186 SENSE OF HEARING. 



well as those that proceed along the lines D F and F C ; or in other 

 words, it will hear both the sound and its echo ; and, if the surfaces on 



which the sonorous undula- 



Fi s- 75 - tions impinge be favourably 



disposed, the echoes may be 

 very numerous. The utility 

 of the ear trumpet, and 

 speaking trumpet, is to be 

 explained by this law of the 

 reflection of the aerial undu- 

 lations; and some physiolo- 

 gists are of opinion, that the 

 external ear is inservient to 

 audition on similar principles. 



K / The ear trumpet is a tube, 



narrow at one extremity, so 



E as to enter the concha; and 



Reflection of Sound. expanded at the other like a 



trumpet. It is also curved, 



so that it may be easily directed to objects. All the sonorous rays, 

 that enter the expanded extremity, are brought after various reflections 

 to a focus in the auricular end; and the intensity of the sound is, in 

 this way, so much augmented, that a person who, without it, is entirely 

 deaf to common conversation, may enjoy it. A sheet of paper, folded 

 like a cone, the apex of which is placed in the concha, and, to a less 

 extent, the hand held concavely behind the ear, serve a like purpose. 



Air is not the only, nor the most perfect, vehicle of sound. The 

 personal experiments of divers show, that it can be conveyed through 

 water. The blows of workmen around a diving bell are distinctly 

 heard above; and fish have manifestly an acute sense of hearing, 

 although this was at one time denied. Aiy experiment, made by the 

 Abbd Nollet, and repeated by Dr. Franklin, proves, that water trans- 

 mits a much stronger vibration than air. When two stones were struck 

 together under water, a shock was given to the ear, which was almost 

 insupportable. The latter philosopher found by experiment, that 

 sound, after travelling above a mile through water, loses but little of 

 its intensity. According to Chladni, its rate of progression in water is 

 about 4900 feet in a second, or between four and five times as great as 

 in air. Solids, too, are much better conductors of sound than air. If 

 we scratch one end of a wooden rod, the sound is distinctly heard by 

 the ear applied to the other; although it may be inaudible through the 

 air. Savage tribes are in the habit of discovering the advance of 

 enemies, or of their prey, by applying the ear to the ground ; and 

 watchmen, in some towns, instead of springing a rattle, and alarming 

 offenders, strike the pavement with a staff, the sound of which is heard 

 by their fellow-watchmen at a considerable distance. It is a common 

 practice to ascertain, whether a kettle boils, by putting one end of a 

 poker on the lid, and the other to the ear. The difference between 

 simmering and boiling is in this way detected. A knowledge of the 

 ready communication of sound through solids, has given rise to a valu- 



