400 SPECIAL PHYSIOLOGY. 



concussion, friction, or other evidences of force; these disturbances 

 have been proved to be delicate undulations, capable of regular propa- 

 gation in all directions. The presence of some matter to be thrown 

 into vibration is indispensable ; for sounds can neither be produced in, 

 nor propagated through, a vacuum. 



Sonorous vibrations may originate in solid, liquid, or aeriform bodies. 

 Their propagation to the ear may take place through either of these 

 media. The rate at which sound travels in air is about 1050 feet per 

 second. In water it travels about 4 times, and in highly elastic solid 

 bodies from 7 to 18 times, as rapidly as in air. Sounds are transmitted 

 most readily from solids to solids. In passing from solids to water 

 they undergo a certain loss, and from solids to air a much greater 

 loss. From water to solids they pass easily ; but from water to air and 

 from air to water, with very great difficulty. In their passage from 

 air to solids they undergo very considerable loss in their intensity. 

 It will thus be seen that the principal impediments to transmission 

 occur in the passage of vibrations to and from solids and air, and to 

 and from water and air ; but by the interposition of certain peculiar 

 arrangements of solid materials, in the form of elastic membranes, these 

 impediments are almost entirely overcome ; for a tense and dry mem- 

 brane is easily made to vibrate, by sonorous undulations in air, and 

 can, in return, readily excite them in air; moist membranes, on the 

 other hand, scarcely vibrate under these circumstances. A tense mem- 

 brane, however, placed between water and air, facilitates very consider- 

 ably the passage of sonorous undulations in either direction. The trans- 

 mission of the sound-waves, in both cases, is rendered easier, when 

 some perfectly solid body is combined with the membrane, though still 

 it is less easy than the transmission of sonorous undulations from water 

 to solids, or from solids to water. The physical action of such mem- 

 branes is of great importance in reference to the passage of sonorous 

 vibrations through the acoustic apparatus of the ear. Membranes, 

 whether tight or loose, conduct sounds in water without loss. In their 

 passage from solids to water, sonorous undulations seem to reinforce 

 the undulations in the water itself; this is more particularly the case 

 in the vicinity of the solids. 



In being propagated through the air or other medium, sounds lose 

 their intensity, according to the distance through which they travel. 

 From their source they are propagated equally in all directions ; and 

 therefore, like light, they diminish in force or intensity, according to 

 the square of the distance. Thus, sounds heard at 2, 3, or 4 times a 

 certain distance, are diminished in intensity in the ratio of 4, 9, or 16. 



When atmospheric sound-waves meet with the surface of any oppos- 

 ing body, they are in part returned or reflected from it; the angle at 

 which the reflection takes place is equal to the angle at which the sound- 

 waves strike the surface, i. e., the angle of reflection is equal to the 

 angle of incidence. The reflection is sometimes almost entirely com- 

 plete, as happens when the opposing body is fixed, solid, and very rigid. 

 Sounds are reflected in water as well as in air. Some of the atmos- 

 pheric sound-waves, instead of undergoing reflection, communicate to, 



