which cause the long-continued reverberation familiar to all. Among well-known echoes may be mentioned that of the Lorelei rock on the Rhine, which is said to repeat sounds fifteen times; that of the tomb of Metella, near Rome, which will repeat a hexameter line; and that of Simonetta, near Milan, which gives sixty definite echoes of the report of a pistol.