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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 widemouthed 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 larynx, 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.
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 bent. The barometer shows that the density of the atmosphere decreases as the height above the earth increases; and direct experiments prove, that the refractive power of the air increases with its density; it follows, therefore, that if the temperature be uniform, the refractive power of the
air is greatest at the earth's surface and diminishes upwards.
A ray of light from a celestial object falling obliquely on this variable atmosphere, instead of is grabeing refracted at once from its course, dually more and more bent during its passage through it, so as to move in a vertical curved line, in the same manner as if the atmosphere consisted of an infinite number of strata of different densities. The object is seen in the direction of a tangent to that part of the curve which meets the eye, consequently the apparent altitude of the heavenly bodies is always greater than their true altitude. Owing to this circumstance, the stars are seen above the horizon after they are set, and the day is lengthened from a part of the sun being visible, though he really is behind the rotundity of the earth. It would be easy to determine the direction of a ray of light through the atmosphere, if the law of the density were known; but as this law is perpetually varying with the temperature, the cause is very complicated. When rays pass perpendicularly from one medium into another, they are not bent; and experience shows, that in the same surface, though the sines of the angles of incidence and refraction retain the same ratio, the refraction increases with the obliquity of incidence. Hence appears, from what precedes, that the refraction
is greatest at the horizon, and at the zenith there is none; but it is proved that at all heights above ten degrees, refraction varies nearly as the tangent of the angular distance of the object from the zenith, and wholly depends upon the heights of the barometer and thermometer; for the quantity of refraction at the same distance from the zenith varies nearly as the height of the barometer, the temperature being constant; and the effect of the variation of temperature is to diminish the quantity of refraction by about its 480th part for every degree in the rise of Fahrenheit's thermometer. Not much reliance can be placed on celestial observations within less than ten or twelve degrees of the horizon, on account of irregular variations in the density of the air near the surface of the earth, which are sometimes the cause of very singular phenomena. The humidity of the air pro-duces no sensible effect on its refractive power.
Bodies, whether luminous or not, are only visible by the rays which proceed from them; and as the rays must pass through strata of different densities in coming to us, it follows that, with the exception of stars in the zenith, no object either in or beyond our atmosphere is seen in its true place; but the deviation is so small in ordinary cases, that it causes no inconvenience, though in astronomical and trigonometrical observations a due allowance