RESEARCHES IN SOUND. 557 



struments at the same time, it was fouud tliat two sounds apparently of 

 the same loudness were heard at very dilierent distances. 



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

 A condition most favorable to the transmission of soimd is that of per- 

 fect stillness and uniform density and tcmj)erature throughout. This 

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

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

 downward refraction of sound due to the gTcater coldness of the lower 

 strata of air, as first pointed out by Professor Eeynolds. Air however is 

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

 due to contact with i^ortions 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 

 effi?ct 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 an 

 increase of velocity of the sound-wave in one direction and a diminution 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 dimin- 

 ish the velocity of the sound-wave only one per cent, while the effect ob- 

 served is in some cases several hundred per cent. It is however due to 

 a change in its direction. Sound moving with the wind is refracted or 

 thrown down toward the earth 5 while moving against the wind it is 

 refracted upward and passes over the head of the observer, so as to be 

 heard at a tlistance at an elevation of several hundred feet when in- 

 audible at the surface of the earth. 



4. Although, as a general rule, the sound is heard farther when mov- 

 ing with the wind than when moving against it, yet in some instances 

 the sound is heard farthest against the wind; but this phenomenon is 

 shown to be due to a dominant upper wind, blowing at the time in an 

 opposite direction to that at the surface of the earth. These winds are 

 not imaginary productions invented to explain the phenomena, but 

 actual existences, established by observation, as in the case of the 

 experiments made at Sandy Hook, in 1874, by means of balloons, and 

 from the actual motion of the air in the case of northeast storms, as 

 observed at stations on the coast of Maine. 



o. Although sound issuing from the mouth of a trumpet is at first con- 

 centrated in a given direction, yet it tends to spread so rapidly that at 

 the distance of three or four miles it fills the whole space of air inclosed 

 within the circuit of the horizon, and is heard behind the trumpet nearly 

 as well as at an equal distance in front of its mouth. This fact i)reeludes 

 the use of concave reflectors as a means of increasing the intensity of 

 sound in a given direction ; for although at first they do give an increase 



