1875] WRITINGS OF JOSEPH HENRY. 485 



normal direction of the sound-wave in still air, instead of 

 proceeding horizontally, should be turned upward on account 

 of the greater velocity of sound near the earth, due to the 

 greater heat of the strata in that position than of those 

 above. This principle, which indicates the existence of a 

 true refraction of sound independent of the motion of the 

 medium, is undoubtedly applicable as a modifying influence 

 to the phenomena we have recorded. It produces however 

 only a slight effect in the case we have last mentioned, 

 since the observation on board the Cactus shows the condi- 

 tion of the air was that of little acoustic absorption. It 

 would nevertheless favor the hypothesis that sound in per- 

 fectly still air of homogeneous density could be heard farther 

 than sound in a moving medium, or in one of unequal tem- 

 perature. This is also in accordance with the fact repeatedly 

 observed in arctic regions, in which the sound of the human 

 voice is heard at great distances during times of extreme 

 cold. In this case the air is of a uniform temperature above 

 and below, but of diminished elasticity, and should on this 

 account transmit sound with less intensity ; and yet the 

 audibility is increased, which is explained by the assumption 

 that its stillness and uniformity of temperature more than 

 compensate for the diminished elasticity. The same ma}^ be 

 said with regard to the audibility of sound during a fog, 

 which usually exists during extreme stillness of the air. 



Whatever be the cause of the variation in the limit of 

 audition as exhibited in the diagrams, it is less efficient than 

 the ordinary action of the wind in producing the same phe- 

 nomena. This is evident from the fact that while the ratio 

 of the extreme variation in the limits of audition in the first 

 case is not more than 1 : 3, in the second it is that of 1 : 5. 



Moreover, when the effect of the wind on the audition of 

 sound in relation to elevation is considered, we think we are 

 fully warranted in asserting, as we did in our last report, that 

 the wind is a more efficient cause of the variable penetration 

 of sound than the invisible acoustic clouds postulated by Pro- 

 fessor Tyndall for the explanation of the phenomena. 



The object of these investigations, as stated at the begin- 

 ning of this report, was to obtain facts which might serve to 



