But even if the ventriloquist has the power of speak ing without moving the muscles of his throat or face, and has the most complete power of imitating sounds under all possible modifications, another condition is necessary to the success of his performances. A line drawn from the mouth of the ventriloquist to the ear of any of his auditors, must not be greatly inclined to the line drawn from the object from which he wishes the sound to appear to proceed to the ears of any of his auditors. If the ventriloquist, for example, were placed to the south of his auditors, it would be in vain for him to attempt to cause any sound to proceed from an object to the north of the auditors, or even from an object east or west of them. The dullest ear is capa ble of distinguishing the direction from which sounds actually proceed to a greater degree than this. There is, however, a certain angle within which the ear can not distinguish differences in the direction of sounds. Thus if a sound issued exactly from the south point of the horizon, and the same sound from a point of the compass to the west of south, an ordinary ear could not determine which of the two sounds came from the south, and which from the west of south. The ventriloquist must therefore take care not to place the object, from which the sounds are to appear to come, without the range of this angle.
The angle within which we cannot judge of the di rection of sounds, depends on the state of the ear, and varies with different individuals, and with the condi tion of the air, as well as with the nature of the sound.
7. On the Polarisation of Sound.
We have already seen in our article on OPTICS, that when light passes through crystallized bodies, or is reflected at a particular angle from transparent sur faces, it receives a particular modification called po larisation, in virtue of which it passes more freely through transparent bodies, when presented to it in one way than when presented to it in another. Mr. Wheatstone of London, has discovered an analogous phenomenon with respect to sounds, which he de scribes as follows: " I connected a tuning fork with one extremity of a straight conducting rod, the other end of which com municated with a sounding board; on causing the tun ing fork to sound, the vibrations were powerfully transmitted ; but on gradually bending the rod the sound progressively decreased, and was scarcely per ceptible when the angle was a right one. As the angle was made more acute, the phenomena were produced in an inverted order; the intensity gradual ly increased as it had before diminished, and when the two parts were nearly parallel, it became as pow erful as it was when the rod was straight. By mul
tiplying the right angles on a rod, the transmission of the vibration may be completely stopped.
In these experiments, the axes of the oscillations of the tuning fork should be perpendicular to the plane of the moveable angles; for if they be parallel the vi brations will still be transmitted. In order to prove this, Mr. Wheatstone placed a tuning fork perpendi cularly on the side of a rectilineal rod. The vibra tions were therefore communicated at right angles: When the axis of the oscillations of the forks coin cided with the rod, the intensity of the vibrations was a maximum.
In proportion as the axis deviated from parallelism the intensity diminished, and when it became perpen dicular the intensity was a minimum. The phenome na of polarisation may be observed in many corded instruments. The cords of a harp are attached to a conductor, which has the same direction as the sound ing board. If any cord be altered from its quiescent position, so that its axis of oscillation shall be paral lel with a bridge or conductor, its tone will be full; but if the oscillations be excited, so that their axis be at right angles to the conductor, the tone will be feeble. See the ilnnals of Philosophy, No. xxxii. p. 37.
8. 11.1. rentau's Gigantic Meteorological Eoliara Harp.
Captain Haas of Basle, has designated by these names an apparatus which emits of itself a variety of sounds during a change of weather. Since the year 1787, he had stretched above his garden fifteen iron wires, 320 feet long, and at the distance of about two inches from one another; the largest were two lines in diameter, the smallest one line, and those of inter mediate size one and a half line. They were situated towards the south, and inclined 20° or 30° to the ho rizon, being stretched by means of rollers properly arranged for the purpose. 'Whenever the weather changes, these wires sound with such loudness, that it was impossible to go on with a concert in the house. The sounds sometimes resembled the hissing noise of water in rapid ebullition, sometimes that of a har monicon, and sometimes that of a distant chime or an organ.
The inventor of this curious apparatus is M. Ven tau, provost at Burkli, not far from Basle. He some times shot at a mark from his window; and in order that he might not go to the mark after each shot, he attached to it a long wire to draw it to him at plea sure. He remarked more than once, that the wire sounded exactly an octave; and he found that every iron wire, stretched in a direction parallel to the south, emitted this tone at every change of weather.
A brass wire did not produce any sound, nor did an iron wire, when it is stretched from east to west.