£CHO. 68 



tlistances; If a ship at sea in distress fires a gun, the light 

 of which is seen on shore twenty seconds before the report 

 is heard, it is therefore known to be at the distance of twenty 

 times eleven hundred and forty-two feet, or a little more than 

 four miles and one third. By counting the number of seconds 

 elapsed between the flash of lightning and the clap of 

 thunder, you may ascertain how far distant you are from 

 the cloud. 



When the aerial vibrations meet with an obstacle, having 

 a hard and regular surface, such as a wall or rock, they are 

 reflected back to the ear, and produce the same sound a 

 second time ; but the sound will then appear to proceed from 

 the object by which it is reflected. If the vibrations fall 

 perpendicularly on the obstacle, they are reflected back in 

 the same line ; if obliquely, the sound returns obliquely in 

 the same direction. This reflected sound is called an echo. 

 At Rosneath, near Glasgow, there is an echo that repeats a 

 tune, played with a trumpet, three times, completely and 

 distinctly. At Brussels there is an echo that answers fifteen 

 times ; and in Italy, near Milan, the sound of a pistol is re- 

 turned fifty-six times. Speaking trumpets, and those made 

 to assist the hearing of deaf persons, depend on the reflec- 

 tion of sound from the sides of the trumpet, and also by its 

 being confined and prevented from spreading in every di- 

 rection. 



Questions. — 1. From what does sound arise ? 2. Upon what three 

 circumstances does the production of sound depend ? 3. What is 

 sound, strictly speaking ? 4. How can it be shown that air is neces- 

 sary to the production of sound ? 5. Why cannot a bell be heard in 

 an exhausted receiver ? 6. What are conductors of sounds besides the 

 atmosphere ? (Ans. water, wood, flannel.) Tie a piece of iron or any 

 metal to the middle of a strip of flannel, 2 or 3 ft. long. Press the 

 ends of the flannel in your ears, and if the metal be struck against iron, 

 you will hear a sound like that of a heavy church bell. 7. How is the 

 tremulous motion of the air as produced by a sonorous body illustrated .'' 

 8. What is said of the velocity of sound ? 9. Ship at sea ? 10. Dis- 

 tance of lightning ? 11 . How is the sound of an echo produced ? 12. 

 Describe the speaking trumpet, fig. 20. 



Note. The science which treats of sound in general is called 

 acoustics. 



