108 



Mr. E. A. Milne on 



When all points on the ground are considered (instead 

 of points on Ox only) there will in general exist a curve of 

 audibility* delimiting the region in which the source can be 

 heard. This curve may be closed or unclosed ; in the latter 

 case it will extend to infinity. The general case is hardly 



Limiting form as 



W-> CO 



Carves of audibility (Z = 10,000 feet ; a -d- 

 scale. 1 unit to 10,000 feet). 



18 ft. sec. 



worth considering in detail, but in the example just taken 

 (where the wind is in the direction Ox and is a linear func- 

 tion of the height) let R be the range of audibility in the 



* For curves of audibility arising' from sources of sound on the ground, 

 see for example Fujiwhara, Proc. Tokyo Math. Phys. Soc. vi. p. 132(1911), 

 and Van Everdingen, Proc. Acad. Sci. Amsterdam, 18, p. 923 (1915). 



