530 Royal Society :^ 



of a rarefaction aloue, it will be transmitted with the velocity « (1 + e) 

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

 It is here sho\^^l also incidentally, that whetlier 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 aifecting 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 « liriori 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, iiowever, 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 

 accordingly endeavours to establish, by arguments derived from the 

 structure of the ear, that aerial rarefactions are alone capable of 

 stimulating that organ in man. These arguments are briefly as 

 follows : — 



1 . The tympanal membrane being convex inwards, a condensation 

 could only affect the air in the tympanal cavity by stretching the 

 membrane, which would cause an expenditure of force, whereas a 

 rarefaction would produce the effect by a simple flexure of the 

 , membrane. 



2. The sense of hearing being certainly produced by the motion 

 of the fluid in the labyrinth, which is a closed vessel filled with an 

 incompressible fluid, the requisite motion could not be produced by 

 a compression of the atmosphere. 



3. The disposition of the muscles of the ear is such as is calculated 

 to assist and regulate the impressions produced by rarefactions rather 

 than those produced by condensations. 



4. The existence of the Eustachian tube is indispensable to the 

 action of the organ (when all its other parts are in a normal state), 

 on the supposition that sound is occasioned by rarefaction, whereas 

 its uses are not satisfactorily predicable on the contrary hypothesis. 



