The faculty of hearing which I have been investigating, betrays men under certain circumstances into errors, that appear the more surprizing, because the judgement relies on the admonition of the ears with the greatest confidence. The theory of these deceptions will therefore form a proper supplement to this essay. Mention has been already made of the sudden change that takes place in the sensible direction of a sound, as soon as the direct communication with the sounding body happens to be broken by the intervention of a loft obstacle, provided the sound in question be loud enough to produce an echo from another quarter. Any person who has had occasion to walk along a valley obstructed with buildings, at the time that a peal of bells was ringing in it, will assent to the truth of the circumstance here alluded to. For the sound of the bells instead of arriving constantly, at the ears of a person so situated, in its true direction, is frequently reflected in a short time from one or two different places. These deceptions are in many cases so diversified by the successive interposition of fresh objects, that the steeple appears, in the hearer's judgement, to perform the part of an expert ventriloquist on [sic] a theatre, the extent of which is adapted to its own powers, and not to those of the human voice. The phaenomenon has often attracted my attention; and the similarity of effect which connects it with ventriloquism, convinces me every time I hear it, that what we know to be the cause in one instance is also the cause in the other: I mean that the echo reaches the ear, while the original sound is intercepted by accident in the case of the bells, but by art in the case of the ventriloquist. In order that the cause which gives rise to the amusing tricks of this uncommon talent may be pointed out with the greater clearness, it will be proper to describe certain circumstances that take place in the act of speaking, because the skill of the ventriloquist seems to consist in a peculiar management of them. Articulation is the art of modifying the sound of the larynx, by the assistance of the cavity of the mouth, the tongue, teeth, and lips. The different vibrations, which are excited by the joint operation of the several organs in action, pass along the bones and cartilages, from the parts in motion to the external teguments of the head, face, neck, and chest; from which, a succession of similar vibrations is imparted to the contiguous air, thereby converting the superior moiety of the speaker's body into an extensive seat of sound, contrary to general opinion, which supposes the passage of the voice to be confined to the opening of the lips. There are but few persons, I imagine, who have not some time or other witnessed an incident, which shews the vulgar notion to be erroneous in this particular. For if a man standing in a close apartment should happen to apply his face to a loop-hole, or narrow window, in order to speak to some person in the open air, a by-stander will hear his voice, not indeed in its natural tone, but as if it were smothered by being forced to issue from a hollow case; but the circumstance of his words being heard distinctly, by one who cannot receive them from the mouth, proves the vibrations requisite for their production to be conveyed through the solid parts of the speaker's body, agreeably to the preceding assertion. The reason why we generally conclude the voice to be confined to the opening of the mouth, appears to be this. Those pulses which escape from this aperture are the strongest, they therefore surpass the weaker vibrations of the contiguous parts; for when a number of sounds moving in different directions strikes the ear at the same instant, the hearer does not notice their several places, but refers all of them to the quarter in which the most powerful is perceived. For instance, when a man stands at a sufficient distance from an extensive obstacle, his words are answered by an echo; but let him make a loud uninterrupted noise, neither he nor any body near him hears two voices whilst his continues, but as soon as the noise ceases the echo is perceived. This does not happen because the one begins the moment the other ends; but the reflected sound being the weaker of the two, it is smothered by that which precedes it. 

We have seen in what manner secondary or reflected sounds are smothered by their principals; but though the places of such sounds are not recognized by the ear, their effects do not die away unnoticed: for the reverberated pulses mingle with those which come immediately from the sounding body, and thereby alter the sensation, which, without their interference, would be less compounded. This is the reason why the same musical instrument has one tone in a close chamber, where its notes undergo a multiplicity of reverberation, and another in the open air, where the reflections are few in comparison. 

But it is time to apply the preceding facts to the subject in hand; and it will be proper to begin with a familiar example. When an orator addresses an audience in a lofty and spacious room, his voice is reflected from every part of the apartment, of which all present are made sensible by the confused noise that fills up every pause in his discourse; nevertheless, every one knows the true place of the speaker, because his voice is the prevailing sound at the time. But were it possible to prevent his words from reaching any one of his audience directly, what then would follow? Undoubtedly a complete case of ventriloquism would be the consequence, and the person so circumstanced would transport the orator, in his own mind, to the place of the principal echo, which would perform the part of the prevailing sound at the instant. This he would be obliged to do, because the human judgment is bound, by the dictates of experience, to regard the person as inseparable from the voice; and the deception in question would be unavoidable, being produced by the same concurrence of causes which makes a peal of bells, situated in a valley, seem to change places in the opinion of a traveller. It is the business of the ventriloquist to amuse his admirers with tricks resembling the foregoing delusion; and it will be readily granted, that he has a subtle sense, highly corrected by experience, to manage, on which account the judgment must be cheated as well as the ear. This can only be accomplished by making the pulses, constituting his words, strike the heads of his hearers, not in the right lines that join their persons and his. He must therefore know how to disguise the true direction of his voice, because the artifice will give him an opportunity to substitute almost any echo he chuses in the place of it. But the superior part of the human body has been already proved to form an extensive seat of sound, from every point of which the two pulses are repelled, as if they diverged from a common centre. This is the reason why people, to speak in the usual way, cannot conceal the direction of their voices, which in reality fly off towards all points at the same instant. The ventriloquist therefore, by some means or other, acquires the difficult habit of contracting the field of sound within the compass of his lips, which enables him to confine the real path of his voice to narrow limits. For he, who is master of the art, has nothing to do, but place his mouth obliquely to the company; and to dart his words,if I may use the expression, against an opposing object, whence they will be reflected immediately, so as to strike the ears of the audience from an unexpected quarter, in consequence of which the reflector will appear to be the speaker. Nature seems to fix no bounds to this kind of deception, only care must be taken not to let the path of the direct pulses pass too near the head of the person who is to be played upon; for, if a line joining the exhibitor's mouth and the reflecting body approach one of his ears too nearly, the divergency of the pulses will make him perceive the voice itself instead of the reverberated sound. 

The only ventriloquist I ever attended, acted in strict conformity to the preceding theory of this curious paradox in the science of acoustics. His audience was arranged in two opposite lines, corresponding to the two sides of a long narrow room. The benches on which they were seated reached from one end of the place to the middle of it, the other part remaining unoccupied. The feats exhibited by him were the three following. First: he made his voice come from behind his audience, but it never seemed to proceed from any part of the wall, near the heads of the people present; on the contrary, it was always heard resembling the voice of a child, who seemed to be under the benches. He stood during the time of speaking in a stooping posture, having his mouth turned towards the place from which the sound issued; so that the line joining his lips and the reflecting object, did not approach the ears of the company. Second: advancing into the vacant part of the room, and turning his back to the audience, he made a variety of noises, that seemed to proceed from an open cupboard which stood directly before him, at the distance of two or three yards. Third: he placed an inverted glass cup on the hands of his hearers, and then imitated the cries of a child confined in it., His method of doing it was this; the upper part of the hearer's arm laid close along his side; then the part below the elbow was kept in an horizontal position with the hand turned downwards, which was done by thew operator himself. After taking these preparatory steps, the man bent his body forwards in a situation which presented the profile of his face nearly to the front of his hearer, whilst his mouth pointed to the cup; in which posture, he copied the voice of a confined child so completely, that three positions of the glass were easily distinguished by as many different tones, viz. when he pressed the mouth of the cup close to the palm, when one edge of it was elevated, and when the vessel was held near the hand but did not touch it. The second and third instances of ventriloquism afford strong proofs, that this delusive talent is nothing more than the art of substituting an echo for the primary sound; for, besides the change perceivable in the direction of the voice, it was found to be blended with a variety of secondary sounds; such as we know by experience are produced as often as a noise of any kind issues from a cavity. I have already made some remarks on this species of knowledge; but it would be improper to dismiss the subject without noticing the accuracy , with which the ear recognizes the finer modifications of sounds, and their causes. I have frequently observed, that a certain waterfall makes a flatter and duller noise when the ground is covered with snow, than that which it affords at other seasons. The human voice also undergoes a similar change within doors, by striking a multiplicity of soft bodies, such as a number of piles of wool, or a crowded congregation in a church. 

The method of preventing the vibration of the vocal organs from reaching the external teguments, is still wanting to complete this theory of ventriloquism; and I presume it can only be supplied by an adept in the art. I must therefore dismiss the subject unfinished, because I have no pretension to that character. 

John Gough, `An Investigation of the Method Whereby Men Judge, By the Ear, of the Position of Sonorous Bodies Relative to their Own Persons', (Read November 27 1801), Proceedings of the Literary and Philosophical Society of Manchester, 5 (1802), pp. 644-52 [622-52].