philosophical society of washington. 51 



93d Meeting. October 23, 1815. 



The President in the Chair. 

 Thirty-two members and visitors present. 

 Mr. Joseph Henry made further remarks on 



SOUND IN connection WITH FOG SIGNALS, 



referring particularly to echo, or reflection of sound, as observed 

 in experiments under the U. S. Light-House Board. 



(jT'Aese experiments are described in the Report of the Light-House Board for 



1875.) 



Mr. W. B. Taylor made a communication on 

 acoustic refraction. 



(abstract.) 



Sound, though differing from light in the character of its 

 waves and their order of magnitude, yet moves like light in 

 radial lines, and, like light, is diverted from its rectilinear course 

 whenever its waves undergo an unequal retardation or accelera- 

 tion. 



There are three different methods in which sound-waves passing 

 through a gaseous medium may suffer such unequal disturbance 

 of velocity : first, by variations of density in the medium — sound 

 moving more slowly as the square root of the density, the pres- 

 sure being the same. Second, by variations of elasticity in the 

 medium, sound moving more swiftly as the square root of the 

 elasticity, the density being the same. Third, by variations of 

 motion or current in the medium — sound travelling (by convec- 

 tion) faster with the wind by a small percentage according to its 

 velocity— and conversely. 



There is no doubt that light also is liable to all three of these 

 forms of refraction ; as its velocity is necessarily retarded by an 

 increase of density in the medium, by a reduction of its elasticity, 

 and by an adverse motion in the medium. 



A fourth cause of velocity disturbance in the case of sound 

 exists in the temperature of the medium — sound moving more 

 swiftly in a heated atmosphere, in proportion to the square root 

 of the absolute temperature. As the only dynamic effect of heat 

 on a gas is to increase its elasticity by confinement, if the volume 

 be constant, or to increase its volume if unconfined, this cause 

 of acoustic refraction, important as it is practically, may be theo- 

 retically resolved into one of the preceding conditions. 



1. The refraction of sound resulting from differences of density 



