CURIOSITIES OF SOUND. To our limited understandings it sometimes seems that Nature delights in curious freaks; but when we come to analyze her apparent vagaries they resolve themselves into mere instances of the working of simple laws. Echoes are reflections of sounds; a flat surface like a blank wall is to sound what a looking-glass is to light. A sounding-board placed over a speaker's head catches the sounds that would otherwise be dispersed in the space above him, and reflects them down upon the audience beneath. The voice is echoed, but we do not hear both the direct and reflected sound because the interval between them is too short. The reflecting surface must be at some distance to allow an appreciable time to elapse for the sound to travel to it and come back again to the ear. The travelling rate of sound in air is about 1100 feet a second, and reflected sound travels at the same speed as direct; hence by noticing the time which elapses between a sound and its echo we may estimate how far off the echoing surface is situated. Of remarkable echoes many are known. There is the celebrated one in the Gap of Dunloe, where the sounds are reflected again and again, so that when a trumpet is blown at the proper place the return notes reach the ear in succession after one, two, three, or more reflections from the adjacent cliffs, and thus die away in the sweetest cadences. Alpine travellers, too, tell of wonderful warblings of echoes in the Swiss mountains. The rolling and pealing of thunder is due to echoes of the primary clap, which are generated in the clouds. A curious echo occurs at the London Colosseum. Mr. Wheatstone found that a syllable pronounced close to the upper part of the wall of this structure was repeated a great many times. A single exclamation sounded like a peal of laughter, and the tearing of a sheet of paper like the patter of hail. We have said that sound travels through the air at the rate of about 1100 feet a second; but this speed depends upon the elasticity and density of the air; and as the elasticity depends upon temperature, it follows that sound travels differently, according as the weather is warm or cold. At freezing temperature its rate is 1090 feet a second; at 80° Fahrenheit, it is 1140 feet. So that sound travels slower in winter than in summer. Its velocity through other substances than air is also very dif ferent. Through hydrogen gas it is 4160 feet a second, and through water a little greater than this. Iron conveys it at nearly four times this speed. In travelling through space, sound diminishes in intensity, and, like light and other actions, it does this in proportion to the square of the distance traversed. A man two yards from a bell only hears onefourth of the sound that reaches an ear distant one yard. A man three yards off only catches one-ninth of it; another four yards distant a sixteenth, and so on. The reason of this rapid rate of diminution, and of this invariable proportion, is obvious. If a certain sound will fill a sphere one yard in diameter with a certain intensity, that same sound, dispersed through a sphere six yards in diameter, and therefore spreading over thirty-six times as much space, will be, as it were, diluted to a thirty-sixth of its strength. But this decrease only takes place in free air. In a room the sound is confined, and its lateral diffusion is prevented, so that the rule, although perfectly true as regards the sound coming directly from the musical source, is not quite applicable to the general effect produced by the reflection and dispersion of the sonorous waves. Indeed, sound confined, or prevented from dispersing, may be conveyed to great distances. There seems to be no limit to the actual distance to which it may be carried in a tube. The French philosopher Biot, experimenting on the transmission of sound through the empty water-pipes of Paris, found that he could hold a conversation in a low voice through an iron tube 3120 feet in length; the lowest possible whisper could be heard at this dis tance. The leading of sound through tubes was practiced in early times, and no doubt speaking images and oracular responses depended upon this acoustical phenomenon. In our own time we have had talking heads. But modern sight-seers know all about tubes; so the heads have been isolated from solid supports, and carried by suspending chains. No matter. The mouth of the figure has been made hollow, or a trumpet bell has been placed in it; the sound has been led by a tube to some concealed orifice directly in front of this bell-mouth, and being as it were injected thereinto, has been thrown out again towards the astonished audience, who have thus been made to believe that the talking has been the result of some highly ingenious mechanism contained within the image. Nevertheless, successful attempts have been made to imitate the human voice by mechanical instruments. In the last century, the Academy of Sciences at St. Petersburg proposed as a prize subject an inquiry into the nature of the vowel sounds and the construction of an instrument for artificially imitating them. The question was solved by M. Kratzenstein, who showed that all the vowels could be pronounced by blowing through a reed into tubes or chambers of various forms. At about the same time a Viennese mechanician, M. Kempelen, made a series of elaborate experiments which led to the construction of a machine that could be made to utter not merely vocal sounds, but words and even some few complete sentences, such as opera, astronomy, Constantinopolis, vous êtes mon ami, Romanorum imperator semper Augustus, &c. Sound is produced by certain vibrations or pulsations communicated to the atmosphere. When we pluck a harpstring we set it quivering, and cause it to give to the adjacent air a rapid succession of blows: the number of these blows in a second depending upon the length and tension of the string. If the string only gave one push to the air we should hear but one noise or blow: but as in vibrating it gives a rapid succession of pushes, we experience a rapid succession of noises, and these resolve themselves into a continuous sound. Noises may become musical if only they succeed each other at equal intervals of time and with sufficient rapidity. If a watch could be caused to tick a hundred times in a second, the ticks would lose their individuality and blend into a musical tone. If the flapping of a pigeon's wings could be accomplished at the same rate the bird would make music in its flight. The humming-bird does this, and so do thousands of insects whose wings vibrate with great rapidity. The highness or lowness, what we call the pitch, of a sound, depends upon the rapidity with which these pulses fall upon the air. When they come at the rate of fifty or sixty a second we have a deep growling bass sound; when at the rate of from twenty to thirty thousand in the same interval, the sound is a piercing treble. The human ear becomes deaf to such high sounds as result from these extremely rapid pulsations. It seems that the tympanic membrane is incapable of receiving and communicating more than about 20.000 blows in a second. But the limit varies with different persons; the squeak of a bat, the chirrup of the house-sparrow, the sound of a cricket are unheard by some people who possess a sensitive ear for lower sounds. The ascent of a single note is sometimes sufficient to produce the change from sound to silence. Since the pitch of a sound depends upon the number of pulsations reaching the ear in a given time-suppose that we run towards a source of sound, what is the consequence? Evidently the vibrations are crowded upon the ear more quickly than they would be if we stood still, and conversely, if we run away from a sound they come upon us more slowly. Hence arises the curious phenomenon, that in the first case the sound is sharpened, and in the second case flattened by our motion. This may be observed at any railway station during the passage of a rapid train. As the engine approaches, the sonorous waves emitted by the whistle are virtually shortened, a greater number of them being crowded into the ear in a given time. As it retreats the sonorous waves are virtually lengthened. The consequence is, that in approaching the whistle sounds a higher note, and in retreating a lower note, than if the train were stationary. Although a plucked string, or a string otherwise made to vibrate, produces sound by beating the air, it must be observed that a string is too small a thing of itself to set in motion such a mass of air as is necessary to fill a room with sound. Hence to make strings available for musical instruments they have to be so connected with larger surfaces as to set them in vibratory motion. These surfaces we call sound-boards, and in every stringed instrument the most important feature is this sonorous medium. The quality of this part of a piano, harp, violin, or lute, determines the entire goodness of the instrument. The sound-board must be able to take up and give out to perfection every vibration that every string offers to it, or it will not do its duty properly, and the instrument, of which it is almost body and soul, will be a bad one. The high value set upon venerable violins is not entirely fanciful. The molecular changes that age works in the nature of the wood they are made of have an impor tant influence over their sounding qualities. The very act of playing has a beneficial effect; apparently constraining the molecules of the wood, which in the first instance were refractory, to conform at last to the requirements of the vibrating strings. When a string, or a column of air in a pipe, is put in vibration, it not only vibrates as a whole, but it subdivides itself into proportional parts, each of which has its own time of vibration, and gives forth its own sound. These supplementary sounds are called harmonics; and it is the mingling of these with the fundamental note produced by the vibration of the whole string or air-column that determines the quality of the emitted tone, or what we, following the French, call the timbre. A violin and a clarinet may give forth the same note; yet their sounds will be quite different in tone, because the auxiliary vibra tions accompanying the fundamental note in each are different. Vibrations imparted to the air are frequently taken up by solid is being played, it is not uncommon to hear the lamp-glasses or other sounding bodies in the room join in the concert. In those cases the glass picks out from the general clamor that particular set of vibrations which it is capable of taking up, and rings in harmony with the note producing them whenever that note is sounded. A sounding tuning-fork will thus excite a silent one to play with it. Two pendulumclocks fixed to the same wall, or two watches lying on the same table, will take the same rate of going, through this sympathetic communication of vibrations; and what is more remarkable, if one clock be set going and the other not, the ticks of the moving clock, transmitted through the wall, will start its neighbor. He bodies at a distance. When music might have seen the harmony." By experiment, he found that the vibrations were not due to the shaking of the walls and floor of the room, but were communicated directly from the music to the flame. This interesting subject has been followed up extensively. It has been found that those flames only are sensitive which are on the point of flaring, or roaring, as some would term it. A common fishtail burner, just at the point of fluttering, but still giving out a clear sheet of flame, is thrown into a state of commotion, spurting out quivering tongues, when a whistle is blown in its neighborhood. A bat's-wing jet behaves in a similar manner, throwing forth its tongues whenever an anvil is struck with a hammer. Professor Tyndall makes. flames almost dance to music. places a long rod-like flame and a short one side by side; upon blowing a whistle, the long flame becomes short, forked, and brilliant,' and the short one long and smoky. The most marvellous flame exhibited at his lecture on this subject was a long thin one, twenty-four inches high. The slightest tap on a distant anvil reduced the height But the most beautiful instances of this flame to seven inches. At of sympathy in sound are afforded the shaking of a bunch of keys by the phenomena of musical or it became violently agitated, and sensitive flames. To Professor Le- emitted a loud roar. The creaking conte belongs the honor of first of a boot set it in commotion; so calling attention to these curiosi- did the crumpling of a piece of ties. The Professor was assisting paper, or the rustle of a silk dress. at a musical party one evening, The patter of a raindrop startled and he says, "Soon after the music it. At every tick of a watch held commenced, I observed that the near it, down it fell. The twitter flames of a fishtail gas-burner ex- of a distant sparrow, or the chirp hibited pulsations which were ex- of a cricket, produced the same actly synchronous with the audible effect. When Professor Tyndall beats. This phenomenon was very striking to every one in the room, and especially so when the strong notes of the violoncello came in. It was exceedingly interesting to observe how perfectly even the trills of this instrument were reflected on the sheet of flame. A deaf man It is in consequence of this property that the sound of a particular organ-pipe will sometimes break a particular window-pane, and that a powerful voice can crack a wineglass by singing near it. The story goes that the Swiss muleteers tie up their bells at certain places, lest the tinkle should bring an avalanche down. recited Spenser's verse, commencing, "Her ivory forehead full of bounty beams," the flame seemed to show its appreciation of the language; it noticed some sounds with a slight nod, to others it bowed more distinctly, and to certain others, again, it made a profound obeisance. To the performance of a musical box, the flame behaved like a sentient being. Jets of smoke are acted upon like flames, and so are jets of water, under certain conditions. The loud noises which caves and rocky inclosures give forth when low sounds are uttered in them are well known. Bunsen has noticed that when one of the steam jets of Iceland breaks out near the mouth of a cavern, a thunder-like sound is produced. When a hollow shell is placed close to the ear, a low, murmuring noise is heard, which little children readily believe is the rolling of the sea. These phenomena are the effects of resonance, and resonance is the reinforcement of one sound by echoes of itself. If we speak into the mouth of a hollow tube the sound vibrations of the air pour down the tube to the bottom; striking against this, they are reflected, and turn back again; on their way back they meet others going down, and, union giving strength, they reinforce each other, and a doubled sound issues from the tube; it may be that several reflections conspire to reinforce the original sound several times, and then for a light whisper we have a loud roar. The channel of the ear itself is a resonant cavity. Every one is familiar with the experiment of holding a poker by two strings, one in each hand, thrusting the fingers in the ears, and striking the poker against some hard substance. A sound is experienced by this means which is as deep and sonorous as a cathedral bell. It is due to the reinforcement of the vibrations of the poker in the hollow cavity of the ear. When we blow gently across a closed tube, such as the pipe of a key, the gentle fluttering of our breath is so reinforced by the resonance of the cavity that a whistle is produced. An organpipe gives forth its powerful note on the same principle; the prime source of the sound is only a gentle puff of wind blown against a sharp edge; this produces a flutter in the air, and some particular pulse of this fluttering is converted into a musical sound by the resonance of the associated column of air. If a tuning-fork be sounded and held in front of the slit near the bottom of an organ-pipe, the pipe will resound as if it had been blown into. But the pipe and the fork must yield the same note, or the former will not "speak." Any cavity will not fully resound to any sound; it is only when the note the pipe would give if blown into is the same as that given by the fork, that the resonance is perfect. But while sound will augment sound, the opposite is likewise the case; sound will destroy sound. As this curiosity brings us to silence, it shall be the last mentioned here. Sound consists in waves or pulses travelling through the air. Now a wave consists of an elevation and a depression. Suppose that two waves come together. If elevation meet elevation they augment each other, and a double elevation is the consequence; if depression meet depression, the effect is similar; we have a depression of double the depth. But if elevation meet depression, what follows? Clearly they destroy each other, and the result is, nothing. So it follows, that when two sounds meet in such a manner that the elevations of the waves of one meet the depressions of the waves of the other, silence is produced. Just the same thing occurs in the case of light, which is also a wave motion. An optician (we do not mean a spectacle and telescope maker, but a scientific student of optics) can make two rays of light so clash that darkness is the result. In an ordinary tuning-fork the vibrations of one prong do really, to a considerable extent, |