1874] WRITINGS OF JOSEPH HENRY. 445 



he has stated are (with a single exception as to the direction 

 of the echo) in strict accordance with what we have repeat- 

 edl}' observed. We regret to say however, that we cannot 

 subscribe to the conclusions which he draws from his ex- 

 periments as to the cause of the retardation of sound, that it 

 is due to a flocculent condition of the atmosphere, caused by 

 the intermingling with it of invisible aqueous vapor. 



That a flocculent condition of the atmosphere, due to the 

 varying density pro'^uced by the mingling of aqueous vapor, 

 is a true cause of obstruction in the transmission of sound is 

 a fact borne out by deduction from the principles of wave- 

 motion, as well as by the experiments of the distinguished 

 physicist of the Royal Institution of Great Britain ; but from 

 all the observations we have made on this subject, we 

 are far from thinking that this is the efficient cause of the 

 phenomena under consideration. A fatal objection we think 

 to the truth of the hypothesis Professor Tyndall has advanced, 

 is that the obstruction to the sound — whatever may be its 

 nature, is not the same in different directions. We think 

 we are warranted in asserting that in the cases of acoustic 

 opacity which he has described, if he had simultaneously 

 made observations in an opposite direction, he would have 

 come to a different conclusion. That a flocculent condition 

 of the atmosphere should slightly obstruct the sound is not 

 difficult to conceive; but that it should obstruct the ray in 

 one direction and not in an opposite, or in a greater degree 

 in one direction than in another, the stratum of air be- 

 ing the same in both cases, is at variance with any fact in 

 nature with which we are acquainted. We would hesitate 

 to speak so decidedly against the conclusions of Professor 

 Tyndall, — for whose clearness of conception of physical prin- 

 ciples, skill in manipulation, and power of logical deduction, 

 we entertain the highest appreciation, were the facts which 

 have been obtained in our investigations of a less explicit 

 character. 



While the phenomena in question are incompatible with 

 the assumption of a flocculent atmosphere as a cause, they 

 are in strict accordance with the hypothesis of the refraction 



