558 RESEARCHES IN SOUND. 



of sound in the direction of the axis, it is only for a comiDaratively short 

 ! distance. 



6. It has been established, contrary to what has formerly been thought 

 to be the case, that neither fog, snow, hail, nor rain, materially interferes 

 with the transmission of loud sounds. The siren has been heard at a 

 greater distance during the prevalence of a dense and widely-extended 

 fog than dimng any other condition of the atmosphere. This may 

 however be attributed to the uniform density and stillness of the air at 

 the time. 



7. In some cases sound-shadows are i^roduced by projecting portions 

 of land or by buildings situated near the origin of the sound, but 

 these shadows are closed in by the spread of the sound-waves, and thus 

 exhibit the phenomenon of sound being heard at a distance and after- 

 wards lost on a nearer approach to the station. 



8. It frequently happens on a vessel leaving a station, that the sound 

 is suddenly lost at a point in its course, and after remaining inaudible some 

 time, is heard again at a greater distance, and is then gradually lost as 

 the distance is fartlier increased. This phenomenon is only observed 

 when the sound is moving against the wind, and is therefore 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 refraction will be re- 

 versed and the sound sent downward to the earth ; or the elfect 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, 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 suggested in the 

 Appendix to the Eeport of the Light-House Board for 1875. {See page 

 513.) Such a serpentine course would result from successive layers of 

 unequal 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 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 all combined. That it is not due to the obstructing or 

 screening eft'ects 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 si)read of the soujid. 



9. The existence of a remarkable i)henomenon 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 consists 



