THE SENSE OF HEARING. 235 



acquainted with this fact, for they apply their ears to the 

 ground to hear the approaching footsteps of their enemy or 

 their prey. An easy way to tell whether a tea-kettle boils, 

 is to touch it with a stick, and place the other end to the 

 ear. On this principle, I suppose, watchmen strike the curb 

 stone with their clubs, as the sound is communicated much 

 farther than it was formerly by means of their rattle. So 

 also a musical box, when held in the hand is scarcely heard, 

 but when placed on a bureau or sounding board, it " dis- 

 courseth most eloquent music." The stethoscope now used 

 by physicians to assist in determining the nature of many 

 diseases is derived from a knowledge of this principle. It is 

 merely a cylinder of wood, about an inch in diameter, one 

 end of which is placed on the surface of the body over the 

 diseased part, and the other applied to the ear ; sound is thus 

 carried along the tube, and most important aid is derived to 

 the skilful physician, in ascertaining the nature and seat of 

 all diseases of the heart and lungs. 



18. The transmission of sound is affected materially by 

 the condition of the atmosphere, with respect to tempera- 

 ture, moisture, &c. During the night, when the air is still, 

 and of uniform density and temperature, sounds are heard 

 to a great distance ; but when it is loaded with vapors, as in 

 a fall of snow or rain, sounds are more limited, and rendered 

 confused and indistinct. 



19. The density of the air has a great effect upon the 

 transmission of sound. In a dry, cold atmosphere, at the 

 level of the sea, sounds are transmitted to vast distances, 

 while on high mountains, such as the Andes, or even Mont 

 Blanc, the report of a pistol is not louder than that of an 

 Indian cracker. The wind, also, has a great influence in 

 aiding or retarding the transmission of sound. Sound is 

 also reflected like light, and indeed is subject to the same 

 laws, for the angle of reflection is always equal to the angle 

 of incidence. Reflected sound is termed an echo. The 

 rolling of thunder is supposed to depend partly on the sound 



