ACOUSTICS. 51 



scarcely distinguishable. Sound is said to travel in water 

 at the rate of nearly 50OO feet in a second ; and a bell 

 sounded under water may be heard at a very considerable 

 distance : it does not produce a tone, but a noise like 

 the clashing of knives. 



Stone is, next to water, the best conductor of sound. 

 Brick is nearly as good a conductor : a soft whisper has 

 been conveyed by a garden-wall, so as to be distinctly 

 heard at the distance of 200 feet. Wood is also a good 

 conductor ; and its vibratory nature renders it pecu- 

 liarly fit for musical instruments, as well as for the 

 lining of theatres, &c. 



Sound, like light, may be collected into one point as a 

 focus, and will be more audible there than at any other 

 point. In buildings of an elliptical shape, a whisper in 

 one of the foci may be distinctly heard in the other focus. 

 The concert-rooms at Edinburgh are of an elliptical 

 shape, and are so contrived that the musicians sit in one 

 focus of the ellipse, and the audience in the other. A 

 person speaking in the lowest tone, in the whispering- 

 gallery of St. Paul's, is distinctly heard on the opposite 

 side : also a person in one of the recesses of Westminster 

 Bridge readily hears another person speaking on the op- 

 posite side. 



When sound strikes any object, and is reflected back, 

 it forms an Echo. Caverns, mountains, and buildings 

 are favourable to this reverberation ; but unless a person 

 stands more than 60 feet from the reflecting object, he 

 will not be able to hear the echo of his own voice dis- 

 tinctly. The echo is in some situations repeated several 

 times successively. Near Milan there is an echo that 

 returns the sound of a pistol more than 50 times. An 

 echo in Woodstock Park returns 17 syllables in the day- 

 time, when the air is brisk, and 20 in the nieht-time. 

 The air at night being denser, the vibrations become 

 slower, and a repetition of more syllables is heard. 



For the purpose of the augmentation of sound, an in- 

 strument called a Speaking-trumpet has been invented, by 

 means of which the sound is reflected from the sides of 

 the tube, and prevented from spreading in the open air : 

 it is much used at sea for hailing vessels at a distance. 

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