MUSICAL AND HARMONIC SOUNDS EXPLAINED. 245 



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 

 fefit. Along fluid and solid bodies its progress is still 

 more rapid. Through water it moves at the rate df 4708 

 feet in a second, through tin at the rate of 8175 feet, and 

 through iron, glass, and some kinds of wood, at the rato 

 of 18,530 feet. 



When a number of single and separate sounds follow 

 each other in rapid succession, they produce 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 from 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 A B, 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 A B, 

 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 vibrating it will give out a sound 

 corresponding to the rapidity of its vibrations, and arising 

 from the successive blows or impulses given to the air by 

 the string. This sound is called the fundamental sound 

 of the string, and its acuteness or sharpness increases 

 with the number of vibrations which the string performs 

 in a second. 



If we now touch the vibrating string A' B' lightly with 



