220 LETTERS ON NATURAL MAGTC. 



upon its edge, and emits only a disagreeable and 

 puffy sound. This effect will continue while the 

 wine is filled with bubbles of air, or as long as the 

 effervescence lasts ; but 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 re-produce 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 en- 

 camped on each side of a river, the outposts 

 were so near, that the form of individuals could 

 be easily distinguished. An American drummer 

 made his appearance, 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 conver- 

 sation 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 varynig from 120 to 200 miles. Over 

 hard and da(y ground of a uniform character, or 



