34 ANSWERS TO PRACTICAL QUESTIONS 



10. A meteor of Nov. 13, 1868, is said to have exploded al 

 a height oj '60 miles : what time would have been necessary for 

 its sound to reach the earth ? 



5280 ft. x 60 = 310,800 ft. 

 316,800 ft. -*- 1090 ft. = 290 (sec.) = 4 min. 50 gee. 



11. A stone was let*fall into a well, and in 4 seconds was 

 heard to strike the bottom. How deep was the well? 



(See p. 48.) d = 16 x f. d = 16 x 4 2 = 256 ft. 



1 2. What lime would it require for a sound to travel 5 miles 

 in tke still water of a lake ? 



6280 ft. x 5 = 26,400 ft. 

 26,400 ft. -i- 4700 ft. = 5.0 (sec.) 



13. How much louder will be the report of a gun to an 

 observer at a distance of 20 rods than to one at half a mile? 



160 rod? are 8 times 20 rods. The intensity of the sound is inversely as the 

 square of the distance = 1 / 64 . Hence the sound is 64 times louder to the 

 observer at 20 rods that to the one at half a mile. 



14. 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 the 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. 



15. Why is an echo weaker than the original sound? 

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



reflection. 



1 6. Why is it so fatiguing to talk through a speaking- 

 trumpet ? 



Because so much more air must be set in motion by the 

 vocal chords. The column of air in the resonant cavity of 

 the throat is re-enforced by all the air in the trumpet. 



-When we hear a goblet or a wine-glass struck with the 

 blade of a knife, we can distinguish three sounds, the funda- 

 mental and two harmonics. * 



* Is not the ear the most perfect sense ? A needlewoman will distinguish by 

 the pound, whether it is silk or cotton that is torn. Blind people recognise 



