Curiosities of Sound. 47 



vibrate,, " all the liiglier tones which require that point to form 

 a node, vanish from the clang." 



To illustrate these facts, Professor Tyndall used an appa- 

 ratus which our readers can easily imitate. He hsd before 

 him a single chord instrument, with a scale divided into a 

 hundred equal parts. By plucking the string at any division, 

 and touching it (damping it) at any division, he was able to 

 produce a number of easily-repeated effects. Plucking the 

 string at 50, he said, " and now I affirm that the first over- 

 tone, which corresponds to a division of the string into two 

 vibratory parts, is absent from the clang. If it were present 

 the damping of the point 50 would not interfere with it, 

 for this point would be its node. I now damp the 

 point 50 ; the fundamental tone is quenched, and no octave 

 of that tone is heard. Along with its octave, its whole progeny 

 of over-tones, with rates of vibration four times, six times, 

 eight times — all even numbers of times — the rate of the funda- 

 mental tone disappears from the clang I now pluck 



some other point, say 25, and damp 50 as before. The 

 fundamental tone is gone, but its octave, clear and full, rings 

 in your ears." In fact, by damping the string at 50 he made 

 a node there, and it vibrated as two half-length strings, 

 yielding the octave above the whole length. 



The nodal points spoken of may be made visible by several 

 contrivances : thus, when a string is employed, little riders of 

 paper will keep their places where such points occur, and will 

 be thrown off in other positions. By employing a square plate 

 of glass fixed to a stand by its centre, and sprinkling fine 

 sand over the surface, very beautiful figures, known from their 

 discoverer as Chladnr's figures, may be made, the sand being 

 first agitated, and then resting in nodal lines. The plate is 

 vibrated by a fiddle-bow, and damped by touching it at 

 certain points with a moistened finger. Very beautiful and 

 intricate patterns may be obtained in this way. 



Musical effects depend very much upon resonance, or the 

 reinforcement of a feeble sound, by enabling it to associate 

 other and sympathetic vibrations with its own. Thus, a tuning- 

 fork produces a feeble note alone, but when it is able to set 

 another body or mass of air in vibration the sound becomes 

 loud. An experiment referred to by Professor Tyndall is 

 easily made. A tuning-fork is sounded over a tall narrow jar, 

 and water gradually poured in until the air column in the jar is 

 reduced to the length which gives the greatest resonance, when 

 the sound is much louder. Pouring in more water diminishes it, 

 by making the air column too short; the maximum effect being 

 produced when the air column is one-fourth of the length of 

 the sound-wave produced by the fork. An open tube gives a 



