SOUND AND MUSIC 91 



When resined cloth is moved to and fro along it, a 

 musical note is heard much higher than if it were 

 plucked aside. If the wire be divided into two halves, 

 the number of vibrations is doubled, and the octave of 

 the first note is heard. If a third of it be taken, it yields 

 the fifth above the octave ; if a fourth, the second octave. 

 In longitudinal vibrations, as in transversal, the number 

 is proportional to the length of the wire. It is not the 

 wire that gives forth the sound, but the sounding board. 

 The vibrations created by it are at right angles to those 

 in the wire, and so are transversal. In it the particles of 

 the iron are set in rapid motion, to and fro, in the direc- 

 tion of the length, and in that direction are harmonic, 

 are exceedingly rapid, and so yield a high-pitched note. 



An iron rod and a brass will yield the same note if 

 their lengths be in proportion to the velocity of sound in 

 them, i.e. as 11 to 17. 



Notes are also produced in rods of wood by rubbing 

 with resined leather, and they follow the same laws. 



Harmonic motions are produced in rods free at both 

 ends. A glass tube is held at the centre, and one end 

 vigorously rubbed, when a musical note is emitted. 

 Longitudinal vibrations are created from end to end of 

 the rod, and at the further end pieces fall off in rings. 



Resonance. A tuning-fork vibrating, say, 256 times 

 in a second, placed over a glass jar, may yield little or no 

 sound. Pour water into the jar, the sound increases, till 

 reaching a certain height it bursts forth loudly. On 

 measuring the depth to the water, it is found to be 

 13 inches, or one-fourth the length of the sonorous 

 wave of such a fork. The motions from the prong, and 

 those reflected from the water, are timed for each other, 

 and so unite, and the note becomes loud and clear. 

 Every tuning-fork has its own depth of jar, which 



