1877] WRITINGS OP JOSEPH HENRY. 507 



atmosphere. A condition most favorable to the transmis- 

 sion of sound is that of perfect stillness and uniform density 

 and temperature throughout. This is shown by the obser- 

 vations 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 den- 

 sity, but is pervaded by local currents, due to contact with 

 portions of the earth unequally heated, and from the refrac- 

 tions 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 difference in audibility is 

 the direct effect produced by the wind. As a general rule, 

 a sound is heard farther when moving with the wind than 

 when moving against it. This effect, which is in conform- 

 ity with ordinary observation, is not due to an increase of 

 velocity of the sound-wave in one direction and a diminu- 

 tion in the other by the motion of the wind except in an 

 imperceptible degree ; for since sound moves at the rate of 

 about seven hundred and fifty miles an hour, a wind of seven 

 miles and a half an hour could increase or diminish the 

 velocity of the sound-wave only one per cent., while the 

 difference of effect observed is in some cases several hun- 

 dred per cent. The true cause of the remarkable variation 

 in the audibility of sound-beams at a distance — is to be 

 found in the change of their direction. Sound moving with 

 the wind is ordinarily refracted or thrown down toward 

 the earth ; while moving against the wind it is ordinarily 

 refracted upward and passes over the head of the observer, 

 so as to be heard at a distance at an elevation of several 

 hundred feet, when inaudible at the surface of the earth. 



4. Although as a general rule the sound is heard farther 

 when moving with the wind than when moving against it, 

 yet in some instances sound is heard farthest against the 

 wind; but this phenomenon is shown to be due to a domi- 

 nant upper wind, blowing at the time in an opposite direc- 



