990 CONDUCTION THROUGH TYMPANUM. [Book hi. 



that in which the undulations are travelling, are spoken of as 

 'longitudinal' waves. In liquids the transmission of sound also 

 takes place by longitudinal 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 vibra- 

 tions, and striking upon a rod, string or membrane, may throw 

 it into transversal vibrations. And this is what takes place in 

 the ear. Aerial waves of sound, in the form of longitudinal 

 vibrations, alternating condensations and rarefactions, of the 

 air, travelling along the meatus, fall upon the tympanic mem- 

 brane, and throw it into transversal vibrations ; the membrane 

 bends bodily inwards and outwards in time with the condensa- 

 tions 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 vibra- 

 tions ; 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 very large range. It probably has 

 a fundamental tone of its own, but this is kept in the back- 

 ground ; it is not prominent, and does not materially influence 

 our hearing. Were it otherwise, were the tympanic membrane 



