160 



,, same manner as a r.i\ <.>' 

 4j light, because the vibra- 

 tion of sound is propa- 



gated in right lines, and 



therefore the angle of re- 

 flection must always be 

 ^V -I equal to the angle of inci- 

 dence. Hereis a diagram 

 Fig. 45. to ex pi a i n M -hat I mean. 

 Let A B be a wall or any dense surface, 

 and suppose that a sound is propagated, 

 from F and travels in an oblique line, it 

 will impinge upon the wall A B, in the 

 direction F c, and be reflected from c in 

 the line c E, forming an oblique angle with 

 the wall A B. If the sound was emitted 

 from D, and proceeded in the straight line 

 D c, it would return again in the same 

 line, and be heard again at D, producing 

 what is called an echo. 



161. T. What is an echo ? 



p. It is a reflected sound, and may be 

 double, triple, or even quadruple, accord- 

 ing to the number of surfaces from and to 

 which the sound may be reflected. 



162. T. Does the reflection of sound 

 always produce an echo? 



P. No; because instead of producing 

 an echo, it may con- 

 centrate the vibra- 

 tions so as to make 

 the sounds heard 

 very distinctly at a 

 l great distance. For 

 example, if the sound 



is repeatedly reflected li'om a curved sur- 

 face ; so that if the sound is propagated or 

 emitted in the focus of one reflecting sur- 

 face, it will be conveyed to the ear placed 

 in the focus of the other reflecting surface. 



S TI(J6. ItaSOii XIV. 



:e i.N a diagram (fig. 41.) that will 

 plain my meaning. Suppose that : 

 like the tick of a watch could not b<- 

 heard in the line A r,, it may U- so in- 

 creased bv reflection as to be disiineth 

 heard Thus it may go from A to < . and 

 from c to B, by a motion of lesser reil <- 

 j tions from d e c b a, alternately terminal 

 , ing at B. This is the principle of the 

 | Whispering Gallery at St. Paul's Cathedral, 

 j London ; and the speaking-trumpet is alsc 

 I constructed upon the principle of the re- 

 flection of sound. 



163. T. How do you account for dis- 

 cordant sounds like cracked bells? 



P. There is a double vibration, and 

 the sound waves interfere with one an- 

 other ; consequently, we have discord. 



164. T. Why is sound conveyed better 

 by night than by day ? 



P. Because the air is less disturbed by 

 accidental vibrations or currents, and is ol' 

 a more uniform density. 



165. T. Why do windows rattle when 

 a salute is fired? 



P. Because the vibrations of the air 

 are communicated to the glass by imping- 

 ing upon the panes, the same as i 

 wall. 



[See Q. 160.] 



GENERAL QUESTIONS ON LESSON \1\ 



1. How are musical sounds produced? 



2. How many notes are there in music ? 



3. Can you account for the musical 

 notes ? 



4. How does sound travel ? 



5. Explain how echoes are produced. 



(i. Why do we hear discordant sounds ? 



C II E 



A B 



Explanation of Answer No. 15i 



London : Printed by William Tyler, Bolt-court. 



