382 Royal Institution : — Prof. Tyndall on the Acoustic 



Meanwhile this investigation has given us a knowledge of the 

 atmosphere in its relation to sound, of which no notion had been 

 previously entertained. "While the velocity of sound has been the 

 subject of refined and repeated experiments, I am not aware that, 

 since the publication of a celebrated paper by Dr. Derham in the 

 ' Philosophical Transactions ' for 1708, any systematic inquiry has 

 been made into the causes which affect the intensity of sound in the 

 atmosphere. Derham' s results, though obtained at a time when 

 the means of investigation were very defective, have apparently 

 been accepted with unquestioning trust by all subsequent writers — 

 a fact which is, I think, in some part to be ascribed to the a priori 

 probability of his conclusions. 



Thus Dr. Eobinson, relying apparently upon Derham, says, 

 " Pog is a powerful damper of sound," and he gives us physical 

 reason why it must be so. " It is a mixture of air and globules of 

 water, and at each of the innumerable surfaces where these two 

 touch, a portion of the vibration is reflected and lost." And he 

 adds further on, " The remarkable power of fogs to deaden the re- 

 port of guns has been often noticed." 



Assuming it, moreover, as probable that the measure of " a fog's 

 power in stopping sound " bears some simple relation to its opacity 

 for light, Dr. Eobinson, adopting a suggestion of Mr. Alexander 

 Cunningham, states that " the distance at which a given object, 

 say, a flag or pole, disappears may be taken as a measure of the 

 fog's power " to obstruct the sound. This is quite in accordance 

 with prevalent notions ; and granting that the sound is dissipated, 

 as assumed, by reflection from the particles of fog, the conclusion 

 follows that the greater the number of the reflecting particles, the 

 greater will be the waste of sound. But the number of particles, 

 or, in other words, the density of the fog, is declared by its action 

 upon light ; hence the optical opacity will be a measure of the 

 acoustic opacity. 



This, I say, expresses the opinion generally entertained, " clear 

 still air " being regarded as the best vehicle for sound. "We have 

 not, as stated above, experimented in really dense fogs ; but the 

 experiments actually made entirely destroy the notion that clear 

 weather is necessarily better for the transmission of sound than 

 thick weather. Some of our days of densest acoustic opacity 

 have been marvellously clear optically, while some of our days of 

 thick haze have shown themselves highly favourable to the trans- 

 mission of sound. Were the physical cause of the sound-waste 

 that above assigned, did that waste arise in any material degree 

 from reflection at the limiting surfaces of the particles of haze, this 

 result would be inexplicable. 



Again, Derham, as quoted by Sir John Herschel, says that 

 " falling rain tends powerfully to obstruct sound." "We have had 

 repeated reversals of this conclusion. Some of our observations 

 have been made on days when rain and hail descended with a per- 

 fectly tropical fury ; and in no single case did the rain deaden the 

 sound ; in every case, indeed, it had precisely the opposite effect. 



