1877] WRITINGS OF JOSEPH HENRY. 509 



uted to the upward refraction of the sound-wave, which 

 passes over the head of the observer and continues an up- 

 ward course until it nearly reaches the upper surface of the 

 current of wind, when the refraction will be reversed and 

 the sound sent downward to the earth; or the effect may be 

 considered as due to a sound-shadow produced by refraction, 

 which is gradually closed in at a distance by the lateral 

 spread of the sound-wave near the earth, in a direction which 

 is not affected by the upward refraction. Another explana- 

 tion may be found in the probable circumstance of the lower 

 sheet of sound-beams being actually refracted into a serpen- 

 tine or undulating course, as suggested in the Appendix 

 to the Report of the Light-House Board for 1875.* Such a 

 serpentine course would result from successive layers of un- 

 equal velocity in an opposing wind ; as being retarded at and 

 near the surface of the earth, attaining its maximum velocity 

 at a height of a few hundred feet, and then being again re- 

 tarded at greater elevations, by the friction of upper counter 

 currents or of more stationary air. In some cases the phe- 

 nomenon is due to one or the other of these causes, and in 

 other cases to all combined. That it is not due to the ob- 

 structing or screening effects of an abnormal condition of 

 the atmosphere is shown by the fact that a sound transmitted 

 in an opposite direction, through what is called the region of 

 silence, passes without obstruction. It is probable from all 

 the observations, that in all cases of the upward refraction 

 of a sound moving against the wind it tends again to descend 

 to the earth by the natural spread of the sound; though it 

 may generally be so enfeebled by diffusion as well as by 

 distance, as to be inaudible. 



9. The existence of a remarkable phenomenon has been 

 established, exhibited in all states of the atmosphere, — 

 during rain, snow, and dense fog, to which has been given 

 the name of aerial echo. It consists of a distinct echo 

 apparently from a space near the horizon of fifteen or twenty 

 degrees in azimuth, directly in the prolongation of the axis 



*[See PART IV ; ante, p. '450.] 



