140 BULLETIN OF THE 



Mr. Taylor said that he wished to emphasize a single point in 

 Mr. Johnson's communication, namely, that referring to Mr. Price- 

 Edwards' statement in regard to the supposed change of view by 

 Prof. Henry as to the explanation of acoustic disturbances, or, at 

 least, as to the source of the ocean echo. The only thing which 

 could give the slightest color to such a supposition was a purely 

 incidental and wholly unimportant suggestion thrown out by Prof. 

 Henry on this subject. Discarding the proposed explanation of 

 the echo by the presence of a hygroscopic flocculence, or invisible 

 acoustic clouds in the air, as quite insufficient in character, as too 

 indefinite in limits, and as too mutable and evanescent in duration, 

 in a mobile atmosphere, to account for so pronounced, distinct, and 

 uniform a phenomenon, Prof. Henry thought, in the absence of any 

 other sufficient surface, that, in view of the large amount of curva- 

 ture in ordinary sound beams, acoustic waves might be reflected 

 back to the ear from the ocean itself, — probably from the sloping 

 sides of the waves. On having his attention drawn by Prof. 

 Tyndall to the circumstance that the echoes were frequently distinct 

 over a perfectly smooth sea, he admitted that this would invalidate 

 the suggestion of wave crests being concerned in the effect ; but he 

 still believed that, with sounds sufficiently powerful to reach con- 

 siderable distances, it was quite possible for some of the upper 

 sound-beams to be so curved as to be reflected upward from a per- 

 fectly level floor, and still to reach an observer's ear placed near the 

 origin of sound. He had also shown that visible clouds were quite 

 incompetent to return any sensible echo to the loudest sounds. 



So far from receding from his views in regard to the occasions of 

 irregularity in the audibility of sound, in his last Report of the 

 Light-House Board — that for 1877, published but a short time 

 before his death — he announced his previous conclusions as only 

 more confirmed by his later observations ; and a summary of these 

 conclusions was also published in the Smithsonian Report for 1877. 



The ideas of sound transmission promulgated in popular books 

 and lectures, as derived from class-room experiments, are very in- 

 accurate and misleading when applied to any considerable range of 

 sound travel. Were the medium of sound propagation — the atmos- 

 phere — perfectly homogeneous in density, in temperature, and in 

 movement, the beams would indeed travel in sensibly straight lines, 

 but still with a large amount of lateral diffusion bearing no anal- 

 ogy to the diffraction of light. But in distances of several miles — 



