The Speaking-trumpet. 335 greatest known distance to which sound has been carried through the atmosphere is 345 miles, as it is asserted that the very violent explosions of the volcano of St. Vincent have been heard at Demerara. There is no doubt that sound travels to a greater distance and more loudly on the earth's surface than through the air. Thus in the wilds of Africa, the roar of the lion is heard for many miles around. This is owing to the animal placing its nostrils within a short distance of the ground, and the transmission of the sound by the surface. It is stated on good authority that the cannonading at the battle of Jena, in 1806, was heard, though but feebly, in the open fields near Dresden, a distance of ninety-two miles; while in the casemates of the fortifications (underground) it was heard with great distinctness. So it is said that the cannonading of the citadel of Antwerp, in 1832, was heard in the mines of Saxony at a distance of 370 miles. 507. The Speaking-trumpet is precisely the reverse of the eartrumpet, but it is constructed according to the same law of reflected sound, with the view of directing the strength of the voice to a particular point. It is usually of a funnel shape (fig. 142), but its proper form is that of a parabolic curve. The rays of sound are reflected from the interior as the words are spoken at the smaller end of the tube, and are carried in parallel lines, represented by the letters A B C D E, to a considerable distance. They prevent the sound from spreading as it issues from the mouth, and set the air in vibration more forcibly in the direction in which the axis of the trumpet is held, than if they had issued directly from the mouth o' the speaker. This will be understood by reference to the figure. The dotted lines show the course which the rays, A E. would take if the trumpet were not used. A strong man's voice sent through a trumpet from eighteen to twenty-four feet in length has been heard at a distance of three miles. (Peschel.) The longer the trumpet the greater the distance to which the sound is carried. The sea-captain uses it to hail ships at a distance, or to send his orders aloft, where the unaided voice would be lost in the noise of the wind and waves. A similar form of mouth is used for the military trumpet, and it renders sounds audible even amid the uproar of battle. 336 Invisible Girl. Musical Sounds. 508. Some amusing effects have been produced by operating on sounds with tubes and concave surfaces. What was termed the Invisible girl was an arrangement whereby the questions of visitors were received through a kind of ear-trumpet connected with a suspended hollow sphere. The sounds were transmitted by tubes concealed in the framework of the apparatus, and thus conveyed to the ear of a woman who was placed in an adjoining apartment; she returned an answer through the speaking-tube to any question thus put to her. The sound was reflected from the interior of the suspended sphere, and its direction was thereby so changed that it appeared to the listener to issue from its interior. The days of spiritualism had not then commenced, or this ingenious adaptation of scientific principles to a clever deception, might have been fairly classed with some of the optical and other illusions which now impose upon a credulous section of the public. MUSICAL SOUNDS. 509. Aërial impulses, however produced, if they fall on the ear with sufficient rapidity, give rise to the sensation of continuous sound. This sensation is pleasing, and the sound is termed musical when the impulses recur regularly or at equal intervals of time; if they recur irregularly, the effect is displeasing, and the sound is called harsh or discordant. If a toothed wheel be made to turn, and the edge of a piece of quill or of cardboard, be placed against the teeth, every tooth can be distinguished, and every blow will be separately heard so long as the wheel turns slowly; but as the rate is increased, the teeth disappear to the eye; and the ear, failing to detect each separate blow, combines the series into one continued sound or tone, the character of which changes with the velocity of the turning wheel. In like manner the vibrations of a long harp-string, while it is slack, are separately visible, and the air-pulses produced by it are separately audible; but as it is gradually tightened, its vibrations quicken, so that at last, where it is moving, the eye sees only a shadowy line broader near the middle: and the distinct sounds which the ear lately perceived seem to run together, and are felt as one uniform continued tone, which constitutes the note or sound then belonging to the string. 510. It is the elasticity of any string used to produce a tone Musical Sounds. Noises. 337 which causes the repetition of the percussions, and therefore the continuance of the sound. Its vibrations are precisely analogous to those of the pendulum already considered (Arts. 286 and 287), the force of cohesion taking the place of the force of gravity. A large vibration of a string, like a large oscillation of a pendulum, occupies very nearly the same time as a smaller, because the farther the string is displaced, so much more forcibly, and therefore more quickly, is it pulled back again by its elasticity. Hence the uniformity of the sound produced by a musical string, is not affected by the different force with which the finger of the player may touch the string. Where a continued sound is produced by impulses which do not, like those of an elastic body, follow in regular succession, the effect ceases to be a clear uniform sound or tone, and is called a noise. Such is the sound of a saw or grindstone-the roar of waves breaking on a rocky shore, or of a violent wind in a forest—the roar and crackling of houses or of a wood in flames-the mixed voices of a talking multitude-the diversified sounds of a great city, including the rattling of wheels, the clanking of hammers, the voices of street-criers, the noises of manufactories, &c. These rough elements, however, mingle so completely in the distance, that the combined result has been called "the hum of men," from analogy to the smooth mingling miniature sounds which constitute the hum of a bee-hive. For the production of a tone, it is of no consequence in what way the pulses of the air are caused, provided they follow with sufficient rapidity and regularity. The musical sound produced by the motion of a gnat's wing was long supposed to be the voice of the insect; but because it ceases instantly when the fly comes to rest, it is now believed to depend altogether on the motion of the wings. Similar effects are produced by passing a finger-nail quickly across the teeth of a comb or the ribbed surface of a piece of hair-cloth, or along the surface of a large harp-string covered with wire. The flapping of a pigeon's wings, the clacking of a corn-mill, and the noise of a stick pulled along a grating, are not tones, only because the pulses follow too slowly. 511. The most simple and familiar instance of sounding vibrations is that of an elastic cord or wire extended between two fixed points, as in stringed instruments of music, such as the violin, pianoforte, guitar, harp. The vibrations of a solid rod of metal, glass, or any other elastic substance, fixed firmly at one end and left free 338 Means of producing Musical Sounds. at the other, are a source of musical sounds, as in a tuning-fork, or the reed-pipe. A schoolboy thus sticks the point of his penknife into the bench, and by one touch makes it produce a continued uniform sound of considerable duration. In "musical boxes" the notes are produced by the vibration of little rods of steel varying in size, fixed by one end, like the teeth of a comb, and touched by small pins or points projecting from a turning barrel. Any elastic flap, as of metal or of tough wood, fixed over an opening, so as to stand away from it a little when not pressed by passing air, but to close the opening momentarily if so pressed, becomes a sounding reed when air is gently forced through the opening. Thus, the air first pressing on the flap to close it, causes a momentary interruption of the current, but the flap immediately recoiling from the blow by its elasticity, again opens the passage, and the continued rapid alternation of the shutting and opening induces pulsations of the air sufficient to produce a musical note. The reed of a clarionet is a thin plate of elastic wood, made to vibrate in this way. The drone of the bagpipe and the common straw-pipe are reeds of nearly the same kind. The Chinese mouth-organ, and the class of instruments represented by the symphonion and concertina, have reeds which differ from these only by beating through the opening instead of merely on its face. Elastic rods simply resting on supports at both ends, or sus. pended by their middle, will also vibrate: a musical instrument is thus made of pieces of glass laid upon two strings, and struck by a cork hammer: in the island of Java, a rude instrument of the same kind is made of blocks of hard elastic wood. 512. The mere rotation of a toothed wheel in air will produce a musical note, if the teeth be large enough to impel a sufficient quantity of air. Akin to this mode of producing sound is the youthful amusement of whirling by a string round the head a piece of lath or of tin plate notched on the edge. So long as the motion is moderate, the sound is dull and heavy, forming to the mind a sort of miniature thunder: as the speed is increased, the note rises to a shrill scream. A very simple and most instructive mode of producing a note is by a succession of puffs of air. For this purpose we may take a disc of cardboard, having a series of holes punched at equal distances in a circle round the centre of the disc. If, now, by means of a gyroscope or a heavy top, this disc be rapidly whirled, while through The Three Elements of a Musical Sound. 339 a tube or pipe we blow steadily against the circle of holes, it is obvious that the blast of wind will pass through the disc by a succession of puffs, or as each hole comes round opposite the mouth of the pipe. If the rotation of the disc be sufficiently rapid, these puffs combine to produce one continuous sound or note, which rises in height, pitch, or shrillness as the rotation increases; with a suitable rate of rotation any height of note may be obtained. Thus, with the means of indicating the number of turns made by the disc per second, we know the number of air impulses, puffs, or vibrations corresponding to any given height of note. This is the embryo of the instrument called the syren (to be described farther on), which plays a most important part in the modern physics of music. 513. Those sounds which are termed musical, alone merit attention, not only from the pleasure which they are capable of affording to the mind, but from the fact that they alone are regular, and are therefore subject to certain laws which admit of being closely studied. Musical sounds differ among themselves in three particulars, namely, (i.) intensity or loudness, (ii.) pitch or height, and (iii.) quality, colour, character, or timbre, as the French express it. (i.) The loudness or intensity of a tone or note depends, in the first place, on the extent of vibrations which the exciting body makes itself, and communicates to the air. If we fix a piece of steel spring in a vice or to a table, the sound which it sets up when it vibrates, is louder the greater the extent of the excursions which it makes. Similarly, the pitch of a tuning fork remains the same, but the loudness of its note is increased by giving it a harder blow. This makes the legs vibrate to a greater extent, and in consequence the air particles also sway to and fro individually through greater distances. For sounds of the same height, the intensity depends on the energy of the air-vibrations and a little consideration of the laws of vibrations generally, shows that for a double, triple, &c., extent of vibration, the intensity is increased four, nine, &c., times. Intensity depends, secondly, on the mass of vibrating air. Two tuning forks of the same pitch sounding together give a *louder note than either singly; because thereby a greater mass or volume of air is set in pulsation, and the total energy of vibration is correspondingly increased. With the same number of holes in the cardboard disc referred to above, we may vary the intensity of the note by varying the size of the punched holes, and so varying |