528 WRITINGS OP JOSEPH HENRY. [1877 



reflected back to the ear from points at a considerable dis- 

 tance from their origin, (this being especially true of the 

 ocean-echoes,) we are liable to be seriously misled if we rely 

 too confidently on the experiments of the laboratory, and 

 form hasty generalizations from apparent analogies, without 

 carefully considering all the meteorological conditions by 

 which the rays of sound may be deflected, distorted, and 

 diverged. It is now well established by numerous observa- 

 tions and experiments — made independently on both sides 

 of the Atlantic, that the lines of acoustic propagation (con- 

 veniently called sound-beams) which are sensibly very recti- 

 linear for the distance of a hundred or two hundred feet, 

 and which are thus obedient to the katoptric and dioptric 

 laws of precise focal convergence, by means of solid mirrors 

 and of gaseous lenses, are yet at the distance of a few miles 

 so strangely contorted and aberrant as seemingly to contra- 

 dict all the analogies suggested by our experience with the 

 rays of light. It is the accumulation of comparatively slight 

 divergencies continued through many thousands of yards, 

 whether under the influence of constant conditions or of 

 changing and reversed conditions, which produces such 

 marked anomalies at the distance of five or of ten miles, and 

 which makes their investigation as laborious as it is instruct- 

 ive and important. And not until we have mastered all 

 the conditions affecting the transmission of sound through- 

 out its entire sensible range, and have thus become enabled 

 to predict its true course, and to announce its varying limits 

 of audibility at the earth's surface, under given circum- 

 stances, can we be said to have perfected the theory of this 

 most interesting and indispensable agent of communication. 



