Chapter X: A Curious Effect in Acoustics

This experiment, the invention of Mr. Wheatstone, the celebrated English physicist, was exhibited at the London Polytechnic Institution, in the year 1855.[39] The stage arrangements for producing this surprising phenomenon are as follows:--

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In the middle of the stage are placed in a semi-circle four harps, by Erard, which are destined to reproduce the sounds of other instruments played in the spacious cellars of the Institution. These instruments are a piano, a violoncello, a violin, and a clarionet.

In order to produce this result, there are, attached to the sounding-boards of the harps, four small upright deal rods, about three-quarters of an inch in diameter (_see_ Fig. 18) which, travelling vertically downwards, pass through the stage and the ceiling of the rooms below till they reach the cellar; where they are connected, the first with the sounding-board of the piano, the second with the belly of the violoncello, the third with the belly of the violin, and the fourth with the mouth of the clarionet.

In order to be able, when desired, to interrupt the connection between the instruments played and the harps, the rods are cut right through an inch or two above the stage. This, however, does not prevent the ends resting one upon another, and so transmitting the vibrations. By turning the harps round on their own axes, the rods are completely disconnected.

N.B.--The rods must be insulated from the floor boards through which they have to pass, so as to cut them off from any vibrations which might be prejudicial to the desired effect. This insulation is obtained by making the holes somewhat larger than the rods, or by supporting these latter with substances the least susceptible of vibration.

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Professor Pepper conducts the experiment as follows:--By gently striking rod No. 1, he gives the signal to the pianist to begin to play. The sounds conveyed by the wooden rod, and reproduced by the sounding-board of the corresponding harp, are heard with perfect ease by the whole of the audience. They have lost scarce anything of their power, and not a single note of the air played is altered either in tone, in volume, time, or tune. But if, while the musician still plays on, you turn round the harp and thereby break the connection, _i.e._, the contact of the conducting rods, not a sound is heard. All is absolutely silent.

Mr. Pepper proceeds in the same way with the three other instruments and their respective harps, and shows that the sound, which is completely inaudible so long as the rod attached to the sounding-board of the harp is not in contact with that attached to the playing instrument, is heard the moment that contact is re-established, as clearly as if the instruments were actually on the stage. He then, by striking all four conducting rods at once, transmits to the musicians the order to play a quartette. Such quartette is transmitted with the greatest precision to the sounding-boards of the harps; this concert without visible musicians producing a marked sensation among the audience. Finally, in the very middle of the quartette, Mr. Pepper, turning round the four harps one after another, severs their communication with the instruments, and the concert comes to an end, not for lack of performers, since the latter still continue to play upon their respective instruments, but because the sound-vibrations, no longer aided by the sounding-boards, remain in their rudimentary condition of mere mechanical movement, and so are lost in space.[40]

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Regarded as an illustration of a scientific principle, this experiment was very well arranged, and produced a striking effect; but as a piece of stage-magic, it would have been possible (assuming of course that the principle was kept a secret) to have made it extremely illusive, and to have founded on it one or two really capital tricks. To that end, however, it would have been necessary to have made some slight alterations in the _mise en scène_ of the exhibition, and to present it, in fact, from quite a different point of view. I will give a couple of illustrations of these necessary modifications:--

Whatever the form in which the experiment was performed, the most important alteration to make would be _to produce the phenomenon as closely as possible to the spectators_, in order that they might be thoroughly convinced of the reality of the facts as stated to them; for at any considerable distance it is difficult to satisfy oneself on this point, and the public, who are generally somewhat suspicious, might not unnaturally imagine that they were being hoaxed, and that the sounds, instead of really coming from the harps, proceeded simply from behind the scenes, or beneath the stage. Such an idea once being entertained, good-bye to the “miracle” and all illusion to be derived from it, The proper plan would be to erect a raised platform quite close to the spectators, and, if possible, in the very midst of them. The under side of this platform should rest upon deal supports, which should be tolerably numerous, and among which the vibrating rods should be intermixed. This lower portion, by the way, should be hidden by drapery, which might be lifted when desirable, so as to show that neither musicians nor musical instruments were concealed beneath.

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These arrangements being made, and following the method laid down in the experiment already described, two harps would be placed on the platform, connected by wooden rods with two other harps in the cellar, each in the hands of a competent performer.

The experiment might then be announced under the title of--

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_The Eolian Harps made to perform by Spirit Influence._--The reader can readily imagine the astonishment which such a performance would excite; two harps, completely cut off from all mechanical influence, playing a duet, ceasing to play, either one or both, at the command of the pretended medium; and at a fresh command again beginning to play. Such an exhibition would really savour of the miraculous.

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N.B.--In order to transmit secretly to the musicians the command to play or to cease playing, a person placed in one corner of the hall might, upon certain agreed movements of the exhibitor, or upon his express order addressed to the pretended spirits, press, unknown to the spectators, a connecting stud communicating with an electric bell placed in the cellar.

In this form of the experiment there is no necessity that the musicians should be placed in a cellar; they need only take up their position in some lower room whence the sounds of the instruments would not be audible through the floor of the exhibition-room.

Here is another illusion, only differing in _mise en scène_ from the preceding experiments, but very much more mysterious in effect.

At the four corners of the platform of which we have spoken above, are placed four little round tables, the pillar of each being hollow. As in the experiment of Mr. Wheatstone, a piano, violoncello, violin, and clarionet are placed in a lower room; the wooden rods attached to them travel vertically upwards to the platform, pass through the pillars of the small tables we have mentioned, and terminate a little above their upper surface. These rods, as already stated, must throughout their whole course be insulated, so to speak, from any vibrating substance, so that they may lose nothing of their own vibrations. These matters being arranged beforehand, we may proceed as follows to the experiment to which we have referred, and which we may entitle--

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_A Concert by the Spirits of another World._--The performer calls the attention of the spectators to four locked money-boxes, made of very thin deal, explaining that he holds captive therein certain “spirits,” which, without the aid of any instrument, “discourse most excellent music.” With this introduction, he places the boxes each upon one of the rods in connection with the various instruments. The reader will already have perceived that the money-boxes supply the place of the harps, and answer the purpose of sounding-boards. The performer takes care so to place them that the bottom of each rests fairly upon the extremity of the deal rod. Each box has four little feet, so as to keep the bottom clear of the surface of the table, and to prevent the box from rocking.

As in the scientific form of the experiment, the exhibitor raps on the boxes as a signal to the musicians to play on their respective instruments, singly or together, as the case may be. A rap, given while they are playing, serves as a command to cease.

As the spectators completely surround the platform, they are enabled to make quite certain that the sounds really proceed from the money-boxes; and this necessarily produces a very striking effect.

It may, however, suggest itself to some of those present that the boxes contain mechanism specially adapted to produce the sounds, as is the case, in fact, with ordinary musical-boxes. Should such be the case, the remark to that effect which would naturally be made, would tend to enhance the effect of the experiment, inasmuch as, when the concert is over, the boxes are opened to show that there is nothing within, and they are placed in the hands of the spectators, so as to be examined with the greater freedom.

As a corollary to this series of experiments, I may here describe two little tricks, based on the same principles, which I had fitted up in my own house for the amusement of my friends. The first of these bore the title of

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_The Rapping Spirit._--This was a partial imitation of that spiritualistic “manifestation” by means of which, from time to time, certain persons specially gifted by a supreme power have conversed with the invisible spirits of another world.

I must here confess that I have never had the good fortune to assist at one of these mysterious conversations, earnestly as I desired to do so. In the year 1857, a friend of mine, who, by reason of his unquestioning belief in the supernatural powers of Mr. Home, had free access to that gentleman, tried in vain to procure for me an opportunity of being present at one of the private _séances_ of the celebrated American medium. Home invariably shirked the encounter, and I was obliged to rest satisfied with the marvellous accounts which were given me by more privileged persons. Without any desire to dispute the truth of facts of which I only knew by hearsay, I thought I might fairly venture to reproduce one of the marvels in question by artificial means, and I thereupon constructed my “spirit-rapper.” If the spiritual power of the mediums has any foundation in fact, this innocent practical joke can certainly do them no injury.[41]

The appliances of my spirit-rapping trick were fitted up in a dining-room, which served also as an informal reception-room for intimate friends. I gave the preference to this particular room because the table served to conceal a certain portion of the apparatus of the experiment.

The trick was exhibited as follows:--Taking a piece of iron wire of about five feet in length, with a hook at each end, I hooked it into a ring or screw-eye which was attached to the ceiling, and from which it hung. To the lower hook I attached the handle of a small deal box, which was thereby held suspended about twelve or thirteen inches above the table.

The company took their places around the table, joining hands in order to establish a magnetic circle; and, after a few moments of sober meditation, of which I set the example by maintaining a demeanour of unbending seriousness, we proceeded to ascertain the presence of the spirit, which according to my account was imprisoned in the box.

“Spirit, are you there?”

Three smart raps inside the box gave a reply which carried conviction to the minds of the hearers.

Each person then proceeded to put questions, to which the spirit replied as briefly as possible; for, as is well known, in a conversation of this kind, the talkers of the other world are much behind ourselves in respect to linguistic methods. They are obliged, in order to express a word, to take the letters which compose it separately, and to indicate each of them by striking a number of raps corresponding with the numerical position which such letter occupies in the alphabet. One rap for A, two for B, three for C, &c., and so on, a process which for some words becomes of necessity rather long-winded; but one is disposed to be patient when engaged in the investigation of facts of so surprising a character as those now under consideration.[42]

After a few minutes’ conversation, the “spirit” abruptly took leave of the company, answering one of the questions asked by the word “Good-bye;” and, however pressing the company might be for a prolongation of the sitting, not another sound was heard.

The box being just level with the faces of the spectators, each was able to satisfy himself that the raps really proceeded from thence; and the astonishment was therefore great, when, unhooking and opening the box, I passed it round for examination. As a general rule the company departed without knowing quite what to believe as to the real cause of the effect produced.

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The idea of this trick was suggested to me by the experiment of Mr. Wheatstone above described. I had, however, considerably modified its practical working. Thus, instead of causing the sound-vibrations to pass upwards through a deal rod, I made them pass downwards through an iron wire to the money-box, which served as a sounding-board to concentrate the sound.

The mechanical arrangements of the trick were as follows:--The ring attached to the ceiling formed one end of a metal rod which passed through the flooring and terminated in the room above, where it was attached to a small flat piece of deal. This piece of deal was supported by little indiarubber feet, to prevent the communication of its vibrations to the floor of the room. An electro-magnet placed on the little wooden slab (_see_ Fig. 19) was so arranged that its keeper should strike on the continuation of the metal rod. Lastly, an electric connecting stud placed close to my right foot, enabled me, by pressing it, to close the circuit, and so to strike the number of raps appropriate to the answer which I desired to give. It was, therefore, as the reader will perceive, my own foot which played the part of the rapping spirit.

The explanation which I have above given and the annexed diagram will sufficiently indicate what really takes place in this surprising experiment: the sound of the percussion produced by the electro-magnet is concentrated by the sounding-board, and this latter transmits its vibrations to the money-box through the medium of the iron wire. The spectators hear only the sounds which appear to come from the box.

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When spirit-rapping had gone out of fashion, and my trick no longer had the same foundation to rest upon, I transformed it into another, of a still more interesting character, and which I entitled

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_Waves of Sound._--The modification here made was very slight: there was still the money-box suspended from the ceiling as before; but the flat piece of wood above was replaced by one of those large musical-boxes known as “Geneva” boxes, and it was to the bottom of this box, which in this case formed the sounding-board, that the continuation of the screw-eye was attached. The reader will perceive that by this arrangement, whenever the musical-box was made to play, all the sounds were reproduced in the money-box below, by reason of the same principle already explained in the preceding trick.

In order to give a finish to the experiment, I had placed on a shelf in one corner of the room another musical-box, the tunes played by which were precisely identical with those of the box on the floor above.

I introduced the trick as a scientific experiment, and under the title of “the transmission of waves of sound.”

After giving some few explanations as to the nature of sound-waves, I declared that sounds could be made to travel from one sounding-board to another, so long as both were tuned to precisely the same pitch, and that, in illustration of the principle in question, the isolated and suspended money-box would repeat the airs played by the musical-box. (The musical-box below was of course alone in question, the existence of the other being unknown.) I then pressed the stop (of the visible box), but scarcely was the first phrase of the air completed, than I abruptly stopped it. Instantly the company heard repeated by the wooden box that portion of the air which had been played. I resumed, making the musical-box play another phrase which was in like manner repeated, and this was continued to the end of the tune.

This repetition of musical phrases was produced by the musical-box placed on the floor above; and it was by the aid of electricity that this latter was made to play when the other ceased to do so.

This illusion produced a very great effect, particularly when, as in the previous experiment, the box was opened and handed round for examination.

FOOTNOTES:

[39] Under the title of the “Telephonic Concert.”--ED.

[40] The deal rods at the Polytechnic were forty feet in length, and passed through two intervening stories of the building. A still more striking effect was obtained by transmitting (by the aid of a single harp) a vocal quartette, the performers holding their faces down as close as possible to the sounding-board of a piano. The absurdity of their position was occasionally too much for the gravity of the singers, and the quartette came to an abrupt conclusion amid peals of laughter, which were faithfully transmitted by the harp to the astonished audience.--ED.

[41] The reader will note with interest the impartial spirit in which Robert-Houdin approaches the vexed question of spiritualism--in his day a comparative novelty. In default of absolute certainty, he wisely reserves his opinion. Where, however, as in the case of the Davenports, he had an opportunity of personally observing the alleged “phenomena,” he has neither difficulty in penetrating nor hesitation in denouncing the imposture. (_See_ page 181, _et seq._) We venture to believe that _any_ of the so-called spiritualistic manifestations which had come under the test of Robert-Houdin’s examination would have met a similar fate, and that Mr. Home showed a very sound discretion in not submitting his “levitation” and “elongation” to so keen a critic.--ED.

[42] It is to be presumed that the more modern plan (of pointing with a pencil at the letters of the alphabet in succession, a rap indicating when the right one is reached), was not yet in use when Robert-Houdin wrote.--ED.