ADDRESS OF PROF. A. M. MAYER. 501 



These facts demanded explanation, and for a long time remained 

 enigmas to Henry; till one day he met with a short paper by Pro- 

 fessor Stokes, of Cambridge, England, in the Proceedings of the 

 British Association for 1857, " in which the effect of an upper current 

 in deflecting the wave-surface of sound so as to throw it down upon 

 the ear of the auditor, or directing it upward far above his head, 

 is fully explained." In the Report of the Light-House Board for 

 1874 Henry says: "The explanation, [of these phenomena,] as 

 suggested by the hypothesis of Professor Stokes, is founded on the 

 fact that in the case of a deep current of air the lower stratum, or 

 that next the earth, is more retarded by friction than the one imme- 

 diately above, and this again than the one above it, and so on. The 

 effect of this diminution of velocity as we descend towards the 

 earth is, in the case of sound moving with the current, to carry the 

 upper part of the sound-waves more rapidly forward than the lower 

 part, thus causing them to incline toward the earth, or in other 

 words, to be thrown down upon the ear of the observer. When 

 the sound is in a contrary direction to the current, an opposite effect 

 is produced, the upper portion of the sound-waves is more retarded 

 than the lower, which advancing more rapidly in consequence, 

 inclines the waves upward and directs them above the head of the 

 observer. To render this more clear, let us recall the nature of a 

 beam of sound, in still air, projected in a horizontal direction. It 

 consists of a series of concentric waves perpendicular to the direc- 

 tion of the beam, like the palings of a fence. Now, if the upper 

 part of the waves have a slightly greater velocity than the lower, 

 the beam will be bent downward in a manner somewhat analogous 

 to that of a ray of light in proceeding from a rarer to a denser 

 medium. The effect of this deformation of the wave will be cumu- 

 lative from the sound-centre outward, and hence, although the 

 velocity of the wind may have no perceptible effect on the velocity 

 of sound, yet this bending of the wave being continuous throughout 

 its entire course, a marked effect must be produced. A precisely 

 similar effect will be the result, but perhaps in a considerably 

 greater degree, in case an upper current is moving in an opposite 

 direction to the lower, when the latter is adverse to the sound, and* 

 in this we have a logical explanation of the phenomenon observed 



