THE SPECIAL SENSES 305 



86. The Sense of Hearing Sound. Hearing is the special 

 sense by means of which we are made acquainted with sound. 

 What is sound ? It is an impression made upon the organs of 

 hearing, by the vibrations of elastic bodies. This impression 

 is commonly propagated by means of the air, which is thrown 

 into delicate undulations in all directions from the vibrating 

 substance. When a stone is thrown into smooth water, a wave 

 of circular form is set in motion from the point where the 

 stone. struck, which, as it advances, constantly increases in size 

 and diminishes in force. 



87. Somewhat resembling this is the undulation, or sound- 

 wave, which is imparted by a sonorous vibration to the 

 surrounding atmosphere. Its shape, however, is spherical, 

 rather than circular, since it radiates upward, downward, and 

 obliquely, as well as horizontally, like the wave in water. 

 The rate of motion of this spherical wave of air is about 1050 

 feet per second, or one mile in five seconds. In water, sound 

 travels four times as fast as in air, and still more rapidly 

 through solid bodies; along an iron rod its velocity is equal 

 to two miles per second. 



88. The earth, likewise, is a good conductor of sound. It 

 is said that the Indian of our western prairies can, by listening 

 at the surface of the ground, hear the advance of a troop of 

 cavalry while they are still out of sight, and can even discrim- 

 inate between their tread and that of a herd of buffaloes. Solid 

 substances also convey sounds with greater power than air. 



Some have thought, by the early use of spectacles, to be able to preserve 

 their power of vision, and have recommended and employed ' conservative 

 glasses.' If I am not mistaken, self-interest had something to do with 

 this recommendation. So long as the eye does not err, and remains free 

 from fatigue in the work required of it, its own power is sufficient, and it 

 is inexpedient to seek assistance in the use of convex glasses." Dr. 

 B. Joy Jeffries. 



86. Hearing ? What is sound ? How propagated commonly ? 



87. Sound-wave in the atmosphere ? It* shape? Kate of motion ? Sound in air, water, 

 and solid bodies ? 



88. Thfrarth as a conductor of sound ? What has the western Indian been taught? 

 Solid substances as conductors ? As regards sound, in what respect is air necessary ? 

 Sound in a vacuum ? 



