88 INNERVATION. [CHAP. xviu. 



It is necessary for the reader to bear in mind that the organ of 

 hearing may be affected in two ways: first, through the external 

 ear, and secondly, through the bones of the head. Every person 

 must have noticed the difference in the sound of the ticking of a 

 watch if it be held near the ear but not in contact with it, and if it 

 be held between the teeth. The waning note of the vibrating 

 tuning-fork seems revived when the stem of the fork is brought in 

 contact with the teeth or with any part of the head. These differ- 

 ences are due to the difference of the medium through which the 

 sonorous undulations are made to affect the auditory nerve. In the 

 former instance, hearing is excited through the external ear, when 

 the watch is held near that part, and through the bones of the 

 head, when the watch is brought into contact with the teeth. And 

 in the example of the tuning-fork, the sound appears to revive when 

 it is made to affect the nerve through a medium (the bones of the 

 head) which more readily vibrates in unison with the most delicate 

 oscillations of the sounding body. 



I . Of the External Ear. The external ear consists of two parts, 

 the auricle, and the meatus auditorius externus. The complete de- 

 velopment of the former is found only in mammalia, in which class 

 it exists pretty generally; with, however, considerable diversity of 



succeed each other, and any two sounds produced by the same number of vi- 

 brations or impulses in the same time, are said to be in unison. 



The loudness or intensity depends upon the violence and extent of the pri- 

 mitive impulse. 



The quality is supposed by Herschel to depend on the greater or less abrupt- 

 ness of the impulses ; or generally on the law which regulates the excursions 

 of the molecules originally set in motion. 



8. The velocity with which sound travels is, however, quite independent of 

 its intensity or of its tone ; sounds of every pitch, and of every quality, travel 

 with the same speed through the same medium, as is proved by the fact, that 

 distance does not destroy the harmony of a rapid piece of music played by a 

 band. 



9. Water propagates sound with much greater velocity than air does. Col- 

 ladon concludes, from numerous observations, that the velocity of sound in 

 water, at 40 Fahr., was at the rate of 4708 feet in a second. 



10. According to Biot, cast-iron propagates sound at the rate of 11,090 feet 

 in a second. 



I 1. Sonorous undulations in passing from one medium to another always ex- 

 perience a partial reflexion, and when they encounter a fixed obstacle, they are 

 almost wholly reflected. The reflexion of sound occurs according to the same 

 law which regulates the reflexion of light, namely, the angle of reflexion is 

 equal to the angle of incidence. 



12. The phenomena of echoes result from the reflexion of sound from any 

 prominent object. 



