NATURE-STUDY LESSONS. 9 



9. Throw a stone into the water and observe the effect. 



10. Infer from the above experiments the nature of 

 vibrations. 



11. Note also that the wave-vibrations grow less and 

 less until they die away altogether. 



C. Vibrations can be Collected. 



12. Observe whether the sound of the voice can be 

 heard better in a building or in the open air. 



13. Infer the reason. 



14. (a) Observe the shape of the ears of certain ani- 



mals that discover their prey by hearing. 

 {b) Infer the purpose of the shape. 



15. (i^) Place the hand behind the ear. 

 (Jb) Listen to different sounds. 



{c) Note the effect of alternately removing and 

 returning the hand. 



16. (a) If you have the opportunity, examine an ear- 



trumpet. 

 {b) Infer why it is thus formed. 



D. Time Necessary for Sound to Travel. 



17. {a) Observe a carpenter some distance from you 



driving a nail. Is the sound heard at the 

 time you see the hammer strike the nail ? 



(b) A train is approaching a crossing. You see 



the steam from the whistle of the engine. 

 When do you hear the whistle ? 



(c) You see a flash of lightning. Is the thunder 



always heard at once? 

 {d) From these observations, what inference as to 

 time can you make with regard to the vibra- 

 tion of air ? 



