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The transmission of musical sounds through solids received some beautiful illustrations in Professor Tyndall's lectures. The following passage shows one of the magical effects which he produced :
“In a room underneath this,” said the lecturer, “and separated from it by two floors, is a piano. Through the two floors passes a tin tube two and a half inches in diameter, and along the axis of this tube passes a rod of deal, the end of which emerges from the floor in front of the lecture table. The rod is clasped by india-rubber bands, which entirely close the tin tube. The lower rod rests upon the soundboard of the piano, its upper end being exposed before you. An artist is at this moment engaged at the instrument, but you hear no sound. I place this violin upon the end of the rod; the violin becomes instantly musical, not however, with the vibrations of its own strings, but with those of the piano. I remove the violin, the sound ceases. I put in its place a guitar, and the music revives.”
A harp was rendered musical in the same way, and as Professor Tyndall says, an uneducated person might well believe that witchcraft was concerned in the production of such music.
The vibrations of columns of air of various lengths give rise to the notes of organs and wind instruments, while pianos, violins, etc., are an illustration of the operation of vibrating strings, the sound of which would be feeble if it were not reinforced by the elastic wood introduced into the structure of such instruments.
Long strings vibrate more slowly than short ones, and thick ones than thin. If a whole string vibrates with a given velocity, half of it will vibrate twice as quickly, a third three times, and so on.
Strings, or air columns may vibrate as wholes, or may divide themselves into a number of equal parts, each one of which vibrates as if it were a whole; but when strings vibrate as wholes, they vibrate more or less in subdivisions at the same time, and hence arise harmonic notes, higher than the fundamental, or whole string, note. Different sorts of instruments tuned to produce the same fundamental notes, will add to them different harmonics, and thus be characterized by distinct qualities of tone, timbres, as the French call them.
The vibration of a string being compounded of whole length vibrations, and of the harmonic, or part length vibrations, just explained, there will be certain places at which the two sets interfere and produce nodes, or points of comparative (but not complete) rest; and Dr. Thomas Young discovered that when a string is plucked at any point, and caused to vibrate, "all the higher tones which require that point to form a node, vanish from the clang,"
To illustrate these facts, Professor Tyndall used an apparatus which our readers can easily imitate. He had 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 afhrm that the first overtone, 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 fundamental 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 Chladni’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 tuningfork 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
maximum of resonance when twice the length of a closed one. The strengthening by resonance of feeble sounds not previously heard or noticed, is the cause of the sea music,” which children are so fond of when cowry shells are held to their
Caves and rocks resound to the noise of waterfalls, and deep wells to the fall of objects on the water which they contain.
Amongst the most beautiful and curious of the strange ways of sound, we must allude to singing and to sensitive flames. If a glass or metal tube, two or three feet long, is held at a proper height-easily found by trial-over a small jet of gas, a musical note is heard; and if the experiment is made on a large scale, the sound is violent. The musical note arises from a series of impulses communicated by the flame, which is a rapidly twinkling one, to the air. Hydrogen gas answers for these experiments better than coal gas.
The sensitive naked flames are the most extraordinary and apparently magical things. A broad fish-tail flame exhibits the phenomena to a considerable extent. Such a flame will jump and put forth tongues in sympathetic response to a particular set of vibrations. Professor Tyndall showed that, by producing a slight flutter with a blowpipe in a candle-flame, it was made sensitive, and jumped when a whistle was sounded. By adding to the pressure, and obtaining flaring flames from common fish-tail and bat’s-wing burners, they became sensitive to whistles, and put forth curious tongues; and striking a distant anvil with a hammer gave similar results. By using a single orifice burnerand suitable pressure, flames eighteen inches long were obtained, smoking copiously, and remarkably sensitive to sound vibrations of the right quality. By sounding a whistle, Professor Tyndall caused the flames to change instantly, and become short, forked, and brilliant. Another flame exhibited by the professor to his wonderstruck audience was twenty-four inches long, and by the slightest tap on a distant anvil it was reduced to seven inches. Dropping sixpence from the hand twenty yards off was sufficient to knock the flame down; the creaking of boots, the rustle of a lady's dress, the patter of rain, and the tick of a watch-all influenced it in a striking way. When lines of poetry were recited, the flame nodded to some sounds, took no notice of others, and when the tones of the voice were most in sympathy with it, “its obeisance was profound.”
Here we must leave Professor Tyndall and his most interesting book, intending, however, to take another opportunity of recurring to more of the “Curiosities of Sound, to whose wonders, we hope, the present pages may prove an introduction for those who love science in its recreative aspects and lighter moods.