168 EXPERIMENTAL GENERAL SCIENCE 



account. Sound moves most rapidly in elastic bodies. Since 

 adding moisture to the air increases its elasticity, we commonly 

 hear distant sounds more distinctly just before a storm. 

 Sound waves may be thought of as a series of alternating com- 

 pressions and rarifications in the mediums through which they 

 pass. They tend to spread out in all directions from their 

 source and thus, coming constantly into contact with a greater 

 number of molecules which must be caused to vibrate, they 

 gradually lose their energy and diminish in intensity. When 

 sound waves are kept from spreading, as in the speaking-tube 

 or megaphone, they carry much farther. The reason one can 

 usually hear sounds so distinctly across the water is because 

 the layers of denser air above the water prevent the sound 

 from rising. 



140. Echoes. Sound, like light, may be 

 turned or reflected, and can also be brought 

 to a focus. When reflected, the angle of in- 

 -, cidence equals the angle of reflection. 



4LJ When a reflected sound reaches our ears after 



vr the original sound has ceased, we call it an 



FIG. 61. Tun- echo but if it reaches us in a shorter period of 

 01 (Z>wjn at ~ ti me > ^ usually serves merely to strengthen 

 the original sound. Since the sensation of 



sound persists about Ho second, the reflecting surface must be 

 at least 56 feet away to cause an echo. If nearer, the sound 

 waves would have time to go and return before the original 

 sound ceased. Echoes, however, may be produced from sur- 

 faces much nearer the observer, but, in such cases, the waves 

 are reflected from one surface to another just as light may be 

 reflected from one mirror to another. The usual reflecting sur- 

 face is a wall, wood, or cliff, but, on occasion, clouds or layers of 

 air of different density may serve as reflecting surfaces also. A 

 sounding-board is often placed back of the speaker's platform 

 in large rooms to reinforce the tones of the speaker. When 



