EESEARCnES IN SOUND. 



531 



The theoretical conditions in order that the sound may be beard with 

 greater distinctness at an elevation than below are, as we have said 

 before, that the wind be moving with a greater velocity in a given di- 

 rection at an elevation than at the surface of the earth, and that the 

 difference in the velocities may be against the sound-wave, so that its 

 npper part may be more retarded than the lower. In this case the di- 

 rection of a beam of sound will be curved upward, leaving as it were 

 a vacuum of sound beneath. The distance of the origin of sound, how- 

 ever, must not be too great relatively to the elevation of the observer; 

 otherwise it will pass over his head, as well as over that of the observer 

 at the surface of the earth. In most instances the sound was not con- 

 tinuous, but was interrupted — heard for a time, then lost; again be- 

 coming audible, it Avas heard until it fiimlly became imperceptible. 

 Besides this, it was difficult to determine when the sound ceased to be 

 heard, since this depended on the sensibility of the ear and the greater 

 or less attention of the observer at the time of the observation. To 

 obviate these difficulties as well as the unfavorable condition of too 

 great a distance of the origin of sound from the observer, it was con- 

 cluded to adopt as the duration of the sound the elapsed time between 

 its beginning and the period when it was first lost. 



The observer on the tower was Mr. P. Barnard, while the one below 

 was General Woodruff. From the records of the observations of these 

 gentlemen the following tables are compiled, the first of which indicates 

 the relative duration of sound on the top of the tower and at the bottom, 

 the sound moving against the wind; tlie second, the same duration, the 

 sound moving with the wind; and the third, the same with the sound 

 at right angles to.it. 



Table 1. — Sound against the ivind. 



Date. 



1875. 



September 2 



4 



4 



4 



C 



G 



7 



S 



8 



8 



8 



Mean 



Heart! at 

 top of 

 tower. 



mm. sec. 



30 

 30 

 30 

 00 

 00 

 00 

 00 

 00 

 30 

 30 

 00 



Heard at 

 bottom. 



mm. sec. 



00 

 30 

 00 

 00 



2 15 



3 00 



15 

 00 

 45 

 15 

 15 



4 57 3 01 



