ECHO, from the Greek, meaning ((sound? noise; a sound repeated by reflection, or a sound heard again at its source. The waves of sound on meeting the surface are turned back in their course according to the same laws that hold for reflection of light. Even clouds are capable of producing echoes, as is observed at sea when a gun is fired off under a dense cloud. Echoes from the clouds also, in all likeli hood, play an important part in the long rolling of thunder. In order that the echo may return to the,place from which the sound proceeds the reflection must be direct and not at the angle to the line of transmission, otherWise the echo may be heard by others but not by the transmitter of the sound. This may be effected either by a reflecting surface at right angles to the line of transmission or by several reflecting surfaces which end in bringing the sound back to the point of issue. To make a distinct echo the reflected sound must be concentrated rather than diffused. To this result a degree of con cavity in the reflecting body is conducive. If the sound is repeated several times, which is the case when it strikes against objects at different distances, many echoes are heard. The reflect
ing surface must be at a certain distance, in order that the echo may come to the ear after the sound and be distinctly separated' from it. Sound travels about 1,100 feet in a second; con sequently, an observer standing at half that dis tance from the reflecting object would hear the echo a second later than the sound. Such an echo would repeat as many words and syllables as could be heard in a second. This is called a polysyllabic echo. If the distance is less, the echo repeats fewer syllables; if only one is re peated, then the echo is monosyllabic. The most practised ear cannot distinguish in a second more than from 9 to 12 successive sounds, so that a distance of not less than 60 feet is needed to enable a common ear to distinguish between the echo and the original sounds. Echoes at least distances, as from the walls of churches and public halls, are liable to mix with and destroy the distinctness of the original sound.