When a sound, after reflection, is audible it is called an echo. The distinctness with which it is heard depends upon the distance of the ear from the reflecting surface. A very quick, sharp sound may produce an echo even when the reflecting surface is not more than fifty or sixty feet away, but for articulate sounds a greater distance is necessary. (a.) Few, if any, persons can pronounce distinctly more than about five syllables in a second. At the ordinary temperature, sound travels about 1120 feet per second. In a fifth of that time it would travel about 224 feet. If, therefore, the reflecting surface be 112 feet distant, the articulate sound will go and return before the next syllable is pronounced. The two sounds will not interfere, and the echo will be distinctly heard. If the reflecting surface be less than this distance, the reflected sound will return before the articulation is complete and confusedly blend with it. If the reflector be 224 feet distant, there will be time to pronounce two syllables before the reflected wave returns. The echo of both syllables may then be heard; and so on. The echo may be heard sometimes when the direct sound cannot be heard. (b.) Suppose the speaker to stand 1120 feet from the reflecting substance. If then he speak ten syllables in two seconds, the echo of the first will return just as the last is spoken; the echo of each syllable will be distinct. But if he continues to speak, the direct and the reflected sounds will become blended and confused. The reflecting surface should be a large, vertical wall, or similar object, as a huge rock. (c.) When two opposite surfaces, as parallel walls, successively reflect the sound, multiple echoes are heard. Sometimes an echo is thus repeated 20 or 30 times.