PHILOSOPHICAL SOCIETY OF WASHINGTON. 41 



of sounds moving against the wind and at right angles to the wind, 

 that they could both be better heard on the top of a high tower 

 than on the surface of the ground.* 



Baron Humboldt, in observations made on the intensity of 

 sounds at the Falls of the Orinoco, remarked their greater audi- 

 bility by night than by day, and referred their comparative weak- 

 ness by day to the effect of atmospheric disturbances arising from 

 ascending currents of rarified air and descending currents of 

 heavier air, which broke up the homogeneity of the atmosphere, 

 and thereby obstructed the transmission of sound. It is a necessary 

 complement of this hypothesis that sound which fails to be trans- 

 mitted through the atmosphere, because of " the reflections which 

 it endures at the limiting surfaces of the rarer and the denser air," 

 is liable to be returned to the hearer in the shape of aerial echoes 

 rebounding from the acoustic cloud which the primary sound is not 

 able to pierce ; and hence the logical place assigned to echoes by 

 Dr. Tyndall, when, adopting and applying the Humboldt hypothe- 

 sis, he says that " rightly interpreted and followed out, these aerial 

 echoes lead to a solution which penetrates and reconciles the phe- 

 nomena from beginning to end." " On this point," he says, " I 

 would stake the issue of the whole inquiry. * * * The echoes 

 afford the easiest access to the core of this question." f 



The conflicting hypotheses of Humboldt and Stokes, as respec- 

 tively applied by Tyndall and Henry in interpreting the abnormal 

 phenomena of sound, are here cited as prefatory to some much 

 older observations made under the same head by Dr. W. Derham 

 in his elaborate paper entitled " Experiments and Observations on 

 the Motion of Sound, and other things pertaining thereto," as read 

 before the Royal Society in 1708. This paper, written in Latin, is 

 the report of a systematic inquiry into phenomena pertaining to 

 the velocity and motion of sounds, and treats only incidentally on 

 the intensity of sounds ; but, nevertheless, it contains some inter- 

 esting statements under this latter head.| 



The subject of echoes is the first which engages the writer's atten- 

 tion. He says that echoes produced by sound-reflecting objects situ- 

 ated near a sounding body may sometimes be heard through many 



-Rep. of Light-House Boai-d, 1875, P- ll 9- 



f" Sound," p. xxiv. 



J Phil. Trans, of Royal Society, Jan. and Feb., 1708. 



