CHAPTER XVII.
OSCILLATION.—UNITED STRENGTH.—THE DOME
Connection of Oscillation with Centrifugal Force.—Equality of Time in Oscillation.—The Spider.—The Stone and String.—Pendulum of the Clock, and its Effect on the Machinery.—Acceleration and Retardation.—Compensating Pendulums.—The Metronome, and its Use in Music.—A simple Metronome.—Value of the Instrument in War.—The Escapement, and its Connection with the Pendulum.—Mode of Action.—Larva of Burying-beetle.—Earthworms and Serpents.—Union is Strength.—The Hippopotamus Rope and its Structure.—The Spider-web.—Distinction between the Threads.—Principle of the Dome.—The Arch, and its Connection with the Dome.—Esquimaux Huts.—Receiver of the Air-pump, and its Power of Resistance.—The Human Skull and the Egg.—Accidental Resemblance.—The Salad-dressing Bottle.—The Medusa, Strobila, and Hydra.
A portion of our last chapter dealt of Centrifugal Force. We will now proceed to another well-known power, which seems to be a variation, or perhaps a division, of the same power. I mean the principle of Oscillation, which has done so much for the present state of the world. I mention the connection of the two principles because it is evident that, if Oscillation were continued in one direction, it would be converted into centrifugal force. In fact, it can only be considered as centrifugal force interrupted.
The chief point in this subject is the equal time occupied by the oscillating body, no matter what may be the “arc” distance through which it sways, provided that the length of the line remains the same. The discovery of this principle by Galileo in a church at Florence is too well known to need repetition.
This principle may be observed by any one, and at almost any time. The Spider at the end of its line illustrates it, and so does a stone tied to a string, both of which objects are shown in the illustration.
In various departments of Art, Oscillation is absolutely invaluable. We will take, for instance, the best known of these examples, namely, the Pendulum, by which the movements of clocks are regulated. Without some mode of regulation, the works would run down rapidly, and the clock rendered incapable of measuring time. But, in the Pendulum, we possess a means of making a clock go at any desirable rate, and be faster and slower at pleasure; a long Pendulum working slowly, and a short one rapidly.
How the Pendulum affects the working of a clock may be seen by reference to the right-hand figure of the illustration. The movements of the clock are connected with the Pendulum by means of an ingenious piece of mechanism called an “escapement,” because it only allows the wheel shown in the illustration to move one cog at each swing of the Pendulum.
Now, as in the latitude of London a pendulum which is a trifle more than thirty-nine inches in length swings once in a second, it is evident that, by lengthening or shortening the Pendulum, we have the rate of the clock entirely under command.
For example, if a Pendulum be required to swing once in two seconds, it must be four times as long as that which swings once in one second, while to swing once in three seconds it must be nine times as long, the length being measured by the square of the time of vibration.
We are thus able to “regulate” clocks by lengthening the Pendulum if they be too fast, and shortening them if they be too slow. The reader will probably have remarked that the conditions of the atmosphere—such as heat, cold, moisture, or dryness—must have an effect on the length of the Pendulum, and thus alter the rating of the clock. So they do, and in consequence the Compensating Pendulums have been invented, some of them being made of metallic rods of different powers of expansion, mostly brass and steel, while others carry a quantity of mercury in a glass tube near the bottom of the Pendulum.
Another familiar example of the Pendulum is the Metronome, which is simply a Pendulum with a weight at the top as well as counterpoise below the bottom, the weight moving up or down so as to decrease or hasten the pace. Generally a bell is added to it, which is struck at the beginning of each bar.
The exactness of its beats is perfect, as is known to all musicians, and is calculated to take the conceit out of players who are apt to disregard their time. I knew one lady, a really good pianiste, before whom I placed my Metronome. Before she had played many bars she broke down, exclaiming that the horrid bell always said “ting” in the wrong place. However, she soon acknowledged the value of the instrument, and was glad to use it.
A very good Metronome may be made by fastening a bullet to the end of a piece of tape, and swinging it backwards and forwards, regulating the tape according to the time required. Such a Metronome is very portable, and extremely useful where the conveyance of the clockwork instrument would be troublesome. Moreover, its beats can be seen by a great number of persons. I have often used it myself.
Such a Metronome is used in the army, in order to regulate the pace of the soldier’s step, it being of the last importance that the pace should always be the same. Otherwise it would be impossible to calculate the time which ought to be consumed in marching a certain distance, and the military calculations on which depends the success or failure of a campaign would be wholly upset, half an hour too soon or too late meaning failure.
The Escapement
As we are on the subject of the pendulum and Escapement, we will say a few words about the latter piece of mechanism. It is here given on a larger scale than in the previous illustration, so that its action may be more easily understood. Whether in watch or clock, the Escapement is exactly the same in principle.
First there is the escapement wheel, the circumference of which is furnished with a number of very deep cogs, varying as to the work which they have to do. Then there comes the escapement itself, which swings on its pivot, and is regulated in its oscillations by the pendulum. As it swings backwards and forwards, it is evident that only one tooth of the wheel can “escape,” and only that in one direction.
We can reverse a steam-engine, but the man has yet to be found who can reverse a clock, i.e. enable it to continue going in the opposite direction. The only mode would be to enable one set of cogs to flatten themselves, so as to pass the escapement, and a second set to start up in exactly the opposite direction. Or perhaps there might be two parallel escapement wheels, capable of being connected or disconnected with the clock at pleasure. As, however, a reverse movement is quite needless, no such invention seems to have been made.
On the left hand is seen an example of the same principle as shown in Nature. It represents a larva or grub of the Burying-beetle. It has no legs available for locomotion, and yet it can get along with tolerable speed.
Many years ago, when living in Wiltshire, I was much struck with this fact. There had been an epidemic among sheep, which killed them off so fast that the farmers would at last not even bury them, but took off the skins, and left the bodies to moulder as they best might.
It was very unpleasant for the farmers, but just the contrary for the Burying-beetles, which simply swarmed in the deserted carcasses. If one of them were tapped with a stick, hundreds of these larvæ came scuttling out, displaying an activity which was really remarkable in creatures practically legless.
In reality this movement is achieved by an apparatus very similar in its action to that of the escapement. The rings, or “segments,” of which the body is composed, are furnished with rows of sharp points, arranged very like the cogs of the escapement wheel. By alternately elongating and contracting the body, these points catch against surrounding substances, and force the creature onwards, only allowing of movement in one direction.
Perhaps the reader will remember that in an earlier part of this work it has been mentioned that the various worms propel themselves by the same means. So do the Serpents, the edges of the scales serving the same purpose as the hairs of the worms and the hooks of the grub.
Union is Strength
ON the left hand of the accompanying illustration we have an example of the wonderful power obtained by uniting together a number of comparatively weak objects. It represents a portion of the rope attached to the harpoon with which the natives of some parts of Africa attack and kill the hippopotamus.
Considering that a full-grown hippopotamus weighs several tons, and, in spite of its enormous size, is as active as a tiger, we can infer the strength of the rope which must be needed to hold such an animal when excited with rage and pain.
A few years ago the female hippopotamus at the Zoological Gardens, when deprived of her cub, actually tried to leap over the lofty iron barrier, and so far succeeded as to throw her weight on the uppermost bar. Fortunately it was made of well-wrought iron, and was only bent by her weight. Had it been made of cast-iron, like most railings, she would have snapped it like glass.
Now, the fibres of which the rope is composed are individually feeble, but, when they lend their strength to each other, their strength is amazing. It is well shown by a lasso in my possession, made of the fibres of the aloe-leaf. It is scarcely as thick as a man’s little finger, and yet it is strong enough to resist the efforts of the most powerful wild bull. I have some of the separate fibres, and it is interesting to notice how fibres so slight when separate should be so strong when united. Part of the rope has been unlaid, so as to show the manner in which it has been put together.
Towards the harpoon itself, a number of small cords laid loosely side by side are used, so as to prevent the hippopotamus from severing the rope with his chisel-like teeth, which he would assuredly do if it were single. The multitudinous cords become entangled among the teeth, and baffle his efforts; but still their unity is their strength; and, though the animal may sever one or two of them, the others retain their hold until he dies under a shower of spears.
On the right-hand side of the illustration is the Spinneret of the ordinary garden Spider, showing the many orifices from which the silken threads emerge. It is a remarkable point, and one which, I believe, is seldom noticed, that the Spider can at pleasure combine all these fibres into a single cord, or issue and keep them separate, just as is the case with the hippopotamus rope.
The latter operation may be seen whenever a large fly gets into the web. The Spider darts at it, bites it, and then, ejecting a loose mass of fibres, rolls it up in a moment, as in a shroud, carries it off and hangs it in a convenient place, and mends the broken meshes of the web. But both kinds of the cords of the net are made differently from the winding-up fibres, the former being fixed together, and the latter kept separate.
Principle of the Dome
We are all familiar with Domes, especially when the Dome of St. Paul’s is the most conspicuous object in our metropolis. Few persons, however, except professional architects and builders, seem to ask themselves the principle on which the Dome is constructed.
The strength of the arch is well known, and the Dome is practically a number of arches, affording material support to each other, and so enormously increasing the strength of the edifice.
A good idea of the Dome principle may be formed by taking two croquet hoops, placing them at right angles to each other, tying them together at the intersection, and pushing the ends in the ground. Even by this very simple arrangement considerable strength can be obtained; but, if the hoops be sufficiently multiplied to form a close Dome, it will be evident that the strength will be correspondingly increased.
So strong, indeed, is the Dome, that it could be made without mortar or cement, although, of course, its strength is increased by their use. A very good example of a Dome thus constructed is found in the “igloo,” or snow-hut of the Esquimaux, which has already been described.
As to the example which I have selected, it would have been easy enough to have chosen one of the great Domes of the world, such as St. Peter’s at Rome, St. Maria del Fiore at Florence, St. Paul’s of London, or St. Geneviève or the Invalides of Paris.
I have, however, selected the present example on account of the thinness of its walls, the fragility of its material, and the enormous pressure which it has to undergo. This is the “Receiver” of the Air-pump. It is made of glass not thicker than an ordinary tumbler, and yet, even when exhausted of air, it will resist the pressure of the atmosphere for days together.
When it is remembered that the Receiver is deprived of its internal air, and therefore has to resist a pressure equal to fifteen pounds on every square inch of its surface, it may be imagined how strong the Dome is. Were the top or either side to be flat, it would be crushed as soon as a vacuum was formed sufficient to deprive it of the support of the air within.
A glance at the illustration will show how the Receiver is modelled on the same plan as the Human Skull, the outlines being curiously similar. It is this formation which imparts such strength to so thin a set of bones as those which compose the human skull as enables them to protect a sensitive organ like the brain, on which both reason and life itself depend.
Eggs also form good examples of the wonderful strength obtained by this principle, their thin shells protecting the yolk and the white, as well as the chick through its progress to maturity.
The last subject in this chapter is a curious example of an evidently accidental resemblance in form.
The figure on the right of the accompanying illustration will at once be recognised as one of those Salad-dressing Bottles which try to conceal by their shape the small volume of their contents.
That on the left represents one of the many forms through which the Medusa passes before it attains its perfect form. It was long thought to be a separate creature, and was known under the scientific name of Strobila. Modern researches have, however, made the discovery that it is one of the transitional stages between the creature known as the Trumpet-hydra (Hydra tuba) and the Medusa, popularly known as Jelly-fish.
The former almost exactly resembles the Hydra of our fresh waters. It is a tiny transparent gelatinous bag—so transparent as to be scarcely perceptible, and with some thirty or forty long and delicate tentacles hanging from its open end. These tentacles are used in catching the minute creatures on which it feeds. It is fixed, and, to use Mr. Rymer Jones’s simile, looks like a beautiful silk-like pencil waving amidst the water. Its length is not quite half an inch.
That it should be identical with the remarkable form shown in the illustration seems impossible, but such is the case. Its body becomes contracted as if tied with strings, and every segment thus formed develops a set of tentacles, breaks away, and swims off in the form of a Medusa. The upper segment is exhibited as undergoing this process.
The figure is magnified so as to show the structure better, its right length being about one-third of an inch. A full and graphic history of this creature and its manifold changes may be found in Mr. Rymer Jones’s “Aquarian Naturalist.”
It is not likely that the inventor of the Salad-dressing Bottle ever saw a Hydra, but the resemblance is strangely exact.
ACOUSTICS
CHAPTER I.
PERCUSSION.—THE STRING AND REED.—THE TRUMPET.—EAR-TRUMPET.—STETHOSCOPE
The Science of Sound.—Rhythmical Vibrations.—The Drum.—Primitive Drums.—The Solid and Hollow Log.—The Bass Drum and Kettle-drum.—African Drums.—Gnostic Gems and the Ashanti Drum.—Tympanum, or Drum of the Human Ear, and its Mechanism.—An artificial Tympanum.—The String.—The Bow and the Harp.—The Harpsichord and the Zither.—The Bow and the Violin.—The Cricket.—The Vibrator, or Reed.—The Jew’s Harp and Harmonium.—The Cicada and its Song.—Harmonics upon Strings.—The Æolian Harp.—Harmonics upon the Trumpet.—The Trombone.—Trachea of the Swan.—The Ear-trumpet.—The Sea-shell.—The Stethoscope.—Savage Food.—The Aye-aye.—The Siren and its Uses.—Echo and Whispering Gallery.
IN a work of this nature it would be absolutely impossible, not to say out of place, to give an account of so elaborate a subject as Acoustics, i.e. the science of Sound. Suffice it to say, that all sounds are produced by the vibration of air, and that the fewer vibrations, the lower is the sound, and vice versâ.
When such vibrations are produced regularly, they form Musical sounds, but, if irregularly, the sounds can be only distinguished under the term of Noise. The earliest germ of music lies in certain savage races, who, as long as they can maintain a rhythmical beat on any resonant substance, do not particularly care what it is. A hollow tree is a splendid instrument in their opinion, but, if this cannot be had, a dry log of wood will answer the same purpose.
Some tribes, more ingenious than others, cut a deep groove upon the upper surface of a log, hollow it through this groove, and then hammer away at it to their hearts’ content. The next move was to cut off a section of the trunk of a tree, hollow it, set it on end, and then beat it on the sides.
Lastly, some one hit upon the idea that if the open upper part of the hollowed log were covered with a tightly stretched membrane, and that if the membrane, instead of the log, were beaten, the resonance would be increased. In consequence, the real Drum was invented, and seems to have existed from time immemorial in parts of the world so distant that they could not have had any communication with each other.
Take, for example, the well-known “Bass Drum” of our bands, which is shown on the right hand of the figure. We make it a very ornamental article, with frame of metal, and heraldic decorations of all kinds.
Lying against it is one of a pair of Kettle-drums, such as are always seen in mounted bands. They look very easy to play, but, if the reader will try a pair, he will soon find his mistake.
But there are savage tribes of Western Africa who make Drums of such wonderful power that their sullen roar is heard for miles around, as their slow, triple beat summons the tribe to arms like the fiery cross of the Highland clans. As to shape, lightness, and beauty, our Drums are infinitely superior to theirs, but, so far as I can gather from personal and written narratives of African travellers, none of our Drums surpass theirs in richness, depth of tone, and power of carrying sound.
Sometimes these Drums, instead of being mere cylinders, are carved into the most strange and fantastical patterns. I possess one of these curious Drums, brought from Ashanti, and carved out of a solid piece of wood.
The strange point in it is, that it represents a double head carrying, after all negro fashions, a sort of vessel upon it. One part of the head represents a human head (not that of a negro), while the other merges gradually into an eagle’s head and beak. It is, in fact, a Gnostic gem, and would pass muster as such if it had been engraved on chalcedony, cornelian, or other semi-precious stones which are employed in the seal-engraver’s art.
Upon this composite head is placed the Drum itself, which is also cut out of the solid block, and which, after the fashion of West African Drums, has a hole on one side.
This remarkable instrument was given to me by an old merchant captain, who brought it himself from West Africa, and who, when I made his acquaintance, had actually painted it all kinds of colours, planted it in his garden, and was using the Drum as a flower-pot. Of course, as soon as it came into my possession, I put it in “pickle,”—i.e. a strong solution of alkali,—brushed off the paint, and placed it in my museum, where it is now.
On the left hand of the illustration on page 514 is given a sort of map or chart of the human Ear, with its internal Drum, or Tympanum, as it is scientifically termed.
It is by the vibration of this Drum that hearing is made possible, the vibrations of the air being transmitted to the Drum by means of a beautiful bony apparatus, termed the Hammer, Anvil and Stirrup. Sometimes the action of the Drum is partially checked, and then the sufferer is said to be “hard of hearing.” Sometimes it is broken, or its action totally clogged, and then he is said to be “stone deaf.” There have been cases where an artificial tympanum has been inserted, and answered its purpose fairly well.
The String and Reed
It has previously been mentioned that all sounds are owing to vibrations of the air. But there are many ways of producing these vibrations, and each mode gives a different quality of tone. We have already seen, by means of the drum, how sound is produced by percussion. We shall now see how sounds can be produced by the vibrations of a String.
If the string of a bow be pulled and smartly loosed, the result is a distinctly musical sound, higher or lower according to the length and tension of the string. Perhaps some of my readers may recall the passage in Homer’s “Odyssey,” where Ulysses strings the fatal bow:—
“Heedless he heard them; but disdained reply,
The bow perusing with exactest eye.
Then, as some heavenly minstrel, taught to sing
High notes responsive to the trembling string,
To some new strain when he adapts the lyre,
Or the dumb lute refits with vocal wire,
Relaxes, strains, and draws them to and fro;
So the great master drew the mighty bow,
And drew with ease. One hand aloft displayed
The bending horns, and one the string essayed.
From his essaying hand the string let fly,
Twanged short and sharp, like the shrill swallow’s cry.”
The Harp is, in fact, nothing but a magnified bow, with a number of strings of graduated length and tension. Some very beautiful experiments have been made on this subject by the Rev. Sir F. A. G. Ouseley, Professor of Music at Oxford, who stretched a string of sixty-four feet in length, and found that although, when vibrating, it must produce a note, there was no human ear that could distinguish it. Yet, if combined with other musical instruments, it would probably do its work well. The theory of the vibrations will be briefly described on another page.
These vibrations may be produced in various manners. The string may be pulled with the fingers, as in the harp, the guitar, the zither, or even the violin, &c., in pizzicato passages.
The old harpsichord, now an instrument vanished into the shadows of the past, pulled the strings with little strips of quill, acting like the thumb-ring of the zither-player. The “plectrum” of the ancients acted in the same manner, and the Japanese have at the present day a sort of guitar played with a plectrum. I have heard it, but cannot particularly admire the effect, the notes appearing to be without feeling, and as if they were played on a barrel-organ.
Sometimes, as in our modern pianos, the strings are struck by hammers instead of being pulled by fingers, plectrum, or goose-quill.
The most ingenious mode of causing musical vibration is the Bow, which is too familiar to need a detailed description. Suffice it to say that it really is a modified bow, the place of the string being supplied by a flat band of horsehair, which is drawn over the string, and so causes it to vibrate. In order to enable the bow to grip the string, it is rubbed with resin almost as often as a billiard-player chalks his cue.
Some skill is required even in producing a sound by the bow. It looks as if any one could do it, but a novice, if he extorts any sound at all, never rises above a squeak. When I took my first violin lessons, nearly thirty years ago, I was so horrified at the discordant sounds elicited from the instrument, that I retired to the topmost garret of the house in order not to hurt any one’s feelings except my own.
On the left hand of the illustration is seen a well-known example of the imitation of Nature by Art. This is the common Cricket, whose loud shrill call is more familiar than agreeable.
Some years ago, while engaged on my “Insects at Home,” I gave much time to the examination of the structures by which such a sound can be produced. On the under side of the wing-covers, or “elytra,” as they are scientifically termed, are notched ridges, which, when examined with a moderate power of the microscope, have something of this appearance ~~~~~~~. The friction of these notches produces the musical sound, which, as the reader will see, is exactly analogous to the friction of the bow upon the string.
Next we come to the Vibrator, sometimes called the Reed. It is introduced into various musical instruments, such, for example, as the harmonium, the clarionet, the oboe, the bassoon, and various organ pipes.
The simplest form of the Vibrator is shown in the Jew’s Harp, as it is popularly called, though it is not a harp, and has nothing to do with Jews.
The word is really a mistaken pronunciation of “jaw’s harp,” because the instrument is held against the teeth, while its tongue is vibrated by strokes of the finger. These vibrations affect the air within the mouth, and, by expanding or contracting the mouth, the sound is lowered or raised according to the laws of Acoustics. Of course, the range of notes is very small, being limited to those of the common chord, and even they being attainable only by a practised performer. Very good effects, however, have been produced by means of a series of Jew’s Harps, set to different tones by loading the end of the tongue with sealing-wax or similar substances.
An apparatus constructed on the same principle is to be found in the vocal organs of the male Cicada. If one of these insects be examined on the lower surface, two curious and nearly circular flaps will be seen, just at the junction of the thorax with the abdomen. It is by the action of these two little vibrators that the insect is able to produce a sound so loud, that in calm weather it may be heard at the distance of a mile.
The accompanying illustration is, in fact, a sort of chart as to the vibration of sound.
On the right is shown the Æolian Harp, with its upper lid raised, so as to show the structure of the strings. These are all tuned to the same note, the present D being generally accepted as being most free from false tuning, and less liable for the errors of “temperament.” Several of the strings are an octave lower than the others, but the tonic is always the same.
The instrument is placed in a current of air, generally in a window, with the sash let down upon it, and the air-currents set the strings vibrating in a most wonderful manner.
There is no need for human fingers to touch them, but they automatically divide themselves into the component parts of the common chord, and produce octaves, fifths, and thirds ad infinitum.
On the left hand of the same illustration is exhibited a string of the same length and tension, vibrating in two different ways. The upper figure shows it divided into three portions, each of which gives the fifth above the tonic, and all of which, when sounding simultaneously, give a fulness and richness to the tone which could only be attained otherwise by three distinct instruments. All players of stringed instruments know how invaluable are these harmonics, without which many passages of well-known music could not be played, and which are produced by “damping,” and not pressing the strings.
So, if the string be lightly touched, or damped at the crossing portion at either end, the result will be that the string divides itself into three portions, and all three resound simultaneously.
The lower string is vibrating in thirds, having divided itself into four portions. If it were damped in the middle, it would divide itself into two portions, and sound octaves.
The subject is a most interesting one, but our space is nearly exhausted, and we must pass to another branch of it.
In all brass instruments furnished with a mouthpiece, and not with a reed, the notes are obtained by vibrations of the enclosed air, caused by the movement of the lips. They are all set to some definite tonic, sometimes C natural, but mostly to a flat tone, such as B flat or E flat.
Taking the ordinary military trumpet or bugle as an example, we have (when we have learned how to play it), first, the tonic. By alteration of the lips we get the octave above the tonic. Then comes the fifth; then the third, which is, in fact, another octave; and then a few other notes, the truth of which depends on the ear of the player.
Now, all these notes are obtained by means of the lips, which set the column of air vibrating, and divide it into harmonics. The apparently complicated bugle-calls of the army are nearly all formed from four notes only, i.e. (taking C as the tonic) C G C E G.
The Trombone, which is shown on the right hand of the illustration, has the advantage of being lengthened at will, and thus giving the performer a fresh tonic, and consequently another series of harmonics. Valved and keyed instruments have a similar advantage, the one acting by lengthening, and the other by shortening, the column of air. The former is infinitely the better plan, as it sets more harmonics vibrating, and consequently gives a greater richness of tone.
A familiar example of this is to be found in the Ophicleide and Euphonium. The former is eight feet in total length, and alters its tonic by eleven keys, which shorten the column of air. The latter is of the same length, but, by the employment of valves, can be made sixteen feet in length. Consequently the euphonium has practically killed the ophicleide, just as the ophicleide killed the serpent. The cornet-à-pistons, the brass contra-basso, the flugel horn, the tenor sax-horn, &c., are all constructed on the same principle.
On the left hand of the illustration is shown the wonderful apparatus by means of which the Swan produces its far-resounding cry. The windpipe, or “trachea,” as it is technically named, passes down the neck, protected by the bones, until it reaches the chest. There it leaves them, enters the cavity of the chest, and contorts itself in such a manner as to obtain greater length, just as is the case with the trombone and valved instruments.