230 OF SOUND. 



varices as are used in optics. From this property 

 of reflection, it happens that sounds uttered in one 

 focus of an elliptical cavity are heard much mag- 

 nified in the other focus. 



Buildings constructed of certain shapes, and 

 also mountains, have this property of reflecting 

 sounds in a remarkable manner. The whispering 

 gallery of St. Paul's is a well-known instance. At 

 Keswick lake, the v firing of a cannon is repeated 

 from the mountains many times, so as to resemble 

 the noise of a distant battle. At Woodstock park 

 in Oxfordshire, an echo repeats seventeen times in 

 the day, and twenty times in the night. 



Sound is conveyed quicker through solid bodies 

 than through the air. Hassenfratz found that 

 when he struck with a hammer the top of a wall, 

 the sound was heard double by a person at the 

 bottom, one sound travelling through the wall, 

 and the other more slowly through the air. 



Sounds are conveyed to great distances through 

 tubes, and upon this property is founded a very 

 useful contrivance, called acoustic or speaking tubes, 

 which are now fixed up in houses for the purpose 

 of speaking from one story to another. 



The Invisible Girl, with which the public was 

 some time puzzled and amused, was constructed 

 upon a principle nearly similar. This exhibition 

 consisted of a hollow copper ball, to which was at- 

 tached four trumpets, and which was suspended by 

 ribbons from the four corners of a frame resembling 

 a bed-post, and having no other connexion with 

 the frame. The globe was supposed to contain the 

 invisible being, as the voice apparently proceeds 

 from the interior of it. If a question was asked 

 by speaking into one of the trumpets, an answer 

 was returned in a low female voice proceeding 



