THE REFRACTION OF SOUND. 211 



above indicates the direction and force of the wind, and the two smaller 

 arrows below the diminished force of the wind in the same direction, by 

 reason of the increasing resistance and retardation toward the surface 

 of the earth. The result is a flattening of the waves at the left side 

 and a swaying of the waves forward on the right side, thus giving the 

 radial lines or sound-beams a curved form, (as shown bj^ the lower lines 

 I i and i A,) these being always perpendicular to the wave-faces. As 

 this curvature or refraction is necessarily upward against the unequal 

 wind and downward in the direction of the wind, an observer at k will 

 hear with great distinctness the sound emanating from i, while an ob- 

 server on the other side at j will hear nothing, by reason of the sound- 

 beams being tilted above his head. By rising to an elevation, as at I, 

 the observer will hear the sound as well against the wind as with if. 



It is not a little surprising that an explanation of a well-marked and 

 puzzling phenomenon, so elegant and so conclusive as that promulgated 

 by Professor Stokes, should have remained for fifteen years unnoticed 

 and unthought-of by the scientific world. When in 1865 Professor 

 Henry discovered that " a sound moving against the wind, inaudible to 

 the ear on the deck of a schooner, was heard by ascending to the mast- 

 head, this remarkable fact at first suggested the idea that sound was 

 more readily conveyed by the upper current of air than the lower." 

 And this general idea seemed confirmed by the observation that with 

 the upper and lower currents at variance the upper wind appeared to 

 most favor the sound. Nor was it till early in 1872 that the full signifi- 

 cance of all this became apparent on first learning of the explanation 

 given by Professor Stokes. And yet during the same series of obser- 

 vations. Professor Henry discovered that the velocity of wind in the 

 higher regions of the air was much greater than in the lower regions. 



In like manner, Professor Reynolds having, in the si)ring of 1874, 

 independently arrived at the same theory, and undertaken a series of 

 experiments and observations in this direction, remarks: "I had just 

 reached the point of making such tests, when I discovered that the 

 same views had been propounded by Professor Stokes so long ago as 

 1857."— (Proc. R. S., 1874.) Professor Reynolds made the decisive 

 observations that in the direction of the wind, the sound of a bell could 

 be as well heard at a distance, with the head depressed as when stand- 

 ing; while against the wind, the sound at no great distance ceased to 

 be heard, passing over the head, and could be regained in full force by 

 elevation. It was found also that the elevation required to reach the 

 lowest sound-beam increased with the distance. 



If we suppose the wind near the surface of the earth (or at G 'eet 

 above it) to be moving at the rate of six miles i)er hour, (one mile in 10 

 minutes, or 8.8 feet per second,) and at the elevation of 1,000 feet to be 

 moving in the same direction with just double the velocity, then a ver- 

 tical wave-front of sound in moving 4.7 seconds, or one mile, against 

 such wind would be retarded 41 feet near the ground, and 82 feet at 

 the height of 1,000 feet. This difference of 41 feet would so tilt the 



