216 THE SPECIAL SENSES. 



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 discriminate between their tread and that of a 

 herd of 'buffaloes. Solid substances also convey sounds 

 with greater power than air. If the ear be pressed against 

 one end of a long beam, the scratching of a pin at the, 

 other extremity may be distinctly heard, which will not be 

 at all audible when the ear is removed from the beam. 

 Although air is not the best medium for conveying sound, 

 it is necessary for its production. Sound cannot be pro- 

 duced in a vacuum, as is shown by ringing a bell in the 

 exhausted receiver of an air-pump, for it is then entirely 

 inaudible. But let the air be readmitted gradually, then 

 the tones become more and more distinct, and when the 

 receiver is again full of air, they will be as clear as usual. 



88. All sonorous bodies do not vibrate with the same 

 degree of rapidity, and upon this fact depends the pitch of 

 the sounds that they respectively produce. The more fre- 

 quent the number of vibrations within a given time, the 

 higher will be the pitch ; and the fewer their number, the 

 lower or graver will it be. Now, the rate of the successive 

 vibrations of different notes has been measured, and it has 

 thus been found that if they are less than sixteen in a 

 second, no sound is audible; while if they exceed 60,000 

 per second the sound is very faint, and is painful to the 



87. The earth as a conductor of sound ? To what has the western Indian been 

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

 air necessary ? Sound in a vacuum ? 



