IN PHYSICS 215 



17. Why will the report of a cannon fired in a valley be 

 heard on the top of a neighboring mountain, better than one 

 fired on the top of a mountain will be heard in the valley ? 



A sound always has the intensity given it by the 

 density of the atmosphere where it originated, and not of 

 that where it is heard. 



(See Tyndall's Lectures on Sound, p. 40.) 



1 8. Why do our footsteps in unfurnished dwellings sound 

 so startlingly distinct t 



In furnished rooms, the chairs, carpets, pictures, etc. 

 break up the echoes. Then, also, our footsteps are 

 louder on an uncarpeted floor. 



19. Why do the echoes of an empty church disappear 

 when the audience assemble ? 



The audience break up the echoes which interfere with 

 the original sound. Wires strung across a lofty room 

 often serve the same purpose. 



20. What is the object of the sounding-board of a piano ? 



By its vibrations and those of the body of air which 

 it encloses, it reinforces the sound of the wires. 



21. During some experiments, Tyndall found that a 

 certain sound would pass through twelve folds of a dry silk 

 handkerchief, but would be stopped by a single fold of a wet 

 one. Explain. 



(See Tyndall's Lectures on Light, p. 325, for a series of experiments 

 showing the action of moisture in propagating the sound-waves.) 



22. What is the cause of the musical murmur often 

 heard near telegraph lines ? 



