180 LETTERS OX NATURAL MAGIC. 



whether they are musical or discordant, high or 

 low, move through the air of our atmosphere at 

 the surface of the earth with a velocity of 1090 

 feet in a second, or 765 miles per hour ; but in 

 sulphurous acid gas sound moves only through 

 751 feet in a second, while in hydrogen gas it 

 moves with the great velocity of 3000 feet. 

 Along fluid and solid bodies, its progress is still 

 more rapid. Through water it moves at the rate 

 of 4708 feet in a second, through tin at the rate 

 of 8175 feet, and through iron, glass, and some 

 kinds of wood, at the rate of 18,530 feet. 



When a number of single and separate sounds 

 follow each other in rapid succession, they pro- 

 duce a continued sound, in the same manner as a 

 continuous circle of light is produced by whirling 

 round a burning stick before the eye. In order 

 that the sound may appear a single one to the 

 ear, nearly sixteen separate sounds must follow 

 one another every second. When these sounds 

 are exactly similar, and recur at equal intervals, 

 they form a musical sound. In order to produce 

 such sounds frem the air, it must receive at least 

 sixteen equally distant impulses or strokes in a 

 second. The most common way of producing 

 this effect is by a string or wire AB, Fig. 40, 

 stretched between the fixed points A, B. If this 

 string is taken by the middle and pulled aside, or 

 if it is suddenly struck, it will vibrate between its 

 two fixed points, as shown in the figure, passing 

 alternately on each side of its axis AB, the 

 vibrations gradually diminishing by the resistance 

 of the air till the string is brought to rest. Its 

 vibrations, however, may be kept up, by drawing 

 a rosined fiddle-bow across it, and while it is 



