224 LETTERS ON NATURAL MAGIC. 



near Rouen, a curious oblique echo which is not 

 heard by the person who emits the sound. A 

 person who sings hears only his own voice, while 

 those who listen hear only the echo, which some- 

 times seems to approach, and at other times seem 

 to recede from the ear ; one person hears a single 

 sound, another several sounds, and one hears it 

 on the right, and another on the left, the effect 

 always changing as the hearer changes his position. 

 Dr. Birch has described an extraordinary echo at 

 Roseneath, in Argyleshire, which certainly does 

 not now exist. When eight or ten notes were 

 played upon a trumpet, they were correctly re- 

 peated, but on a key a third lower. After a short 

 pause, another repetition of the notes was heard 

 in a still lower tone, and after another short 

 interval they were repeated in a still lower tone. 



In the same manner as light is always lost by 

 reflexion, so the waves of sound are enfeebled by 

 reflexion from ordinary surfaces, and the echo is 

 in such cases fainter than the original sound. If 

 the reflecting surface, however, is circular, sound 

 may be condensed and rendered stronger in the 

 same manner as light. I have seen a fine ex- 

 ample of this, in the circular turn of a garden 

 wall nearly a mile distant from a weir across a 

 river. When the air is pure and homogeneous, 

 the rushing sound of the water is reflected from 

 the hollow surface of the wall, and concentrated 

 in a focus, the place of which the ear can easily 

 discover from the intensity of the sound being 

 there a maximum. A person not acquainted 

 with the locality conceives that the rushing noise 

 is on the other side of the wall. 



In whispering galleries, or places where the 



