SCIENCE OF COMMON THINGS. 



115 



Construction of the ear. 



Air not necessary to sound. 



fig. 29. 



Fig. 30. 



many animals, so that they can direct it to the place from which the 

 sound comes. 



2. The sound concentrated at the bottom of the ear-tube falls upon a 

 membrane stretched across the channel, like the parchment of an ordi- 

 nary drum, over the space called the tympanum, or drum of the ear, b, 

 and causes the membrane to vibrate. That its motion may be free, 

 the air contained within the drum has free communication with the 

 external air by the open passage, /, called the eustachian tube, leading to 

 the back of the mouth. A degree of deafness ensues when this tube is 

 obstructed, as in a cold ; and a crack, or sudden noise, with immediate 

 return of natural hearing, is generally experienced when, in the effort of 

 sneezing or otherwise, the obstruction is removed. 



3. The vibrations of the membrane of the drum are conveyed further 

 inwards, through the cavity of the drum, by a chain of four bones (not 

 here represented on account of their minuteness), reaching from the 

 centre of the membrane to the oval door or window, leading into the 

 labyrinth e. 



4. The labyrinth, or complex inner compartment of the ear, over which 

 the nerve of hearing is spread as a lining, is full of watery fluid ; and, 

 therefore, by the law of fluid pressure, when the force of the moving 

 membrane of the drum, acting through the chain of bones, is made to 

 compress the water, the pressure is felt instantly over the whole cavity. 

 The labyrinth consists of the vestibuk, e, the three semicircular canals, c, 

 imbedded in the hard bone, and a winding cavity, called the cochlea, d, 

 like that of a snail-shell, in which fibres, stretched across like harp-strings, 

 constitute the lyra. The separate uses of these various parts are. not yet 

 fully known. The membrane of the tympanum may be pierced, and the 

 chain of bones may be broken, without entire loss of hearing. ARNOTT. 



*7S4: Is air necessary to the production of sound ? 



' No ; but most sounds owe their origin to the vibra- 

 tions of the air. Sound can be produced under water, 

 and all bodies are, in fact, more or less fitted to produce 

 the sound vibrations ; in many cases air is neither the 

 quickest nor the best conductor of sound. 



735 Upon what does the loudness of sound conveyed by air depend f 



Upon the density of the air ? 



