28 Things not generally Known. 



probably enclosed within the walls of a single room. Lieutenant 

 Foster, on Parry's third Arctic expedition, found that he could 

 converse with a man across the harbour of Port Boweu, a dis- 

 tance of 6696 feet, or about one mile and a quarter. Dr. Young 

 records that at Gibraltar the human voice has been heard at a 

 distance of ten miles. If sound be prevented from spreading 

 and losing itself in the air, either by a pipe or an extensive flat 

 surface, as a wall or still water, it may be conveyed to a great 

 distance. Biot heard a flute clearly through a tube of cast-iron 

 (the water-pipes of Paris) 3120 feet long : the lowest whisper was 

 distinctly heard j indeed, the only way not to be heard was riot 

 to speak at all. 



THE ROAR OF NIAGARA. 



The very nature of the sound of running water pronounces 

 its origin to be the bursting of bubbles : the impact of water 

 against water is a comparatively subordinate cause, and could 

 never of itself occasion the murmur of a brook ; whereas, in 

 streams which Dr. Tyndall has examined, he, in all cases where 

 a ripple was heard, discovered bubbles caused by the broken 

 column of water. Now, were Niagara continuous, and without 

 lateral vibration, it would be as silent as a cataract of ice. In 

 all probability, it has its "contracted sections," after passing 

 which it is broken into detached masses, which, plunging suc- 

 cessively upon the air-bladders formed by their precursors, sud- 

 denly liberate their contents, and thus create the thunder of the 

 waterfall. 



FIGURES PRODUCED BY SOUND. 



Stretch a sheet of wet paper over the mouth of a glass tum- 

 bler which has a footstalk, and glue or paste the paper at the 

 edges. When the paper is dry, strew dry sand thinly upon its 

 surface. Place the tumbler on a table, and hold immediately 

 above it, and parallel to the paper, a plate of glass, which 

 you also strew with sand, having previously rubbed the edges 

 smooth with emery powder. Draw a violin-bow along any 

 part of the edges ; and as the sand upon the glass is made to 

 vibrate, it will form various figures, which will be accurately 

 imitated by the sand upon the paper ; or if a violin or flute be 

 played within a few inches of the paper, they will cause the 

 sand upon its surface to form regular lines and figures. 



THE TUNING-FORK A FLUTE-PLAYER. 



Take a common tuning-fork, and on one of its branches 

 fasten with sealing wax a circular piece of card of the size of 

 a small wafer, or sufficient nearly to cover the aperture of a 

 pipe, as the sliding of the upper end of a flute with the mouth 



