48 REPORT OF THE SECRETARY. 



These investigations have been made along the coast on light-house 

 steamers, with the assistance of members and officers of the light-house 

 establishment. During the last year a series of experiments were made 

 along the coast of Maine, which served to establish some new principles, 

 as well as to confirm previous results. 



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 modification and extensiou 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 

 gut 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 loud- 

 ness 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 vibrating surface, the greater will be the 

 distance to which it is audibly transmitted. , These results are derived 

 from observations on the siren, the reed-trumpet, and the automatic 

 buoy. The effect of quantity of sound is shown in 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. The audibility of sound depends upon the state of the 

 atmosphere. 



2. A condition most favorable to the transmission of sound is that of 

 perfect stillness and uniform density and temperature throughout. 

 This is shown by the observations of Parry and other Arctic explorers; 

 although in this case an efficient and co-operating cause is doubtless 

 the downward refraction of sound due to the greater coldness of the 

 lower strata of air, as first pointed out by Professor Reynolds. Air, 

 however, is seldom in a state of uniform density, but is pervaded by 

 local currents, due to contact with portions of the earth unequally 

 heated, and from the refractions and reflections to which the sound- 

 wave is subjected in its passage through such a medium it is broken up 

 and lost to the ear at a less distance. 



3. But the most efficient cause of the loss of audibility is the direct 

 effect produced by the wind. As a general rule, a sound is heard far- 

 ther when moving with the wind than when moving against it. This 

 effect, which is in conformity with ordinary observation, is not due to 



