506 WRITINGS OF JOSEPH HENRY. [1877 



longed character of the echo, and its angular extent along 

 the horizon. 



While we do not advance this hypothesis as a final solu- 

 tion of the question, we shall provisionally adopt it as a 

 means of suggesting further experiments in regard to this 

 perplexing question at another season. 



General Conclusions. 



From all the experiments which have been made by the 

 Light-House Board in regard to the transmission of sound 

 in free air, and those derived from other observations which 

 can be fully relied upon, the following conclusions may be 

 considered established, subject however to such further modi- 

 fication and extension as subsequent investigation may seem 

 to indicate : 



1. The audibility of sound at a distance (the state of the 

 atmosphere being constant) depends upon the character of the 

 sound. The distance through which a sound may be heard 

 is governed by the pitch, the loudness, and the quantity of 

 sound. The pitch or frequency of the impulses in a given 

 time must not be too high, otherwise the amplitude of 

 vibration will be too small to allow a sufficient quantity of 

 air to be put into motion ; neither must the pitch be too low, 

 for in this case the motion of the atoms of air in the sound- 

 wave will not be sufficiently rapid to convey the impulse to a 

 great distance. Again the greater the loudness of the sound, 

 (which depends upon the amplitude of the vibrations of the 

 sounding-body,) the greater will be the distance at which it 

 will be heard. And finally, the greater the quantity of 

 sound, (which depends upon the magnitude of the vibrat- 

 ing surface,) the greater will be the distance to which it is 

 audibly transmitted. These results are derived from obser- 

 vations on the siren, the reed-trumpet, and the automatic 

 buoy. The effect of quantity of sound is shown by the fact 

 that in sounding different instruments at the same time, it 

 was found that two sounds apparently of the same loudness 

 were heard at very different distances. 



2. The audibility of sound depends upon the state of the 



