RESEARCHES IN SOUND. 541 



of perception is tlie center. From these facts we are inclined to think 

 that sound is not heard as far during a time of high "wind in any direc- 

 tion as it is during' a perfect cahn, and that it is lieard farthest with a 

 gentle wind. This conclusion, which was not anticix)ated at the begin- 

 ning of these investigations, is we think in strict conformity with the 

 hypothesis adopted. In the case of sound moving against a strong 

 wind, the sonorous waves being thrown up above the ears of the ob- 

 server, the sphere of audition in that direction is without question 

 greatly diminished; and that it should be also diminished when sound 

 is moving with a strong wind having a greater velocity above than be- 

 low is not ditticult to conceive. In this case the sound-wave will be so 

 thrown down against the earth, and so much of it absorbed, as to weaken 

 the intensity of that part which reaches the ear, while in the case of a 

 feeble wind, moving faster above than below, the portion of the wave 

 thrown down from above will only be suthcient to compensate for the 

 smaller loss by friction, and thus the sound may be heard at a greater 

 distance than in still air. But on this point, as well as others, further 

 experiments are required. 



While we consider the wind as the principal agent in producing the 

 abnormal phenomena of sound, we do not by any means regard it as 

 the sole agent. Prof. Osborn Reynolds, of Owens College, IManchester, 

 without any knowledge of what was doing in America on this subject, 

 instituted a series of experiments on the eifect of wind upon sound, and 

 finally adopted precisely the same hypothesis which we have used for 

 generalizing the observed phenomena. He has however in a very in- 

 genious and important paper, presented to the Eoyal Society in 1874, 

 extended the same i^rinciple to the effect of heat in changing the form 

 of the sound-wave, and has shown, both by reasoning and experiment, 

 that the normal direction of the sound-wave in still air, instead of pro- 

 ceeding horizontally should be turned upward, on account of the greater 

 velocity of sound near the earth, due to the greater heat of the strata in 

 that position than of those above. This principle, which indicates the 

 existence of a true refraction of sound independent of the motion of 

 the medium, is undoubtedly ai^phcable as a modifying influence to the 

 phenomena we have recorded. It produces however only a slight ef- 

 fect in the case we have last mentioned, since the observation on board 

 the Cactus shows the condition of the air was that of little acoustic ab- 

 sorption. It would nevertheless favor the hypothesis that sound in 

 perfectly still air of homogeneous density could be heard farther than 

 sound in a moving medium, or in one of unequal teuq^eratiu-e. This is 

 also in accordance with the fact repeatedly observed in arctic regions, 

 in which the sound of the human voice is heard at great distances dur- 

 ing times of extreme cold. In this case, the air is of a uniform temper- 

 ature above and below, but of diminished elasticity, and should, on this 

 account, transmit sound with less intensity ; and yet the audibility is 

 increased, which is explained by the assumption that its stillness and 



