the Mode in which it administers to the Perception of Sound. 119 



pagation materially different from that to which a provisional 

 and imperfect theory would conduct us. 



I have shown that the velocity with which a small disturbance 

 is propagated through air of a given density is not, as the exist- 

 ing theory would teach us, invariably the same whatever the 

 character of the disturbance, — that, on the contrary, the disturb- 

 ances capable of such transmission are divisible into two classes, 

 viz. waves of condensation, in which the density is throughout 

 greater, and waves of rarefaction, in which the density is through- 

 out less than the original density of the air through which the 

 propagation takes place* — in waves of the first kind the velocity 

 of propagation being somewhat less, while in waves of the second 

 kind it is somewhat greater than the calculated velocity given by 

 the existing theory f. 



In arriving at these conclusions I was confronted by this great 

 difficulty, viz. that in a great variety of instances sounding 

 bodies give rise to waves of condensation and waves of rarefac- 

 tion simultaneously ; so that in such instances we should have a 

 double sound whenever the distance of the sounding body from 

 the ear is considerable, unless the ear were so constructed as to 

 suppress one of the two classes of waves. 



So incredible did this latter conclusion appear to me, that 

 nothing but the conviction which reiterated examination had 

 wrought in me of the certainty of the results at which I had 

 arrived would have induced me so much as to examine into the 

 evidence upon the subject. 



But, however perfect might be the parallelism which I was 

 disposed a priori to attribute to waves of condensation and waves 

 of rarefaction as agents for the transmission of sound, the slight- 

 est examination of the auditory apparatus was sufficient to show 

 that no such parallelism exists in their modes of action upon 

 the organ of hearing, or in the contrivances by which the latter 

 is adapted to their reception. 



The slightest examination was sufficient to show, as I propose 

 by and by to point out, that some of the most striking and 

 characteristic features of the auditory mechanism are specially 

 calculated to transmit the action of rarefied waves, are essential 

 to such transmission, and can exercise no function in the trans- 

 mission of condensed waves. Nevertheless a long-cherished 



* Although waves of condensation and waves of rarefaction are very 

 commonly called into play simultaneously, it may be shown, even upon 

 the principles of the existing theory, that waves of either kind are capable 

 of transmission when no waves of the other kind are present. 



t I must be understood to refer here to the theoretical velocity of pro- 

 pagation apart from Laplace's correction, which correction, for the reasons 

 stated in the paper of March last before referred to, I cannot regard as 

 otherwise than untenable. 



