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IV". THE EAR. 



The vibrations in the air caused by a sounding body, 

 are communicated to the ear, and from that to the brain, 

 by the connexion of which with the mind, the idea of 

 sound is produced. 



The ear is composed of an external ear, so contrived 

 as to collect the vibrations of the air ; of a tube lying at 

 the root of the external ear, through which these vibrations 

 pass to a thin membrane, called the drum of the ear, on 

 account of its resemblance to the head of a drum. This 

 vibrates, and gives motion to four small moveable bones, 

 which are so connected with each other, and with the 

 drum of the ear, that if the drum of the ear vibrate, these 

 bones are put in motion, at the same moment. The last 

 bone of the four, is called the stapes, which in connexion 

 with some complicated parts whose use is not fully un- 

 derstood, conveys the sound to the auditory nerve which 

 proceeds from the brain. When the vibrations thus come 

 to the sensorium, they there produce found. Strictly speak- 

 ing there is no sound, but only a motion of particles, 

 before. A thousand thunders would produce no sound 

 unless there was a living being to hear. Without this 

 they would occasion only silent agitations of the air. 



V. REFLECTION OF SOUND, OR ECHO. 



Suppose a cannon to be fired at a distance from a per- 

 pendicular wall. This causes vibrations in the air, and 

 the particles conveying sound at length reach the wall, 

 and not being able to proceed any further, return back, 

 and produce what we call a reverberation or echo. 



The echo may be illustrated by the following experi- 

 ment. If a pebble be thrown into the centre of a vessel 

 containing water, circular waves will be immediately 

 formed around the place where it falls, and will gradually 

 extend until they reach the sides of the vessel ; when 

 not being able to proceed farther, they will be reflected, 

 as it were, and a new succession of waves will extend 

 towards the centre. The air moves in precisely the same 

 manner in the production of the echo. 



An echo follows every sound which is made, but it 

 generally follows the principal sound so immediately that 

 we do not perceive it. The more distant a sound is from 



