REMARKABLE ECHOES. 279 



when the effervescence begins to subside, the sound 

 becomes clearer and clearer, and the glass rings as usual 

 when the air-bubbles have vanished. If we reproduce 

 the effervescence by stirring the champagne with a 

 piece of bread the glass will again cease to ring. The 

 same experiment will succeed with other effervescing 

 fluids. 



The difference in the audibility of sounds that pass 

 over homogeneous and over mixed media is sometimes 

 so remarkable as to astonish those who witness it. The 

 following fact is given on the evidence of an officer 

 who observed it. When the British and the American 

 forces were encamped on each side of a river, the outposts 

 were so near that the forms of individuals could be easily 

 distinguished. An American drummer made his appear- 

 ance and began to beat his drum, but though the motion 

 of his arms was distinctly seen, not a single sound 

 reached the ear of the observer. A coating of snow that 

 had newly fallen upon the ground, and the thickness of 

 the atmosphere, had conspired to obstruct the sound. An 

 effect the very reverse of this is produced by a coating 

 of glazed or hardened snow, or by an extended surface of 

 ice or water. Lieutenant Foster was able to carry on a 

 conversation with a sailor across Port Bowen harbour, a 

 distance of no less than a mile and a quarter, and the 

 sound of great guns has been heard at distances varying 

 from 120 to 200 miles. Over hard and dry ground of 

 an uniform character, or where a thin soil rests upon a 

 continuous stratum of rock, the sound is heard at a great 

 distance, and hence it is the practice among many eastern 

 tribes to ascertain the approach of an enemy by applying 

 the ear to the ground. 



Many remarkable phenomena in the natural world are 

 produced by the reflection and concentration of sound. 

 Every person is familiar with the ordinary Echo which 



