198 



WELLS'S NATURAL PHILOSOPHY. 



tween the perception of the original and reflected sounds may be sufficient to 

 prevent them from being blended together. 



"When the original and retlected sounds are blended together, the effect 

 produced is caUed a resonance, and not an echo. 



Thus, the walls of a room of ordinary size do not produce an echo, because 

 the reflecting surface is so near the source of sound that the echo is blended 

 with the original sound ; and the two produce but one impression on the ear. 



Large halls, spacious churches, etc., on the contrary, often reverberate or re- 

 p3at the voice of a speaker, because the walls are so far otT from the speaker, 

 that the echo does not get back in time to blend with the original sound; 

 and therefore each is heard separately. 



The shortest interval sufficient to render sounds distinctly appreciable by 

 the ear, is about l-9th of a second ; therefore when sounds follow at shorter 

 intervals, they wiU form a resonance instead of an echo ; so that no reflecting 

 surface wQl produce a distinct echo, unless its distance from the spot where 

 the sound proceeds is at least 62i feet ; as the sound will in its progress in 

 passing to and from the reflecting surface, at the rate of 1,125 feet per sec- 

 ond, occupy l-9th part of a second, passing over 621X2 = 125 feet. 



•When is (in ^^^' Where separate surfaces are so situ- 



*"ikid? ™"^"' ^^^^ ^^^^^ ^^^^y receive and reflect the sound 

 from one to the other in succession, multiplied 

 echoes are heard. 



Fig. 189. 



An echo in a build- 

 ing near Milan, Italy, 

 repeats a loud sound 30 

 times audibly. A river 

 bounded by perpen- 

 dicular walls of rock, 

 where the sound is re- 

 flected backward and 

 forward over the sur- 

 face of still water, is a 

 favorable situation for 

 the production of re- 

 peated echoes. Pig. 

 189 represents the 

 manner in which the 

 sounds rebound, in such 

 situations, as at 1, 2, 3, 4, from side to side. 



It is not necessary tliat the surfiice producing an echo 

 should be either hard or polished. It is oflen observed at 

 sea that an echo proceeds from the surface of clouds. An 

 echo at sea, however, or on an extensive plane, is heard but 

 rarely, there being no surfaces to reflect sound. To insure a 

 perfect echo, the reflecting surface must be tolerably smooth, and of some 



What condi- 

 tions of surface 

 are requisite to 

 produce a per- 

 fect echo ? 



