214 



GENERAL SCIENCE 



FIG. 190. Diagram Illustrating the Way 

 Sound Travels. 



compressed. As it expands it compresses a second layer 

 and so on, so that the volume of the air affected is made 



up of layers of com- 

 pressed and rarefied 

 air (Figure 190). 



Waves pass over 

 fields of standing 

 grain ; but the grain 

 simply bends under 

 the pressure of the wind and rises again. Wave motion 

 may be shown by the following experiment. 



Experiment 61. Attach one end of a small rope about 25 feet 

 long to some solid object such as the wall. Hold the other end of 

 the rope in the hand and cause waves to run along it by quick 

 movements of the hand. The waves pass from one end of the 

 rope to the other, but the particles of the rope do not move 

 forward. 



Echoes are due to the reflection of sound. When we 

 speak, the sound waves often strike some reflecting sur- 

 face and are returned to the source. If the reflecting sur- 

 face is near, as in the case of the walls of a small room, 

 the echo will not be noticed ; but in large halls the echoes 

 may be so strong as seriously to inconvenience a speaker. 

 Such effects may be remedied by substituting light, porous 

 materials for the hard surfaces of reflecting walls and by 

 hanging curtains in certain parts of the hall to destroy 

 returning sound waves. In hilly districts echoes are 

 sometimes heard several seconds after the original sound. 



Musical Tones and Noises. - - The human ear quite 

 readily distinguishes between pleasing sounds and those 

 that are not. Pleasing sounds are musical and are pro- 

 duced when the vibrations of a sounding body follow each 

 other at precisely equal intervals of time. If the vibra- 



