CHAP, iv.] HEARING. 191 



particles of matter, a series of movements of the particles from 

 and to a fixed point. In air and other gases the movements of 

 the particles lead to alternating condensation and rarefaction of 

 the medium, the sound is propagated as waves of alternating 

 condensation and rarefaction, which since the to-and-fro movement 

 of the particles is in the same direction as that in which the 

 undulations are travelling, are spoken of as ' longitudinal ' waves. 

 In liquids the transmission of sound also takes place by longi- 

 tudinal waves of alternating condensation and rarefaction, and 

 sound may travel through solids in the same way. But solids in 

 the form of membranes or plates, strings, and rods may also give 

 rise to sounds by being thrown into bodily vibrations, a rod for 

 instance bending alternately to-and-fro in rapid succession. In 

 such a case the particles of the rod move sensibly in a direction 

 transverse to the long axis of the rod ; and the vibrations of this 

 kind, thus giving rise to sounds, are spoken of as " transversal " 

 vibrations. It will be understood that a rod, membrane, plate or 

 string, may also be the subject of longitudinal vibrations ; but the 

 sound given out by such longitudinal vibrations differs from that 

 given out by transversal vibrations of the same body. A rod, string, 

 or membrane thrown into sufficiently rapid and strong transversal 

 vibrations, will communicate its vibrations to the surrounding air, 

 and so give forth a sound, which will travel through the air in the 

 form of waves of longitudinal vibrations. Conversely, sound 

 travelling through the air in waves of longitudinal vibrations, and 

 striking upon a rod, string or membrane, may throw it into trans- 

 versal vibrations. And this is what takes places in the ear. Aerial 

 waves of sound, in the form of longitudinal vibrations, alternating 

 condensations and rarefaction, of the air, travelling along the 

 meatus, fall upon the tympanic membrane, and throw it into 

 transversal vibrations ; the membrane bends bodily inwards and 

 outwards in time with the condensations and rarefactions of the 

 air in the meatus on its outer surface. 



The vibrations of a rod, a tuning-fork for example, are com- 

 paratively simple in character ; and we find, correspondingly, that 

 a tuning-fork is very limited in its power of ' taking up ' sounds 

 from the air, of being thrown into vibrations by sounds falling 

 upon it ; it will only take up from the air the particular sounds, 

 the particular tones as we shall presently call them, which it itself 

 gives forth when thrown into vibrations by being struck. The 

 vibrations of a membrane are much more complex ; and for this 

 reason a membrane takes up much more readily a variety of 

 different sounds reaching it through the air. Still every membrane 

 has its fundamental tone or tones, as they are called, those which 

 it naturally gives forth when thrown into vibrations ; and it takes 

 up these from the air much more readily than any other sounds. 

 It is a feature of the tympanic membrane that it takes up, without 

 any marked distinction, a very great variety of sounds within a 



