Part 4

In this new instrument, no tubes are necessary, as in the ordinary opera-glass; their place being supplied by a slender elastic conical spring of wire, into the upper extremity of which is inserted the eye-glass; the object-glass being fixed to the other extremity, as shown in the engraving. The two glasses must, of course, be kept parallel to each other when in use; which is very easily effected.

In using this opera-glass, rest the finger and thumb of one hand on the rim of the object-glass, B, whilst, with the thumb and finger of the other hand you hold the rim of the eye-glass, A. The spring tube may then be drawn out or shut up to very minute distances. Thus, the ordinary sliding tubes are superseded; nor is any external covering necessary, as the hand in grasping the instrument serves the purpose. If, however, a covering be preferred, a piece of silk may be sewn to the spirals of the spring.

This kind of opera-glass may be made very cheaply; it may be shut into a small space for the pocket, merely by pressing the object-glass and eye-glass together.

MULTIPLYING THEATRES.

Place two pieces of looking-glass, one at each end, parallel to one another, and looking over, or by the edge of one of them, the images of any objects placed on the bottom of the box, will appear continued to a considerable distance.

Or, line each of the four sides of the box with looking-glass, and the bottom of the box will be multiplied to an astonishing extent, there being no other limitation to the number of images but what is owing to the continued loss of light from reflection. The top of the box may be almost covered with thin canvas, which will admit sufficient light to render the exhibition very distinct.

The above experiments may be made very entertaining, by placing on the bottom of the box some toy, as sentry soldiers, &c.; and, if these be put in motion, by wires attached to them, or passing through the bottom or side of the box, it will afford a still more entertaining spectacle. Or the bottom of the box may be covered with moss, shining pebbles, flowers, &c.; only, in all cases, the upright figures between the pieces of looking-glass should be slender, and not too numerous, else they will obstruct the reflected light.

In a box with six, eight, or more sides, lined with looking-glass, as above, the different objects in it will be multiplied to an almost indefinite extent.

APPARATUS FOR WRITING IN THE DARK.

In this ingenious contrivance, A is a frame of wood, into the back and front of which are inserted two thin boards, the front one, B, reaching about half the height of the frame, and the back one being movable, by sliding in grooves, for better fixing the paper to be written on, C, to a roller at top, with a handle and ratchet working into a spring.

To use the apparatus, the paper is to be fixed on the roller, and a strip of lead, or other weight, suspended from the bottom of the paper, to keep it smooth: then, by resting the right hand on the edge of the board B, and turning, with the left hand, the ratchet, the distance of the lines may be regulated by the number of clicks caused by the spring on the ratchet. D, is a foot to support the apparatus, which, however, should be light enough to be held in the hand as a slate.

PORTABLE MICROSCOPE.

This cheap and useful instrument consists of a handle of hard wood, _a_, which is screwed into a brass piece, _d_, having, at its top, a ring, with screws on back and front, into which are to be screwed two cells with lenses of different foci. There is also a projecting piece formed on the side of the brass piece, _d_, in which is a hole to receive the screwed end of a cylindrical rod of brass, _c_. Upon this rod, a springing slit socket, _e_, slides backwards and forwards, and is also capable of being turned round. This socket has affixed to it, on one side, a projecting part, with a screwed cavity in it, to receive a short screwed tube, with a small hole in its centre, made to fit the steel stem of the spring forceps; a corresponding hole being made at the bottom of the screwed cavity, where is lodged a piece of perforated cork; which, being pressed upon by the action of the screw, closes upon the steel stem of the forceps, and steadies them, and the objects held in them. The stem of the forceps being removed from its place in the short tube; the handles and lenses, and the rod, _c_, and the sliding socket upon it, being unscrewed from its place in the handle; they can all three be packed in a black paper case, which is only three and a half inches long, one inch broad, and half an inch thick.

This microscope possesses three different magnifying powers, namely, those of two lenses separately, and the two in combination.

Microscopes of a still simpler nature are small globules of glass, formed by smelting the ends of fine threads of glass in the flame of a candle; and small globular microscopes of great magnifying power, made of hollow glass about the size of a small walnut, may be purchased very cheaply at the opticians’.

THE PHENAKISTISCOPE, OR STROBOSCOPE.

This amusing instrument consists of a turning wheel, upon which figures are seen to walk, jump, pump water, &c. The disc or wheel should be of stout card-board, upon which should be painted, towards the edge, figures in eight or ten postures. Thus, if it is wished to represent a man bowing, the first position is a man standing upright; in the second, his body has a slight inclination; in the third, still more; and so on, to the sixth position, where the body is most bent: the four following, represent the figure recovering its erect posture, so that the fifth and seventh, the fourth and eighth, the third and ninth, and second and tenth figures, have the same posture. Between each of the figures on the wheel, should be a slit, three-fourths of an inch long, and one-fourth of an inch wide, in a direction parallel with the radii of the wheel, and extending to an equal distance from the centre.

To work this instrument, place the figured side of the wheel before a looking-glass, and cause it to revolve upon its centre; then look through the slits or apertures, and you may observe, in the glass, the figures bowing continually, and with a rapidity proportionate to the rate at which the wheel turns. The illusion depends on the circumstance, that the wheel between each aperture is covered, while the figure goes further. That the deception may be complete, it is necessary that every part of the figures not bowing shall be at an equal distance from the centre of the wheel, and from the slits; also that the figures possess equal thickness and colour.

TO LOOK AT THE SUN WITHOUT INJURY.

Provide a wine-glass filled with plain water, which will keep off the heat so effectually, that the brightest sun may be viewed some time through it without any inconvenience. If a little black ink be added to the water, the image of the sun will appear through it, as white as snow; and when the ink is still more diluted, the sun will be of a purple hue.

BRILLIANT WATER MIRROR.

Nearly fill a glass tumbler with water, and hold it, with your back to the window, above the level of the eye, as in the engraving. Then look obliquely as in the direction E, _a_, _c_, and you will see the whole surface shining like burnished silver, with a strong metallic reflection; and any object, as a spoon, A C B, immersed in the water, will have its immersed part C B, reflected on the surface, as in a mirror, but with a brilliancy far surpassing that which can be obtained from quicksilver, or from the most highly-polished metals.

OPTICAL ILLUSION UNDER WATER.

Procure a large gallipot; place on the bottom, next the side furthest from you, a sixpence, and next to it, but towards the centre, a shilling; move to such a distance as will render the coins invisible; then let another person pour water gently in, and as it rises in the gallipot, it will cause both the sixpence and shilling to be seen, without your approaching nearer to the gallipot, or moving it towards you.

THE MAGIC WHEELS.

Cut out two card-board cog-wheels of equal size; place them upon a pin, and whirl them round with equal velocity in opposite directions; when, instead of producing a hazy tint, as one wheel would do, or as the two would if revolving in the same direction, there will be an extraordinary appearance of a fixed wheel. If the cogs be cut slantwise on both wheels, the spectral wheel, as it may be called, will exhibit slanting cogs; but if one of the wheels be turned, so that the cogs shall point in opposite directions, then the spectral wheel will have straight cogs. If wheels with radii, or arms, be viewed when moving, the deception will be similar; and however fast the wheels may move, provided it be with equal velocity, the magic of a fixed wheel will be presented.

Or, cut a card-board wheel with a certain number of teeth or cogs at its edge; a little nearer the centre, cut a series of apertures resembling the cogs in arrangement, but not to the same number; and still nearer the centre cut another series of apertures, different in number, and varying from the former. Fix this wheel upon another, with its face held two or three yards from an illuminated mirror; spin it round, the cogs will disappear, and a greyish belt, three inches broad, will become visible; but, on looking at the glass through the moving wheel, appearances will entirely change; one row of cogs, or apertures, will appear fixed, as if the wheel were not moving, whilst the other two will appear as if in motion; and, by shifting the eye, other and new effects appear.

These amusing deceptions were first experimented by Mr. Faraday. The simple apparatus for their exhibition may be purchased, for a trifling sum, of any respectable optician.

ACOUSTIC RAINBOW.

A sounding-plate, made of brass, nine inches long, and half a line in thickness, covered with a layer of water, may be employed to produce a rainbow in a chamber which admits the sun. On drawing a violin bow strongly across the plate, so as to produce the greatest possible intensity of tone, numerous drops of water fly perpendicularly and laterally upwards. The size of the drops is smaller as the tone is higher. The inner and outer rainbows are very beautifully seen in these ascending and descending drops, when the artificial shower is held opposite to the sun. When the eyes are close to the falling drops, each eye sees its appropriate rainbow; and four rainbows are perceived at the same time, particularly if the floor of the room is of a dark colour. The experiment succeeds best, if, when a finger is placed under the middle of the plate, and both of the angular points at one side are supported, the tone is produced at a point of the opposite side, a fourth of its length from one of its angles. An abundant shower of drops is thus obtained.

TRANSMISSION OF SOUND.

Suspend any sonorous body, as a bell, a glass, a silver spoon, or a tuning-fork, from a double thread, and put with the finger the extremities of the thread, one in each ear; if the body be then struck, the apparent loudness and depth of the sound will be surprising.

Again, if you shut your ears altogether, you will yet feel very sensible of the impression of any sound conveyed through the mouth, the teeth, or the head: if you put one end of a small stick or rod in the mouth, and touch with the other extremity a watch lying on the table, the beatings will become quite audible, though the ears be actually shut. So, also, if a log of wood be scratched at one end with a pin, a person who applies his ear to the other end will hear the sound distinctly.

Fogs and falling rain, but especially snow, powerfully obstruct the free propagation of sound; and the same effect is produced by a coating of fresh-fallen snow on the ground, though when glazed and hardened at the surface by freezing, it has no such influence.

Over water, or a surface of ice, sound is propagated with remarkable clearness and strength. Dr. Hutton relates, that on a quiet part of the Thames, near Chelsea, he could hear a person distinctly at 140 feet distance, while on the land the same could only be heard at 76 feet. Lieutenant Forster, in the third Polar expedition of Captain Parry, held a conversation with a man across the harbour of Port Bowen, a distance of 6696 feet, or about a mile and a quarter. This, however remarkable, falls short of what is related by Dr. Young, on the authority of the Rev. W. Derham, _viz._ that, at Gibraltar, the voice has been heard ten miles, perhaps, across the strait.

The cannonade of a sea-fight between the English and Dutch, in 1672, was heard across England as far as Shrewsbury, and even in Wales, a distance of upwards of 200 miles from the scene of action.

At Carisbrook Castle, in the Isle of Wight, is a well 210 feet in depth, and twelve feet in diameter, into which if a pin be dropped, it will be distinctly heard to strike the water. The interior is lined with very smooth masonry.

PROGRESS OF SOUND.

A stretched string, as that of a piano-forte, may be made to vibrate not only from end to end, but in aliquot parts, the portions being separated by points of rest which interrupted the progress of the sound. This kind of effect may be shown by shaking a long piece of cane in the air, when there will be one, two, or three points of rest, according to the mode of vibrating it.

An elastic surface has, likewise, some parts in motion and others at rest; and these parts may be made visibly distinct, by strewing pieces of bristle over them upon the sounding-board of an instrument.

When a bow is drawn across the strings of a violin, the impulses produced may be rendered evident by fixing a small steel bead upon the bow; when looked at by light or in sunshine, the bead will seem to form a series of dots during the passage of the bow.

SOUND TURNING CORNERS.

Take a common tuning-fork, strike it, and hold it, (when set in vibration,) about three or four inches from the ear, with the flat side towards it, when the sound will be distinctly heard; let a strip of card, somewhat longer than the flat of the tuning-fork, be interposed at about half an inch from the fork, and the sound will be almost entirely intercepted by it; and, if the card be alternately removed and replaced in pretty quick succession, alternations of sound and silence will be produced; proving that sound is by no means propagated with so much intensity round the edge of the card, as straight forward. Indeed, to be convinced of this fact, you have only to listen to the sound of a carriage turning a corner from the street in which you happen to be, into an adjoining one. Even where there is no obstacle in the way, sounds are by no means equally audible in all directions from the sounding body; as you may ascertain, by holding a vibrating tuning-fork or pitch-pipe near your ear, and turning it quickly on its axis.

TO TELL THE DISTANCE OF THUNDER.

Count, by means of a watch, the number of seconds that elapse between seeing the flash of lightning and hearing the report of the thunder; allow somewhat more than five seconds for a mile, and the distance may be ascertained. Thus, say the number of seconds is

5)20 —— 4 miles distant;

or the distance may be estimated by remarking the number of beats of the pulse in the above interval; provided, of course, that we know the rate at which the pulse beats in a certain time. In a French work, it is stated that if the pulse beat six times, the distance of the thunder will be about 30,000 feet, or five miles and a half; thus reckoning 5000 feet for each pulsation.

In a violent thunder-storm, when the sound instantly succeeds the flash, the persons who witness the circumstance are in some danger; when the interval is a quarter of a minute, they are secure.

HEARING BY THE TOUCH.

If a deaf person merely place the tips of his finger-nails on the window-shutters or door of a room in which instruments are playing, he may enjoy their concert of harmony.

CONVERSATION FOR THE DEAF.

If two persons stop their ears closely, they may converse with each other by holding a long stick between their teeth, or by resting their teeth against them. The person who speaks may rest the stick against his throat or his breast; or he may rest the stick, which he holds in his teeth, against a glass tumbler or china basin into which the other speaks. The sound may also be heard when a thread is held between the teeth by both persons, so as to be somewhat stretched.

GLASS BROKEN BY THE VOICE.

On vibrating bodies, which present a large surface, the effects of sounds are very surprising. Persons with a clear and powerful voice have been known to break a drinking-glass, by singing the proper fundamental note of their voice close to it. Looking-glasses are also said to have been broken by music, the vibrations of the atoms of the glass being so great as to strain them beyond the limits of their cohesion.

FIGURES PRODUCED BY SOUND.

Stretch a sheet of wet paper over the mouth of a glass tumbler, which has a footstalk, and glue or paste the paper at the edges. When the paper is dry, strew dry sand thinly upon its surface. Place the tumbler on a table, and hold immediately above it, and parallel to the paper, a plate of glass, which you also strew with sand, having previously rubbed the edges smooth with emery powder. Draw a violin bow along any part of the edges, and as the sand upon the glass is made to vibrate, it will form various figures, which will be accurately imitated by the sand upon the paper; or, if a violin or flute be played within a few inches of the paper, they will cause the sand upon its surface to form regular lines and figures.

TRANSMITTED VIBRATION.

Provide a long, flat glass ruler or rod, as in the engraving, and cement it with mastic to the edge of a drinking-glass fixed into a wooden stand; support the other end of the rod very lightly on a piece of cork, and strew its upper surface with sand; set the glass in vibration by a bow, at a point opposite where the rod meets it, and the motions will be communicated to the rod without any change in their direction. If the apparatus be inverted, and sand be strewed on the under side of the rod, the figures will be seen to correspond with those produced on the upper surface.

DOUBLE VIBRATION.

Provide two discs of metal or glass, precisely of the same dimensions, and a glass or metal rod; cement the two discs at their centres to the two ends of the rod, as in the engraving, and strew their upper surfaces with sand. Cause one of the discs, viz. the upper one, to vibrate by a bow, and its vibration will be exactly imitated by the lower disc, and the sand strewed over both will arrange itself in precisely the same forms on both discs. But if, separately, they do not agree in their tones, the figures on them will not correspond.

CHAMPAGNE AND SOUND.

Pour sparkling champagne into a glass until it is half full, when the glass will lose its power of ringing by a stroke upon its edges, and will emit only a disagreeable and puffy sound. Nor will the glass ring while the wine is brisk, and filled with air-bubbles; but, as the effervescence subsides, the sound will become clearer and clearer, and when the air bubbles have entirely disappeared, the glass will ring as usual. If a crumb of bread be thrown into the champagne, and effervescence be re-produced, the glass will again cease to ring. The same experiment will also succeed with soda-water, ginger wine, or any other effervescing liquid.

MUSIC FROM PALISADES.

If a line of broad palisades, set edgewise in a line directed from the ear, and at even distances from each other, be struck at the end nearest the auditor, they will reflect the sound of the blow, and produce a succession of echoes: these, from the equal distance of the palisades, will reach the ear at equal intervals of time, and will, therefore, produce the effect of a number of impulses originating in one point. Thus, a musical note will be heard.

THEORY OF THE JEW’S HARP.

If you cause the tongue of this little instrument to vibrate, it will produce a very low sound; but, if you place it before a cavity, (as the mouth,) containing a column of air, which vibrates much faster, but in the proportion of any simple multiple, it will then produce other higher sounds, dependent upon the reciprocation of that portion of the air. Now, the bulk of air in the mouth can be altered in its form, size, and other circumstances, so as to produce by reciprocation, many different sounds; and these are the sounds belonging to the Jew’s Harp.

A proof of this fact has been given by Mr. Eulenstein, who fitted into a long metallic tube a piston, which, being moved, could be made to lengthen or shorten the efficient column of air within at pleasure. A Jew’s Harp was then so fixed that it could be made to vibrate before the mouth of the tube, and it was found that the column of air produced a series of sounds, according as it was lengthened or shortened; a sound being produced whenever the length of the column was such that its vibrations were a multiple of those of the Jew’s Harp.

MUSIC OF THE SNAIL.

Place a garden-snail upon a pane of glass, and in drawing itself along, it will frequently produce sounds similar to those of musical glasses.

TO TUNE A GUITAR WITHOUT THE ASSISTANCE OF THE EAR.

Make one string to sound, and its vibrations will, with much force, be transferred to the next string: this transference may be seen by placing a saddle of paper (like an inverted v, Λ) upon the string, at first in a state of rest. When this string _hears_ the other, the saddle will be shaken, or fall off; when both strings are in harmony, the paper will be very little, or not at all, shaken.

MUSIC FROM GLASS OR METAL RODS.

Provide a straight rod of glass or metal; strike it at the end in the direction of its length, or rub it lengthwise with a moistened finger, and it will yield a musical sound, which, unless its length be very great, will be of an extremely acute pitch; much more so than in the case of a column of air of the same length, as in a flute. The reason of this is the greater velocity with which sound is propagated in solids than in the air. If the rod be metal, the friction will be found to succeed best when made with a bit of cloth, sprinkled with powdered resin; or, if of glass, the cloth or the finger may be moistened and touched with some very fine sand or pumice powder.

Generally speaking, a fiddle-bow, well resined, is the readiest and most convenient means of setting solid bodies in vibration. To bring out their gravest or fundamental tones, the bow must be pressed hard and drawn slowly; but, for the higher harmonies, a short, swift stroke, with light pressure, is most proper.

THE TUNING-FORK A FLUTE-PLAYER.

Take a common tuning-fork, and on one of its branches fasten with sealing-wax a circular piece of card, of the size of a small wafer, or sufficient nearly to cover the aperture of a pipe, as the sliding of the upper end of a flute with the mouth stopped: it may be tuned in unison with the loaded tuning-fork (a C fork), by means of the moveable stopper or card, or the fork may be loaded till the unison is perfect. Then set the fork in vibration by a blow on the unloaded branch, and hold the card closely over the mouth of the pipe, as in the engraving, when a note of surprising clearness and strength will be heard. Indeed, a flute may be made to “speak” perfectly well, by holding close to the opening a vibrating tuning-fork, while the fingering proper to the note of the fork is at the same time performed.

MUSICAL BOTTLES.