414 WRITINGS OF JOSEPH HENRY. [1874 



the explanation of this might seem to be very simple, namely, 

 that the sound is borne on in the one direction and retarded 

 in the other by the motion of the wind. But this explana- 

 tion, satisfactory as it might appear, cannot be true. Sound 

 moves at the rate of about 780 miles an hour, and therefore 

 on the above supposition, a wind of 7'8 miles per hour could 

 neither retard nor accelerate its velocity more than one per 

 cent. — an amount inappreciable to ordinary observation; 

 whereas we know that a wind of the velocity mentioned is 

 frequently accompanied with a reduction of the penetrating 

 power of sound — of more than 50 per cent. 



The explanation of this phenomenon, 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 

 immediately above, and this again than the one above it, 

 and so on. The effect of this diminution of velocity as we 

 descend toward 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 parts, thus causing 

 their resultant impulse (the sound-beam) 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 por- 

 tion of the sound-waves is more retarded than the lower, which 

 advancing more rapidly in consequence, inclines the rays of 

 sound upward and directs them above the head of the ob- 

 server. 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 per- 

 pendicular to the direction of the beam, — like the palings of 

 a fence. Now if the upper part of the waves has a slightly 

 greater velocit}' than the lower, the beam will be bent down- 

 ward 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 cumulative 

 from the sound-centre onward, and hence — although the 

 velocity of the wind may have no perceptible effect on the 



