225 



Reverting to the equation of sound, which (neglecting terms of the 

 second order) may be put under the form 



the author next shows that if the initial disturbance consist of a con- 

 densation alone, it will be transmitted with the velocity (1 e) in 

 the direction in which its particles are moving ; and that if it consists 

 of a rarefaction alone, it will be transmitted with the velocity a(l + e) 

 in the direction contrary to that in which its particles are moving. 

 It is here shown also incidentally, that whether the resistance be 

 taken into account or not, the particles of a wave of condensation 

 must all move in the same direction, which will be the direction of 

 transmission ; and the particles of a wave of rarefaction will all move 

 in the same direction, which will be contrary to that of transmission. 



In confirmation of the conclusion that waves of rarefaction are 

 transmitted more rapidly than waves of condensation, the author 

 adduces the fact, that when explosions of gunpowder have taken 

 place, the glass in windows has been observed to break outwards 

 rather than inwards. 



It is then suggested, that, as when sound is produced, a conden- 

 sation and rarefaction of air usually occur in immediate succession, 

 if both kinds of disturbance were capable of affecting the human ear, 

 we should hear sounds double ; and as we know practically that this 

 is not the case, it is contended that only one kind of disturbance, 

 i. e. either rarefaction alone, or else condensation alone, can stimulate 

 the ear. 



It is shown to be a priori probable, that if one of the two classes 

 of aerial disturbance is suppressed by the ear, that one would be dis- 

 turbance by condensation, inasmuch as waves of rarefaction being 

 swifter, would better perform the duty entrusted to them : and it is 

 pointed out that if the sensation of sound is produced by aerial rare- 

 factions alone, a difficulty attending the received theory will be ob- 

 viated, by reason of the velocity deduced upon that theory being too 

 small. 



The author considers, however, that the question, whether either 

 and which of the kinds of aerial disturbance is suppressed, can only 

 be satisfactorily determined by examination of the ear itself. He 



R 2 



