NATURE OF SOUND. 281 



FIG. 221. 



(a.) In the experiment above described, it is plain that many of 

 the rays reflected by the first mirror are intercepted before they 

 reach the second mirror. This may be remedied, in part, by the 

 use of an ear-trumpet, the larger end being held at the focus of the 

 second reflector. The ear-trumpet may be a glass funnel, with a 

 piece of rubber tubing leading from its smaller end to the ear. The 

 experiment may be modified by using a single reflector, the watch 

 being placed a little further from the reflector. The proper positions 

 for the watch and the funnel are easily determined by experiment 

 They are conjugate foci ( 602). 



442. Echo. When a sound, after reflection, is 

 audible, it is called an echo. The distinctness with 

 which it is heard depends upon the distance of the ear 

 from the reflecting surface. A very quick, sharp sound 

 may produce an echo even when the reflecting surface is 

 not more than fifty or sixty feet away, but for articulate 

 sounds a greater distance is necessary. 



(a.) Few, if any, persons can pronounce distinctly more than 

 about five syllables in a second. At the ordinary temperature, 

 sound travels about 1120 feet per second. In a fifth of that time 

 it would travel about 224 feet. If, therefore, the reflecting surface 

 be 112 feet distant, the articulate sound will go and return before 

 the next syllable is pronounced. The two sounds will not inter- 

 fere, and the echo will be distinctly heard. If the reflecting sur- 

 face be less than this distance, the reflected sound will return before 



