50 REPORT OF THE SECRETARY. 



attributed to the upward refraction of the sound-wave, which passes 

 over the head of the observer and continues an upward course until it 

 nearly reaches the upper surface of the current of wind, when the refrac- 

 tion will be reversed and the sound sent downward to the earth 5 or 

 the effect may be considered as due to a sound-shadow produced by re- 

 fraction, which is gradually closed in at a distance by the lateral spread 

 of the sound-wave near the earth, on either side, in a direction which 

 is not affected by the upward refraction. Another explanation may be 

 found in the probable circumstance of the lower sheet of sound-beams 

 being actually refracted into a serpentine or undulating course, as sug- 

 gested in the Appendix to the Keport of the Light-House Board for 1875. 

 Such a serpentine course would result from successive layers of unequal 

 velocity in an opposing wind; as being retarded at and near the sur- 

 face of the earth, attaining its maximum velocity at the height of a 

 few hundred feet, and then being again retarded at greater elevations 

 by the friction of upper counter-currents, or stationary air. In some 

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

 other cases to both combined. That it is not due to the obstructing 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 refraction of a 

 sound moving against the wind, it tends again to descend to the earth 

 by the natural spread of the sound. 



9. The existence of a remarkable phenomenon has been established, 

 which is exhibited in all states of the atmosphere during rain, snow, 

 and dense fog, to which has been given the name of aerial echo. It con- 

 sists 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 of the trumpet. The loudness of this echo depends upon the loud- 

 ness and quantity of the original sound, and therefore it is produced 

 with the greatest distinctness by the siren. It cannot be due to the 

 accidental position of a flocculent portion of atmosphere nor the direct 

 reflection from the crests of the waves, as was at first supposed, since 

 it is always heard except when the wind is blowing a hurricane. 



As a provisional explanation^ the hypothesis has been adopted that 

 in the natural spread of the waves of sound some of the rays must take 

 such a curvilinear course as to strike the surface of the water in an 

 opposite direction and thus be reflected back to the station or location 

 of the origin of the sound." 



Eespectfully submitted. 



JOSEPH HENRY, 

 Secretary Smithsonian Institution. 

 Washington, January, 1878. 



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