musical tone, but is called a noise, as, for instance, when we rattle pieces of metal in a box, or let a weight fall to the ground. What, then, is the difference between the two ? When the vibrations succeed one another in a regular and uniform manner, as in the case first mentioned, a perfect sound or tone is produced; but when the vibrations are not isochronous, or when a single explosive disturbance of the air is produced-as, for instance, by a sudden blow or the report of a pistol-or when several sounds interfere with one another so as to produce confused waves in the air, in any such cases a noise is the result. If we examine a few sounding bodies we can easily satisfy ourselves that in every case their particles are thrown into a state of rapid vibration.

In a sounding cord (Fig. 1), or the wire of a piano, these vibrations are easily seen by the eye, and in cases where a flat surface is made to sound they may be rendered manifest by sprinkling a little light powder, as, for instance, lycopodium, on it. The motion will thus be at once rendered visible by the agitation of the dust.

A much more elegant plan of showing the same effect is by means of the apparatus represented in Fig. 2. A hemispherical bowl of thin glass is fixed to a stand, and directly over it is suspended a small frame of six arms, from which hang as many threads, each carrying a small ivory ball. This is so arranged that the balls shall just rest against the rim of the glass.

Now let a violin bow be rubbed with a lump of resin, and then drawn steadily over the edge of the glass. A clear musical note will be produced, but the vibrations of the glass will scarcely be perceived by the eye. The ivory balls will, however, at once act as tell-tales, for they will be violently agitated and swing away from the glass, and the louder the note produced, the greater will be the amount of their oscillation.

We must now see in what way the vibrations which are thus produced are propagated through the air, so as to reach our auditory nerves. The particles of air immediately around the vibrating body are not driven right away so as to strike the tympanum of the ear. Each one is moved a slight distance from its original position, to which, however, it immediately returns, and then recedes almost as far in the other direction. These particles, however, impart a similar oscillating movement to those lying beyond them, which in their turn communicate the movement, and thus the waves produced are conveyed from particle to particle, and travel widely and rapidly.

If we fix one end of a long rope or cord to a staple in a wall, and holding the other end in the hand, shake it, waves will appear to travel from the hand to the staple and back again. We know, however, that in reality each portion of the cord merely moves up and down in an almost straight line, and the successive movements of the single portions produce the appearance of a wave. This affords a good idea of the mode in which sound-waves are propagated by the oscillations of different layers of air.

By standing at the head of a pier, and watching the waves rise and fall in the sea, we get a further illustration of the same fact. They appear to be travelling along and coming ashore in rapid succession; but if we drop a piece of wood on the surface in a part where it is not affected by the breaking of the waves against the pier, we shall find that it scarcely moves along at all, but merely rises and falls on their surface.

So, too, if we drop a stone into the middle of a pond of water whose surface is quite calm, we shall see the waves produced by it gradually enlarging and spreading in all directions towards the sides. As, however, they recede and become wider, they diminish in height, till in a large pond they are quite lost. In just the same way a bell or any sounding body produces waves in the air around it, which extend further and further, diminishing in intensity as they travel, till at last they become too faint to affect the ear, or else are overpowered by the multitudes of other vibrations which exist in the air.

By taking a shallow rectangular vessel of water, and watching the waves produced in it when we touch its surface, we shall be able to understand many things in connection with the diffusion and reflection of sound that would otherwise appear difficult. A moment's consideration will easily show us why it is that a sound diminishes so rapidly in intensity as we recede from the sounding body. Since the waves are propagated equally in all directions, it is clear that the mass of air set in vibration in

creases very rapidly; the original vibration has therefore to be spread over a much larger area, and its intensity is diminished in the same proportion.

If

It is

From this we see that it is necessary to have some substance to convey the vibrations from the vibrating body to the ear. the atmosphere were entirely removed, no sound would ever reach us; all would be continual unbroken silence. We can easily obtain an experimental illustration of this fact. An alarum (Fig. 3), made so as to continue striking for some little time, is placed under the receiver of an air-pump, a layer of wadding being placed between it and the pump-plate to prevent the vibrations being communicated to the air in that way. now set in action, and the pump rapidly worked; as the air under the receiver becomes more and more rarefied the sound becomes feebler and feebler, till at last it almost entirely ceases, though we can see by the eye that the hammer still continues to strike on the bell. A better way of performing the experiment is to suspend the alarum by means of threads from four supports, as in this way all the vibrations are kept from the pump-plate. A rod is then made to pass air-tight through the top of the receiver, and by pressing this down a detent can be moved, so as to stop or start the bell at pleasure. When a nearly perfect vacuum is attained, no sound whatever will be heard even when the ear is applied closely.

Now admit hydrogen gas into the receiver in place of common air, and allow the alarum to strike as before; the sound will be heard, but it will be faint and peculiar in tone. If we inhale hydrogen gas (which for this purpose must be quite pure), and then attempt to speak, the voice likewise will be found greatly changed in character, having become hollow and thin, at the same time being considerably higher than usual, so as to resemble a squeak. We see then that the intensity of any sound depends upon the density of the air in which it is generated, and not of that in which it is heard.

When at great elevations on the sides of mountains, all sounds are wonderfully diminished in intensity in consequence of the rarefied state of the air. Saussure says that on the summit of Mont Blanc the report of a pistol was not louder than that of an ordinary cracker, and the travellers were obliged to speak in a louder tone than usual in order to be heard.

The rate at which the sound-wave travels through the air does not depend at all upon the intensity or the pitch. If it did, music when heard at a little distance would be quite changed into discord, since the louder notes would outstrip the others.

In the case, however, of extremely loud sounds, such as, for instance, the report of a heavy piece of ordnance, there seems to be a slight departure from this law.

Sound is conducted by liquids or solids, as well as by gases. When two stones are struck together under water, the sound is conveyed a considerable distance. Divers, too, can communicate with those on the surface by striking the sides of the diving-bell with a hammer or stone. If a watch be laid upon one end of a plank, and the ear applied to the other end, the ticking will be heard much further off than it would otherwise be. In a similar way the earth acts as a conductor of sound, for if the ear be applied to its surface, the footsteps of men or horses approaching may be heard at a very great distance. So, too, by laying the ear upon the metal rails, the sound of a train can be heard much further off than it can by any person merely standing up and listening.

Many very interesting experiments can be tried to illustrate the conduction of sound. One of the simplest is to suspend a common poker by a piece of string or list. Wind the ends of this round the forefinger of each hand, and, having put the fingers in the ears, make the poker swing so as to strike against the fender or some piece of metal. Instead of the sound usually heard we shall now hear one almost resembling that of a church bell. The vibrations are conveyed so much more plentifully along the string than through the air, that the sound is very greatly increased in intensity, and is heard for a longer period. In a similar way we can easily conduct sound from place to place. Let a thin wooden rod some twelve or fifteen feet long be rested on the tips of the fingers of two people, and against one end of it let there be held a thin sounding-board, or a box of thin wood, or, better still, a violin. Now strike a tuning-fork, and place it against the other end of the rod. The sound will at once fill the room, but will appear to proceed, not from the

tuning-fork, but from the sounding body at the other end of the rod. Every vibration of the former is conveyed along the rod, and accurately reproduced at the other end. It is heard much more distinctly there because it is distributed over the surface of a large sounding body, and thus the waves of sound produced are much more distinct. If two forks sounding different notes be struck and placed together at the end, both sounds will be conveyed along the rod, the vibrations of the one appearing not to interfere with those caused by the second.

A very interesting modification of this experiment was introduced by Professor Wheatstone at the 'London Polytechnic Institution some years ago, and has been many times repeated since. It was an arrangement known as the Telephonic Concert. Long deal rods were made to pass up from the basement of the building through the different ceilings to the floor of the lecture hall, above which they projected a little distance. The lower ends of these were made to rest upon various musical instruments; the end of one being pointed and made to rest upon the sounding-board of a piano, while another was in contact with a violin, and so on. On the upper ends of these harps were placed, so that the rods were in contact with their sounding-boards. They were, however, so arranged that they could very easily be removed from the rods when necessary. A gentle tap conveyed to the performer below intimation that all was ready, and every sound emanating from the instruments was faithfully conveyed along the rods, and appeared to issue from the harps resting upon them. It can easily be understood what a strange effect was produced by the sound of a piano, violin, or other musical instrument appearing in this way to issue from a harp, especially as no performers could be seen. If the harp were moved at all, SO as to break the contact between it and the rod, every sound at once ceased, though the performers still continued to play upon the various instruments. On renewing contact, the sound continued as before. The experiment is a very remarkable one, all the different vibrations produced by the various wires of the piano being conveyed along the one rod without interfering at all with one another.

plenty of room all round it. In the top of this make a small hole through which the rod may pass. If two boxes can be procured, one of which can go inside the other, the musical-box being placed in the inner one, the result will be still more satisfactory. Now line or pack them carefully with wadding or baize, so as completely to drown the sound of the music, taking care, however, to leave room for the rod to pass quite down to the box, and also to arrange for winding it up, when required. No sound will now be heard, the vibrations being completely muffled by the non-conducting materials employed. If, however, we insert the rod, and place on it a thin piece of board, the music will become distinctly audible. A spiral spring placed on the board, as shown in Fig. 4, will increase the sound considerably. A violin, being specially constructed for the purpose of spreading sound, answers the purpose still better, and may be

Fig. 2.

Fig. 1.

This experiment has been carried even further than this. The attempt was made to convey the music of the human voice in the same way. The performers were placed with their mouths very close to a sounding-board connected with the rod, and, as they sung, the music was conveyed along the rod, and produced the remarkable phenomenon of a singing harp. The success of this experiment was even more complete than could have been anticipated. The performers were obliged to be so close together, and to remain in such a ludicrous and confined position, that often they bumped their heads together, and the music ended in a peal of laughter, which was, of course, reproduced by the harp, to the no small astonishment of the

audience.

This experiment is rather difficult and costly to repeat in a private house. A very similar one, showing fully the principle on which it depends, and creating much astonishment, may, however, easily be tried with a small musical box. Let a box be procured large enough to contain the musical box, with

used in place of the thin wood.

Another point illustrated by these experiments is the effect of sounding-boards in increasing the volume of sound produced by musical instruments. It is well known that the sound of a tuning-fork will be heard much more distinctly if its end be placed upon a thin box or piece of wood. In stringed instruments this is especially important. If the cord be merely fixed to firm supports, and set in vibration, the note will be faint and indistinot; but if a thin piece of board be connected with it, or, better still, if the cord be stretched on one side of a hollow wooden case, as is done in a violin, the volume of sound is immensely increased. For this reason a sounding-board is placed in the harp, the piano, and most other stringed instruin ents.

We may now collect and review the main causes which influence the intensity of any sound.

The first, as has already been explained, is the distance of the sounding body from the ear, the sound being found to diminish in intensity inversely as the square of the distance; that is, a sound when heard at double the distance has only one-fourth the intensity.

Another cause is the density of the air in which the sound

is produced. This is shown by the bell under the exhausted receiver. As air is gradually admitted, the sound becomes more and more distinct; and if the receiver be filled with carbonic acid gas, the density of which is half as great again as that of air, the sound will be rendered much more intense. The intensity of any sound is further dependent upon the amplitude or extent of the vibrations of the sonorous body. When a stretched cord is first plucked or struck, its vibrations are much more extensive than they are when the sound grows fainter. So, too, if a tuning-fork (Fig. 5) be violently struck the sound will gradually become feebler as the vibrations of the limbs become more and more limited.

The next cause to which we must refer is the motion of the atmosphere and the direction of the wind. On a calm day sound is always conveyed better than when the air is disturbed. A gentle wind, too, causes the sound to be more intensely heard in the direction in which it is blowing.

The proximity of a sonorous body also serves to increase the power of sound. Illustrations of this have been given in the case of musical instruments, and we shall meet with several others as we proceed.

VOLTAIC ELECTRICITY.-XII.

ELECTRO-MOTIVE MACHINES-ALARUM-ELECTRIC CLOCK.

MANY different attempts have been made to employ electricity as a moving power, but the great expense of maintaining a sufficiently powerful current has hitherto proved the great obstacle to its practical employment. Great advantage would be derived from the facility of conducting the current from place to place, but as zinc has to be consumed in the battery in place of the coal used by the steam-engine, the advantage in a pecuniary point is altogether in favour of steam. It is quite possible, however, that some cheaper mode of generating electricity may be discovered, and then, doubtless, many electro-motive machines would be employed.

Most of the machines already made have been constructed on

during the rest it is separated from it. This spring is interposed in the course of the current, so that when B and C are in contact the circuit is complete, and the magnet then attracts the keeper. As soon, however, as the keeper approaches the magnet, the wheel has turned so far that contact is broken, and the weight of the keeper, together with the momentum acquired, carries the wheel round till contact is again made, and the circuit completed as before. Its action is thus very similar to that of a single-action steam-engine.

Two magnets are sometimes used instead of one, and are then so arranged as to face one another and act alternately. The keeper in this case is attracted by turns in each direction, and thus the machine may be compared to a doubleaction steam-engine. In many of these engines the magnets are made in three pieces, the two sides being fixed on to a

a very small scale, so that they may almost be ranked as philosophical toys; a few experiments have, however, been made on a much more extensive scale-in one case a small steamer, and in another a railway carriage, having been propelled by means of electricity. These experiments, though they conclusively proved the possibility of employing the electric current as a prime mover, showed likewise the fact that in an economical point of view it totally failed.

The simplest way of giving motion to a machine by means of an electric current is to place the keeper of an electro-magnet that its own weight or a spring may keep it a little distance away from the poles. As soon, then, as the contact is made, and the current passes round the coils, the keeper is drawn to the magnet. The machine is so constructed that the current is alternately established and broken, and in this way a vibrating motion is given to the keeper, which then drives a wheel in the same way as the treadle of a lathe turns the driving-wheel.

The breaking of the current is usually effected by placing a projecting piece of metal, B, on the axle, A (Fig. 78), of the fly-wheel. During a part of its revolution this comes in contact with another piece of metal, c, supported on a thin spring, but

VOL. VI.

cross-bar, as seen in Fig. 79; the action, however, is exactly the same as in the ordinary horse-shoe form, the alteration being merely made for the sake of greater convenience in fitting up the apparatus.

In Fig. 80 we have an illustration of another kind of magnetic machine, constructed by M. Bourbouze. This greatly resembles in form an ordinary beam-engine. Four hollow bobbins are employed, two of which only are seen in the drawing, the others being directly behind them. Solid plungers are made to fit loosely into these, and each pair is alternately set to work, so that first one end of the beam and then the other is drawn down. This oscillating movement is easily converted into a rotatory one by means of a crank, and thus the machine is worked.

An eccentric is fixed to the axle of the large wheel, and serves to make the connections between the battery and the alternate bobbins. The annexed detailed view of a part of the machine will explain the manner in which this is accomplished. A separate wire passes round each pair of bobbins, and one end of each wire is connected to one battery screw; the other ends are connected to the brass plates, A and B (Fig. 81), A being 137

connected with the right-hand pair, and B with the left-hand pair. These plates are separated from each other by a piece of ivory, c. The second battery wire, E, is fixed to a screw seen near c, and then connected with a metal slide, D, moved backwards and forwards by the eccentric.

It will now be seen that, when the slide is in the position shown, the right-hand pair of bobbins will be in action, and the cores will be drawn into them. As soon, however, as they have reached the bottom, the slide will have been drawn back by the eccentric so far that it will be in contact with B. The other pair of bobbins will therefore commence to act, and in this way a continuous motion will be produced.

There are many different forms of electro-motive engines besides those we have already mentioned, most of them being made as scientific toys. The principle of action is, however, almost the same in all. Fig. 82 shows a mode of construction not unfrequently adopted. In this the ends of the keeper, A, are bent at right angles to its centre portion, and it is then mounted on two pivots placed at the angles, so as to oscillate, and thus, through the medium of a crank, drive the pump or other machinery.

On the axis of the fly-wheel is fitted a small wheel, divided into segments by pieces of ivory or other non-conducting material. This serves to make and break contact twice in each revolution, and is so arranged that the circuit is completed the moment when the ends of the keeper are farthest from the poles of the core. They are then attracted, but as soon as they come opposite to these ends, the current is interrupted, and the momentum of the fly-wheel carries it on through the next quarter of a revolution. In this way a constant motion is maintained. Electro-magnets are applied to very many different purposes apart from giving motion to machines of this description. In using the electric telegraph, some mode of calling the attention of the clerk at the receiving-station to the message is required, and this may be easily supplied in this way :-An ordinary alarum-bell, set in action by clockwork, is placed near the instrument. Against one of the wheels of this a small catch or detent is placed, so as to interrupt the motion of the clockwork, and prevent its running down till this catch is removed. electro-magnet is then fitted, so that when the current passes round it the catch is attracted and the mechanism set free. The sender, therefore, has only to transmit an electric current through this magnet, and the bell at once commences to ring, and continues to sound while the current is maintained, thus attracting the attention of the clerk to the message.

An

A self-acting alarum is frequently made, in which the clockwork is entirely dispensed with, and the bell rung by means of the electric current alone; and this apparatus is sometimes employed in place of that already described. The principle on which it acts is very simple and ingenious, and a somewhat similar arrangement is adopted in other pieces of apparatus that will be described hereafter.

It is clear that a hammer can easily be attached to the keeper of an electro-magnet by means of a piece of spring or wire, in such a way that, when the current is completed, it may strike a bell placed near it. A single stroke of the bell might, however, easily escape notice, and it would be tedious to keep on interrupting the current, so as to cause a series of strokes. What we want, then, is an arrangement by which the bell shall continue to ring as long as the current passes. The apparatus by which this is accomplished is represented in Fig. 83. A horseshoe electro-magnet, e, is firmly mounted in a case, the ends of the wire being brought to the binding-screws, p and p'. The keeper, c, is fixed to a piece of flat spring, m, so that when at rest it may be about ths of an inch away from the poles of the magnet. Above c is a hammer, K, supported by a piece of wire in such a way that, when the keeper is drawn to the magnet, K may almost touch the bell, T. As soon as the current passes round the bobbins, c is attracted to the poles; and when it strikes against them the elasticity of the wire causes K to strike the bell loudly, but not to remain in contact with it, and so damp the sound. The continuous ringing is produced by means of the spring, J. One battery wire is connected with one end of the wire wound on the magnet, by means of the binding. screw, p'. The other end of this wire is connected to p, and thus to the keeper, c; when this keeper is in its natural position, the current passes from it along the spring g to m, and thence to the battery, the circuit being thus completed. The

keeper is, however, immediately attracted, and thus removed from contact with y, so that the circuit is interrupted there, and e at once ceases to be a magnet. Thereupon the spring, m brings the keeper back to its original position, and again makes a contact with g. In this way the keeper is kept in a state of rapid oscillation, and the hammer, K, keeps on constantly striking the bell. A piece of paper is frequently placed on the poles of the magnet, to prevent absolute contact with the keeper, as otherwise it sometimes sticks for a moment or two, and thus impedes the motion. Very frequently, too, the spring, 9, is replaced by a screw tipped with platinum, and mounted on a small brass support. A platinum plate, or a piece of platinised silver. is then let into the keeper at the point where it meets the screw, and thus all rust at this point is avoided. The advantage of the screw lies in the fact that, by means of it, the play of the keeper can be more easily regulated, so as to suit the strength of the current.

Many other practical applications of electro-magnets are frequently made. At Greenwich Observatory, and several other places, time-balls are fixed. These are large balls, sliding upon a lofty pole, and at five minutes before one they are raised to the top of the pole, and kept there by a small catch attached to the keeper of an electro-magnet. Precisely at one an electric current is sent round this magnet; this releases the catch, and the ball instantaneously begins to fall, so that all vessels within sight may regulate their chronometers by it. Several of these balls are now fixed at various places along the coast, and all are dropped by an electric current from Greenwich. Formerly the circuit was completed by hand, a person watching the standard chronometer, and touching a spring exactly at the hour; but the apparatus is now made entirely self-acting, so as to avoid the possibility of error.

Many different electro-magnets may be placed in the same circuit, and all will be acted upon simultaneously. In some places a number of clocks are thus connected with a central regulator, electric currents being transmitted at frequent is tervals, so that they all indicate exactly the same time. Clocks have been constructed which are driven by electricity. In some of these the bob of the pendulum consists of a coil of insulated wire, which at the extreme of each oscillation passes on to the pole of a bar magnet, and a commutator is arranged so that the coil is alternately attracted and repelled by these magnets. The pendulum thus, instead of merely restraining the motion of the wheels, as in an ordinary clock, drives them, the pallets forcing on the teeth of the scape-wheel. With a constant battery an electric clock will keep in action for a very long time without being touched. It is found, however, that the variations in the temperature and in the intensity of the current somewhat interfere with the regularity of its movements.

By means of electricity very minute intervals of time can be measured: an apparatus of this kind, known as a chronoscope. was invented by Mr. Wheatstone for the purpose of measuring the speed of a cannon-ball or other projectile. Two screens. separated by a known distance, are placed in the path of the shot. one being usually placed near to the cannon's mouth. Across each of these a string is placed, which is severed by the shot in its progress.

An insulated wire is now carried from the battery to hoth the screens, and then back again, the recording apparatus being included in the circuit. This part of the arrangement consists of some clockwork, which is capable of very rapid motion, bat is stopped by means of a detent, connected with an electromagnet. At each screen a contact key, similar to that shown at Fig. 84, is placed in the circuit. The screw A is, however, removed, and a spring placed under that end of the key against the first screen, and under the other end of the second.

In this way, the one nearest the cannon would have its circuit completed by the hammer pressing on the anvil seen under B, while in the other they would be separate.

The strings across the targets are, however, attached to B and A respectively, so as to act against the springs, and thus interrupt the circuit at the nearer one and complete it at the further.

Now let us see the action of the whole arrangement. As soon as the shot severs the first string, the circuit is completed in the contact key there, and the detent in the recording apparatus is immediately freed. The clockwork continues,