280 NATURE OF SOUND. 



greater distance is very much diminished. This lesser energy is 

 unable to excite the auditory nerve to action, and the ticking of the 

 watch is unheard. 



440. Reflection of Sound. When a sound ray 

 strikes an obstacle, it is reflected in obedience to the prin- 

 ciple given in 97. This fact is turned to account in the 

 case of "conjugate reflectors" of sound. Fig. 220 repre- 

 sents the section of two parabolic reflectors mn and op. 

 It is a peculiarity of such reflect- 

 ors that rays starting from the JP 



focus, as F, will be reflected as > 

 parallel rays, and that parallel rays 

 falling upon such a reflector will 

 converge at the focus, as F' . 

 Hence, two such reflectors may 

 be placed in such a position that FlG 22Q 



sound waves starting from one 



focus shall, after two reflections, be converged at the other 

 focus. Two reflectors so placed are said to ~be con- 

 jugate to each other. This principle underlies the 

 phenomena of whispering galleries. 



(a.) " The great dome of St. Paul's Cathedral in London is so con- 

 structed that two persons at opposite points of the internal gallery, 

 placed in the drum of the dome, can talk together in a mere whisper. 

 The sound is transmitted from one to the other by successive reflec- 

 tions along the course of the dome." A similar phenomenon is 

 observable in the dome of the Capitol at Washington. 



441. Experiment. At the focus of a curved re- 

 flector, place a watch or other suitable sounding body. 

 Directly facing it, but at a distance so great that the 

 ticking is unheard, place a similar reflector. When the 

 ear is placed at the focus of the second mirror, as shown in 

 Fig. 221, the ticking is plainly heard. 



