1877] WRITINGS OF JOSEPH HENRY. 495 



Dearest land consists of a series of broken surfaces not rising; 

 above the ocean enough to reflect sound or in any way to 

 produce sound-shadows in the region through which the 

 plienomena are observed. This hypothesis therefore is inad- 

 missible. 



Another hypothesis is that of invisible acoustic clouds (as 

 they have been called), or portions of atmosphere existing 

 over the water in the region of silence, which might absorb 

 or variously reflect the sound. That such a condition of a 

 portion of the atmosphere really exists in some cases is a 

 fact which may be inferred from well-established principles 

 of acoustics, as well as from experimental data. They would 

 occur especially in the case of dissolving clouds, which 

 would be accompained by local diminutions of temperature, 

 and also from portions of air which have been abnormally 

 heated by contact with warm earth. But if the pheno- 

 mena in question were produced by a cloud of this kind, 

 its presence ought to be indicated by carrying through it 

 the usual set of meteorological instruments. This was done 

 in the foregoing experiments, but no change was observed 

 in the indications either of the thermometer or barometer. 

 Unfortunately we had not a hygrometer in our possession, 

 but this observation was less necessary, since from abundant 

 testimony it is established that the same phenomena are 

 exhibited during a dense fog, in which all parts of the 

 atmosphere for miles in extent must be in a homogeneous 

 condition. Furthermore, a local cloud could not continue 

 to exist in a given space for more than an instant while a 

 wind was blowing with a velocity of from ten to twelve 

 miles an hour. Again, this hypothesis ftiils entirely to 

 explain the fact that this phenomenon is always observed 

 at nearly the same place, especially during a fog, when the 

 wind is in a southerly direction. Finally, it is impossible to 

 conceive of a cloud so arranged as a screen producing a 

 sound-shadow of greater intensity on one side than on the 

 other. 



Another hypothesis is that of the refraction of sound due 

 to the action of the wind. It is an inference from well- 



