292 THE SPECIAL SENSES 



85. 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. 



86. 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. 



87. 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. 



business, any more than it is the apothecary's to know about disease. 

 The advice of the ophthalmic surgeon will also be found invaluable as to 

 how to wear glasses, whether springs or spectacles, so as not to fatigue 

 the eyes by straining them from improper use of these invaluable aids to 

 man's happiness. We conclude by saying, that all advice in this article 

 applies to those having normal, healthy, strong eyes." Dr. B. Joy 

 Jeffries. 



85. Hearing ? What is sound ? How propagated commonly ? Stone thrown in water ? 



86. Sound-wave in the atmosphere ? Its shape ? Kate of motion ? Sound in water, air, 

 and solid bodies ? 



87. The earth 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 ? 



