1904.1 



on Shadows. 



433 



menon — the ease with which waves travel round the inside of a 

 curved surface. This is the case of the whispering gallery, of which 

 there is a good example in St. Paul's Cathedral. The late Sir George 

 Airy considered that the effect could be explained as an instance of 

 concentrated echo, the sound being concentrated by the curved walls, 

 just as light may be brought to a focus by a concave mirror. From 



Fig. 3. 



-To Illustrate a Simple Mechanical Application of 

 THE Principle of Reciprocity. 



my own observations, made in St. Paul's Cathedral, I think that 

 Airy's explanation is not the true one ; for it is not necessary, in 

 order to observe the effect, tliat the whisperer and the listener should 

 occupy particular positions in the gallery. Any positions will do 

 equally well. Again, whispering is heard more distinctly than 

 ordinary conversation, especially if the whisperer's face is directed 



Fig. 4. — Model Illustrating the Peculiarities op a 

 Whispering Gallery, 



along the gallery towards the listener. It is known that a whisper 

 has less tendency to spread than the full-spoken voice ; thus a 

 whisper, heard easily in front of the whisperer, is inaudible behind 

 that person's head. These considerations led me to form a fairly 

 satisfactory theory of the whispering gallery, nearly twenty-five 

 years ago.* The phenomenon may be illustrated experimentally 



* ' Theory of Sound,' § 287. 



