1882.J Note on General Duanes Soundless Zones. 19 



signals, and has been at times observed at all the stations, at one of 

 which the signal is situated on a bare rock 20 miles from the main- 

 land, with no surrounding objects to affect the sound." 



For a long time past, I have thought that this disappearance of the 

 sound was due to the interference with the direct waves, of waves 

 reflected from the surface of the sea. This explanation is capable of 

 very accurate experimental illustration. Placing, for instance, a 

 sensitive flame at a distance of 3 or 4 feet from a sounding reed, the 

 flame exhibits the usual agitation. Lifting a light plank between the 

 flame and reed, a position is easily attained where the sound, reflected 

 from the plank, increases the flame's agitation. Lifting the plank 

 cautiously still higher, a level is attained, reflection from which com- 

 pletely stills the flame. By slightly raising or lowering the plank, or 

 by its entire removal, the flame is once more agitated. In these 

 experiments a high pitched reed was used, so that it was easy to 

 produce by the motion of the plank the retardation of half a wave- 

 length requisite for interference. 



In General Duane's case, a fairly smooth sea would be required for 

 the reflection ; while the position of the zone of silence would be 

 determined by the height of tne signal on the one hand and the 

 height of the observer on the other above the surface of the sea. The 

 position would also, of course, depend on the pitch of the note of the 

 whistle. 



The preparation of some lectures on the " Resemblances of Sound, 

 Light, and Heat," has recently brought this subject to mind, hence 

 the present communication. 



