1874.] On the Refraction of Sound by the Atmosphere. 295 



III. " On the Refraction of Sound by the Atmosphere/' By Prof. 

 OSBORNE REYNOLDS, Owens College, Manchester. Communi- 

 cated by Prof. STOKES, SeC.R.S. Received March 18, 1874. 



(Abstract.) 



The principal object of this paper is to show that sound is refracted 

 upwards by the atmosphere in direct proportion to the upward diminu- 

 tion of the temperature, and hence to explain several phenomena o sound, 

 and particularly the results of Prof. Tyndall's recent observations off the 

 South Foreland. 



The paper commences by describing the explanation of the effect of 

 wind upon sound, viz. that this effect is due to the lifting of the 

 sound from the ground, and not to its destruction, as is generally sup- 



The lifting of the sound is shown to be due to the different velocities 

 with which the air moves at the ground and at an elevation above it. 

 During a wind the air moves faster above than below, therefore sound 

 moving against the wind moves faster below than above, the effect of which 

 is to refract or turn the sound upwards ; so that the* " rays " of sound, 

 which would otherwise move horizontally along the ground, actually move 

 upwards in circular or more nearly hyperbolic paths, and thus, if there 

 is sufficient distance, pass over the observer's head. This explana- 

 tion was propounded by Prof. Stokes in 1857, but was discovered inde- 

 pendently by the author. 



The paper then contains the description of experiments made with a 

 view to establish this explanation, and from which it appears that : 



1. The velocity of wind over grass differs by one half at elevations of 

 1 and 8 feet, and by somewhat less over snow. 



2. When there is no wind, sound proceeding over a rough surface 

 is destroyed at the surface, and is thus less intense below than above. 



3. That sounds proceeding against the wind are lifted up off the ground, 

 and hence the range is diminished at low elevations ; but that the 

 sound is not destroyed, and may be heard from, positions sufficiently 

 elevated with even greater distinctness than at the same distances with 

 the wind. 



4. That sounds proceeding with the wind are brought down to the 

 ground in such a manner as to counterbalance the effect of the rough sur- 

 face (2) ; and hence, contrary to the experiments of Delaroche, the range 

 at the ground is greater with the wdnd than at right angles to its direc- 

 tion, or where there is no wind. 



On one occasion it was found that the sound could be heard 360 yards 

 with the wind at all elevations, whereas it could be heard only 200 yards 

 at right angles to the wind, standing up ; and, against the wind, it was lost 

 at 30 yards at the ground, 70 yards standing up, and at 160 j r ards at an 



2 A2 



