Discoveries and Inventions of the Nineteenth Century

the intervention of appropriate apparatus, a similar record may be obtained from all sounding bodies. From observations of this kind, and others in which totally different methods are used for counting the number of vibrations per vibrations made in a given time, it is known that the pitch of the sound or note depends on the rapidity of the vibrations—the pitch rises with the number of them per second, and the relationship between the notes of a musical scale depends entirely on these numbers. Thus, when the vibrations for the eight notes of an octave are counted, the numbers always have this proportion, beginning from the lowest note-–24, 27, 30, 32, 36, 40, 45, 48. Thus of the two notes— as produced on musical instruments tuned to the concert pitch of the present day, the lower corresponds with 264 complete vibrations per second, the higher with 528. It will be observed, too, that all the harmonies are determined by some simple ratio in the rates of vibration: the interval of the fourth is 3, 4; that of the fifth is 2, 3, etc. Another easily discoverable fact is that the loudness of the sound depends upon the amplitude of the vibrations. This is sufficiently obvious by a few experiments with a tuning fork; and by close examination of such tracings as have been mentioned, we shall soon become aware of another circumstance—namely, that the vibrations not simple, but that the larger or general movement has one or more sets of small vibrations within it. In Fig. 319a, A is the curve that would be traced by the tuning-fork in a state of simple vibration; B and C are tracing such as are given by a fork in two of its modes of vibration. The fork gives out its proper or fundamental note in both cases; but the ear recognizes a difference in the quality of the sound due to the smaller and more numerous vibrations. Differences of the same kind are recognized in the notes produced by different musical instruments; but these are usually more complicated, and their forms are characteristic of the particular quality of the tone, 667which is thus shown to be due to the superposing of several related systems of vibration upon the fundamental one. Thus three out of the four qualities of sound recognized by the ear have had their physical causes assigned. As for the fourth—namely, the distinction we perceive among the different vowels sung on the same note—it has a physical origin identical with the last. For since parts of the vocal organs assume different positions in enunciating the different vowels, they constitute for the time being so many varied musical instruments, and the graphical traces of the sounds (for they can be obtained) show a corresponding modification. Here, in Fig. 319b, for example, are represented the tracings of the vibrations given to the air in various vowel sounds. It is also through the vibrations conveyed by the air to the little membrane called the drum of the ear that the sensations of sound are received, and of the nature of these vibrations a few words must be said presently. In considering the different qualities of sound, we have so far confined ourselves to sustained musical notes, as, for instance, the vowel sounds in singing. This has been done to show the relations of rapidity of vibration to pitch and for simplicity of illustration of the superposition of vibrations, etc. Two other remarks must be added—viz., that the vibrations of musical tones are isochronous—that is, whether the note be loud or soft, the same time is taken up in each vibration corresponding with the same fundamental note. Other vocal sounds than sustained vowel notes are found to be due to still more complicated combinations of vibrations of shorter duration, and noises, as distinguished from musical sound, are formed also by the superposition of a greater or less number of systems of vibrations, the rapidities of which are wanting in harmonic relations such as we have pointed out belong to the musical scale. Even in noises, however, there is often one or more predominant systems of vibrations 668which a musical ear can detect. If the reader will hold a pencil or penholder at one end and tap with it on the edge of the table, passing in quick succession to parts progressively nearer where the pencil is held, he will hardly fail to recognize a rising pitch in the little noises. Fig. 319a.—Traces of Vibrations of a Tuning-Fork. Fig. 319b.—Phonautographic Tracings of Different Vowel Sounds. Fig. 319c.—Diagram. Fig. 319d.—Phases of Sound Waves. A word or two must be said as to the way in which sound is transmitted through the air. This progression is commonly spoken of as a wave motion, but it must not be thought of as taking place in the form familiar to us as waves on water; still less must the reader confound it with the sinuous lines shown in the graphical representations of vibrations given in the figures. It is rather a series of rapid pulsations of the particles of the air taking place in the direction in which the sound is propagated, and resembling waves on water only by presenting periodical phases in uniform succession. The difference may be illustrated from what may be seen in a field of wheat when the wind is blowing over it. The stalks bend down, and rise again when the breeze has passed, and thus the general appearance of the waves of the sea is produced. If we confine our attention, however, to the motion of the several ears of the wheat in a file of stalks, we shall obtain a clearer notion of what takes place in the so-called waves of sound. The positions of the stalks at some one instant of time may be represented by the diagram, Fig. 319c. Each stalk is swinging backwards and forwards like an inverted pendulum, and the successive phases of these vibrations bring the adjacent ears nearest to each other about i, and farthest apart at a and a´. The places of these, and of all the intermediate degrees of approximation and retreat, pass along the file. Instead of the ears of wheat swinging on their elastic stalks, suppose particles of air approaching and receding by virtue of the elasticity by which they resist compression and recover from it, and you will obtain an elementary idea of what takes place in the transmission of sound. Fig. 319d is a picture of a column of air acted upon by a tuning-fork. The swiftly advancing prong is compressing the air in front of it, and in swinging back it will tend to leave a vacuum behind it by which the air is partially rarefied; and these alternate condensations and rarefactions will travel 669along through the air by virtue of its elasticity, and the mechanical action by which they are able to agitate a stretched membrane (or other elastic body), so that its vibrations will correspond with them in period and magnitude, may be easily understood. The vibration produced is a simple one, but any number of other systems may pass at the same time, and each one will be propagated as if the rest did not exist, just as we may see different systems of undulations moving on the surface of water. It should be observed that the velocity of propagation is the same whatever may be the period or the magnitude of the vibrations. The high and the low, the loud and the soft notes of a piece of music played at a distance, all take the same time in reaching the ear. Light as are the particles of air, the mechanical actions which a number of them carrying strong vibratory impulses will produce, may be illustrated by the rattling of window panes by a loud peal of thunder, and may be bodily felt by a person standing close to a very large bell while the hour is striking. We have referred to instruments for registering sound, and even vocal sounds, before anything has been said of the construction of the phonograph, and it is, in fact, many years since the problem was solved of recording the vibrations produced by speech. Mr. Leo Scott, in 1856, invented an instrument, called the Phonautograph, which did this. It consisted of a cone of sheet zinc like a large ear-trumpet, across the smaller end of which was stretched a membrane, having attached to it a very light style, which left a record of the vibrations of the membrane on a blackened cylinder properly disposed to receive the tracing. When any sound was produced near the open end of the cone, the impulses reflected from its internal surface were concentrated on the membrane, throwing it into corresponding vibrations. Now, this process could be reversed if the tracing could be made to give back again to the style its original movements, these transferred to the membrane would throw the air within the cone into corresponding vibrations, and the sounds that gave rise to the tracing would be reproduced. Yet Mr. Scott seems to have suggested no such possibilities for his instrument; but a few years after the invention of the phonautograph, M. Cros deposited at the Academy of Sciences in Paris a sealed paper, which was opened after Mr. Edison had patented the phonograph (1877), and found to contain suggestions of how this might be done, but describing no experiments in which any approach had been made towards realizing the conditions laid down. To Mr. Edison belongs the honour of solving the problem by the invention of the phonograph, which was patented by him in 1877. The device which he happily hit upon for converting the phonautograph into a phonograph was very simple in principle, and consisting merely in substituting a sheet of tinfoil for the blackened paper in Scott’s apparatus, the mechanism required for reproducing articulate human speech was thus found, contrary to all expectations that had previously been entertained, to be essentially of a remarkably simple character, for the arrangement of the parts was even more direct than in the phonautograph itself. This is not derogatory to the merit of the inventor, for every invention depends upon something previously attained, and the discovery of suitable materials for the various parts of the machine, and the many delicate adjustments of their forms and disposition to secure the required object, demanded the application of very remarkable experimental skill. The phonograph differs from the phonautograph by giving up what it has registered in the original form and material, and thus it is a speaking machine. It is a 670speaking machine which reproduces articulate speech, not produces it. Much ingenuity has been devoted to the construction of speaking machines which should be capable of producing the sounds of the human voice. By throwing into vibration the air contained in cavities of certain shapes, it was long ago found possible to produce sounds closely resembling those of a voice singing particular vowels, and a real speaking machine that could articulate words was exhibited in America before the phonograph had been brought out. It was constructed by Mr. Faber, and formed a very complicated arrangement, in which all the organs of human speech were imitated. There were bellows acting like the lungs; a larynx with various diaphragms, a mouth with movable tongue and lips, and a tube to resemble the cavity of the nose. The positions and connections of these parts were determined by levers acted from a key-board, like that of a piano, and by certain pedals. By moving these in proper order, the machine pronounced words distinctly enough, but in a strange drawling tone. So like, however, were the sounds to those of the human voice, that some accused the exhibitors of imposition, and unjustly credited ventriloquism instead of mechanism with the results. It will be observed that it is the function of the phonograph to reproduce, not produce, human speech, and the mechanical arrangements of the instrument are simplicity itself compared with Faber’s speaking machine. Fig. 319e.—Edison’s Original Phonograph. Fig. 319f.—