40 ANSWERS TO PRACTICAL QUESTIONS 



12. What time would be required for a sound to travel 

 five miles in the still water of a lake ? 



s 5x5,280 

 * = = -= 5-6 seconds. 



13. Does sound travel faster at the foot or at the top of 

 a mountain? 



The density and elasticity of the air vary in the same pro- 

 portion ; hence if the temperature were the same on top of a 

 mountain that it is at the foot, the velocity of sound would be 

 the same, but as it is always colder, the velocity is less. 



14. IVJiy is an echo weaker than the original sound ? 



Because the intensity of the sound-wave is weakened at each 

 reflection. In addition to this, the sound which is perceived as 

 an echo has traveled over a much greater distance than that 

 which comes directly from the sonorous body. 



15. IVlvy is it so fatiguing to talk through a speaking - 

 trumpet ? 



Because it is unusual, and unusual effort is necessary to se- 

 cure adaptation to unusual conditions. 



16. WJiy 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.) 



17. Why do our footsteps in unfurnished dwellings 

 sound so startlingly distinct ? 



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

 up the echoes. Then, also, our footsteps are louder on an un- 

 carpeted floor. 



18. Why do the echoes of an empty church disappear 

 when the audience assemble ? 



