On the Refraction of Sound by Wind. 159 



air, furnishes the remaining resistance. When the gas is 

 flowing across this surface the molecules of the gas are too 

 greatly agitated, too much stirred by convection-currents, to 

 group themselves into anything like the semblance of a film. 

 In the case of a gas quiescent, the C0 2 meeting a limiting 

 surface and not being disturbed, a film of carbon dioxide 

 forms, which is superposed over the air-film. The fact that 

 the upper portions of both branches are parallel indicates 

 that they differ, in this region at least, by an additive 

 constant. Several readings taken after the C0 2 had remained 

 in the vessel for about thirty minutes, seemed to indicate a 

 greater resistance of the dielectric for a given distance. It 

 seems probable that a time-factor enters into this con- 

 sideration. The readings, however, are too few in number 

 to base any further statement thereon. 



In conclusion, it becomes my pleasure to thank Prof. A. A. 

 Michelson not only for suggesting this experiment, but for 

 encouragement and advice always freely given throughout 

 the course of the work. It is also my pleasure to thank 

 Prof. S. W. Stratton for many courtesies rendered during 

 the progress of this experiment. 



Ryerson Laboratory, 



University of Chicago. 



Sept. 13th, 1900. 



XII. On the Refraction of Sound by Wind. By Edwin 

 H. Barton, D.Sc, F.R.S.E., Senior Lecturer in Physics at 

 University College, Nottingham *. 



IN his treatise on Sound (vol. ii. pp. 132-4), Lord Rayleigh 

 discusses the refraction of sound by w T ind where the rays 

 are everywhere but slightly inclined to the wind, and obtains 

 an approximate expression which, in the numerical illustration 

 adduced, gives a result differing by only a i'ew minutes of arc 

 from the strict value. The theoretical interest of the wave 

 propagation in this case seems, however, to warrant a slightly 

 fuller examination of the problem on the basis of Huyghens's 

 principle of wavelets and envelopes. Let us retain Lord 

 Rayleigh's assumption as to the distribution of the wind, 

 namely, that it is everywhere horizontal and does not vary 

 in any one horizontal plane but is different at different levels. 

 Then, confining our examination to rays in the same vertical 

 plane as the wind, we find the following results : — 



(1) The direction of propagation is not usually at right 

 angles to the wave-front where there is a wind, conse- 

 quently the cosecant law for the wave-front needs 

 * Communicated by the Physical Society : read Nov. 9, 1900. 



