verse.

produced on the principle of interferences, by the reflection of the longitudinal undulations, at the closed end or ends of the pipe, as in the musical string, only that in the one case the undulations are longitudinal, and in the other transGlass and metallic rods, when struck at one end, or rubbed in the direction of their length with a wet finger, vibrate longitudinally, like a column of air, by the alternate condensation and expansion of their constituent particles, which produces a clear and beautiful musical note of a high pitch, on account of the rapidity with which these substances transmit sound. Rods, surfaces, and in general all undulating bodies, resolve themselves into nodes; but in surfaces, the parts which remain at rest during their vibrations are lines, which are curved or plane according to the substance, its form, and the mode of vibration. If a little fine dry sand be strewed over the surface of a plate of glass or metal, ground smooth at the edges, and if undulations be excited by drawing the bow of a violin across its edge, it will emit a musical sound, and the sand will immediately arrange itself in the nodal lines, where alone it will accumulate and remain at rest, because the segments of the surface on each side will be in different states of vibration, the one being always elevated while the other is depressed, and as these two motions meet in the nodal lines, they neutralize one another. These lines vary in form and position with the part where the bow is drawn across, and the point by which the plate is held being at rest, must necessarily be in a nodal line; the smallest variation in the pitch changes the nodal lines. A sound may thus be detected though inaudible. The motion of the sand shows in what direction the vibrations take place if they be perpendicular to the surface, the

sand will be violently tossed up and down, till it finds the points of rest; if they be tangential, the sand will only creep along the surface to the nodal lines. Sometimes the undulations are oblique, or compounded of both the preceding. The air of a room, when thrown into undulations by the continued sound of an organ-pipe, or any other means, divides itself into masses separated by nodal curves of double curvature, such as spirals, on each side of which the air is in opposite states of vibration.

All solids which ring when struck, as bells, drinkingglasses, gongs, &c. have their shape momentarily and forcibly changed by the blow, and from their elasticity, or tendency to resume their natural form, a series of undulations take place, owing to the alternate condensations and rarefactions of the particles of solid matter. These have also their harmonic tones, and, consequently, nodes. Indeed generally when a rigid system of any form whatever vibrates either transversely or longitudinally, it divides itself into a certain number of parts, which perform their vibrations without disturbing one another. These parts are at every instant in alternate states of undulation, and as the points or lines where they join partake of both, they remain at rest because the opposing motions destroy one another.

All bodies have a tendeney to impart their undulations both to the air and to substances in contact with them. A musical string gives a very feeble sound when vibrating alone, on account of the small quantity of air set in motion; but when attached to a sounding-board, as the harp and piano-forte, it communicates its undulations to that surface, and from thence to every part of the instrument, so that the whole system vibrates isochronously, and by

exposing an extensive undulating surface, which transinits its undulations to a great mass of air, the sound is much reinforced. It is evident that the sounding-board and the whole instrument are agitated at once by all the superposed vibrations excited by the simultaneous or consecutive notes that are sounded, each having its perfect effect independently of the rest. The air, notwithstanding its rarity, is capable of transmitting its undulations when in contact with a body susceptible of admitting and exciting them. It is thus that sympathetic undulations are excited by a body vibrating near insulated tended strings, capable of following its undulations, either by vibrating entire, or by separating themselves into their harmonic divisions. When a tuning-fork receives a blow, and is made to rest upon a piano-forte, during its vibration every string which, either by its natural length, or by its spontaneous subdivisions, is capable of executing corresponding vibrations, responds in a sympathetic note. Some one or other of the notes of an organ are generally in unison with one of the panes, or with the whole sash of a window, which consequently resound when these notes are sounded. A peal of thunder has frequently the same effect. The sound of very large organ-pipes is generally inaudible till the air be set in motion by the undulations of some of the superior accords, and then its sound becomes extremely energetic. Recurring vibrations occasionally influence each other's periods. For example : two adjacent organ-pipes, nearly in unison, may force. themselves into concord, and two clocks, whose rates differed considerably when separate, have been known to beat together when fixed to the same wall.

Every one is aware of the reinforcement of sound by

the resonance of cavities. When singing or speaking near the aperture of a wide-mouthed vessel, the intensity of some one note in unison with the air in the cavity is often augmented to a great degree. Any vessel will resound if a body vibrating the natural note of the cavity be placed opposite to its orifice, and be large enough to cover it; or, at least, to set a large portion of the adjacent air in motion. For the sound will be alternately reflected by the bottom of the cavity and the undulating body at its mouth. The first impulse of the undulating substance will be reflected by the bottom of the cavity, and then by the undulating body, in time to combine with the second new impulse; this reinforced sound will also be twice reflected in time to conspire with the third new impulse; and as the same process will be repeated on every new impulse, each will combine with all its echos to reinforce the sound prodigiously.

Several attempts have been made to imitate the articulation of the letters of the alphabet. About the year 1779, MM. Kratzenstein, of St. Petersburgh, and Kempelen, of Vienna, constructed instruments which articulated many letters, words, and even sentences; Mr. Willis, of Cambridge, has recently adapted cylindrical tubes to a reed, whose length can be varied at pleasure by sliding joints. Upon drawing out the tube, while a column of air from the bellows of an organ is passing through it, the vowels are pronounced in the order i, e, a, o, u; on extending the tube, they are repeated, after a certain interval, in the inverted order u, o, a, e, i; after another interval, they are again obtained in the direct order, and so on. When the pitch of the reed is very high, it is impossible to sound some of the vowels, which is in perfect correspondence

with the human voice, female singers being unable to pronounce u and o in their high notes. From the singular discoveries of M. Savart, on the nature of the human voice, and the investigations of Mr. Willis on the mechanism of the larnyx, it may be presumed that ultimately the utterance or pronunciation of modern languages will be conveyed, not only to the eye, but also to the ear, of posterity. Had the ancients possessed the means of transmitting such definite sounds, the civilized world would still have responded in sympathetic notes at the distance of hundreds of ages.

SECTION XIX.

The action of the atmosphere on light is not less interesting than the theory of sound, for in consequence of the refractive power of the air, no distant object is seen in its true position.

All the celestial bodies appear to be more elevated than they really are, because the rays of light, instead of moving through the atmosphere in straight lines, are continually inflected towards the earth. Light passing obliquely out of a rare into a denser medium, as from vacuum into air, or from air into water, is bent or refracted from its course towards a perpendicular to that point of the denser surface where the light enters it. In the same medium, the sine of the angle contained between the incident ray and the perpendicular is in a constant ratio to the sine of the angle contained by the refracted ray and the same perpendicular; but this ratio varies with the refracting medium. The denser the medium the more the ray is