LESSONS IN PHYSICS. 57 



syllables, and many other localities furnish interesting echoes. 

 Echoes are sometimes interesting; but sometimes they are 

 annoying, and the usefulness of many large halls or assembly 

 rooms is much diminished by conflicting echoes. Theceilings and 

 walls of such rooms should be so broken up by arches, balconies, 

 beveled angles, etc., that no large areas of plane surface appear; 

 drapery, furniture, etc., tend to break up and disperse the sound 

 waves so as to lessen the effect of reflections. People do not 

 intentionally build whispering galleries, but there are many 

 rooms in which at certain points the slightest noise is conveyed 

 with great intensity to certain other points, while it may not be 

 heard at all at intermediate points. The Mormon Tabernacle, 

 and the dome of St. Paul's Cathedral, London, are illustrations. 

 Sounds or vibrations are not only reflected, but are also re- 

 fracted. The laws of reflection and refraction of vibrations 

 causing sound are similar to those of vibrations causing light, 

 and may be considered in connection with the study of light. 



