Light from the partial reflection of opake bodies. Reflection from metallic mirrors. makes a sensible angle with the principal section. If the ray just mentioned be made to fall in this plane, under an angle of 56° 30', or near it, it will comport itself at the first surface as in the preceding case, it will traverse it without any reflection: but at the second surface it will be reflected in two pencils, which will attain their maximum of intensity, when the plane of incidence is perpendicular to the principal section. It is obvious, that the light reflected at the second face does not comport itself here as in the preceding case, because in the first experiment the incident ray refracted and reflected is still in the same plane, while in the last the repulsive force, that produces the extraordinary refraction, turns the light away from the plane of incidence, so that it ceases to be similarly circumstanced with respect to the forces that act on it. If we examine the light that proceeds from the partial reflection of opake bodies, as black marble, ebony, &c., we shall equally find an angle, at which this light enjoys the properties of that which has traversed a crystal of Iceland spar. Polished metals appear to be the only reflecting substances, that do not seem capable of producing this phenomenon: but, if they do not impress this peculiar disposition on luminous rays, they do not alter it, when they have already acquired it by the influence of another sub stance. This property is preserved also by pencils, that traverse substances which refract light singly. In the second part of this paper* I shall describe the circumstances, under which, by means of reflection from metallic mirrors, the mutual disposition of the particles of a ray, either ordinary or extraordinary, may be so changed, that some shall always be refracted ordinarily, while the others are refracted extraordinarily. The examination of these different circumstances will lead us to the law of these phenomena, which depends on a general property of the repulsive forces that act on light. This will appear in our next. C. IV. Experiments on the Transmission of Sound through solid IT has long been known, that air is not the only medium, Sound proin which the phenomenon of sound may be produced and duced and transmitted by transmitted. All bodies enjoy this property, when they other bodies enter into a vibratory motion: and as, even in the most beside air: solid substances, the elasticity of the ultimate particles appears to be extremely great, it follows, that sound may be produced and transmitted in all bodies, when they are suitably agitated. This result is confirmed by a great number feet. of daily observations. The miner, when excavating his as the ground, gallery, hears the strokes of the miner opposed to him: and thus judges of his direction. Stone, wood, metals, and even water, transmit sound: and Franklin assures us, that water, he has heard under water, at the distance of half a mile, the sound of two stones struck against each other. Several too have observed, that the velocity of sound is much greater in solid bodies, than in the air. Experiments of this kind a wire of 600 were made in Denmark on a wire extended horizontally 600 feet. A piece of sonorous metal, suspended from one extremity of this wire, was struck gently; and a person at the other extremity holding it between his teeth, or applying it to some solid part of the organ of hearing, heard two distinct and successive sounds. The first and most rapid was transmitted by the wire: the second through the air: and from their interval, compared with the known velocity of sound in air, it was found, that the sound transmitted by the metal arrived almost instantaneously. These experiments were repeated in England by the Royal Society, and similar results were obtained, but I do not know the precise quantities found. Mr. Hassenfratz too made experiments Experiments on the same subject in the quarries at Paris, with Mr. Gay- ries. Mém de la Soc. d'Arcueil, vol. II, p. 405. Read to the Instituté November, 1808. Lussac. in stone quar None of these show the precise velocity in solids. Attempt to ascertain it by their vibrations. Lussac. A stroke of a hammer against the side of the gallery produced two sounds, which separated at a certain distance, and that transmitted by the stone arrived first. This separation too was observed, when the sound was transmitted through iron bars, or wooden rails of different lengths, and no perceptible interval could be distinguished between giving the stroke and hearing the sound. All these experiments are well adapted to show the great velocity, with which sound is conveyed through solid bodies, but they were made on lengths not sufficient to afford a measure of this velocity, or even to give a precise idea of it. An ingenious philosopher, whom we have now the pleasure of having at Paris, Mr. Chladni, author of some very fine experiments on the vibrations of solids, has proposed a method of estimating the transmission of sound through their substance. It consists in causing a rod of any substance, of a given length, to vibrate by friction: when the tone produced by the rod, compared with that of a column of air of the same length, will give the ratio of the velocities of the transmission of sound through air, and through the substance of which the red is formed. In fact, we readily perceive from the theory, that the velocity of the longitudinal oscillations of a body and that of the sound transmitted, through it are proportional to one another: but it is necessary to be certain, that the whole rod vibrates so as to give its fundamental note, without dividing itself into its aliquot parts: for such a separation, heightening proportionally the tone, would give a velocity of sound proportionally 16 or 17 times above the truth. In this way Mr. Chladni found, that the velocity of sound in certain solid bodies is 16 or 17 times as great as in air. The most elastic substances are iron, and fir with very straight fibres, when it is rubbed longitudinally. as great as in air. Experiments made in the aqueducts forming at Paris, The construction of the aqueducts and conduits, which is at present carrying on for the embellishment of the capital, has furnished me with means of making experiments of this kind on a much greater length, than any of those who have gone before me have had at their disposal. It was besides a subject of curiosity, to learn the effects and reach of the human voice in very long cylindrical tubes. Such were were the objects of the following experiments. Some of them were made by Mr. Bouvard and me, others by one of us alone. Mr. Malus, colonel of engineers, was likewise present at many of them. In all of them we were assisted by Mr, Martin, maker of nautical watches, a very ingenious and attentive artist, who was particularly appointed to give instantaneously, at determinate seconds, the stroke that was to produce the sound. The sonorous body, on which we operated, was formed by which consist a series of cylindrical tubes of cast iron, of as equal die of a series of iron pipes. mensions as possible, and the mean length of which I found to be 2.515 met.* [8 feet 3 in, nearly]. This I found by mea suring the whole length of twelve cylinders placed end to end, The tubes are separated by leaden rings covered with tarred fustian: but they are pressed together by strong screws, so that the rings are forcibly compressed, and so close a contact produced, that no water can escape. The mean thickness of each ring is 0014286 met. [0·562 of an inch], as I found by measuring twelve. The whole series of cylinders forms a curved line, which has two inflexions. about the middle of its length: but they were not all joined together at once, and we made our experiments on different successive lengths, as will be seen in my report of them. The first were made by Mr. Bouvard: and myself on 78 1st set of expe. cylinders, forming a length of 19617 met., to which must length of 213 riments, on a be added 11 for the 77 rings, giving a total length of yards. 197-27 met. [215'587 yds]. The following were the phe nomena we observed. In the last cylinder was placed a ring of iron, of the Apparatus. same diameter as the cylinder, and having in its centre a bell without a clapper, and a hammer that could be let` fall at will. The hammer, as it struck the bell, struck also. the cylinder, with which it formed a communication by means of the iron ring. Two sounds must therefore be heard, one transmitted by the cylinder, the other by the air. Mode of exIn fact they were heard very distinctly by applying the perimenting. ear to the cylinder, and even without this. They appeared All the measures employed in this paper are expressed in metres; and the time in seconds of the sexagesimal division. sensibly sensibly in unison. The first and more rapid was transmitted by the substance of the cylinder, the second by the air. Strokes of a hammer on the last cylinder likewise produced this transmission. We observed attentively with half second chronometers the intervals between the two sounds transmitted. We even employed successively sexagesimal and decimal watches, to vary the numbers observed. Thus we found Velocity of transmission through the solid calcu lated. 2d set of experiments, on a 20 53 observations. 0.555". Hammer 0.544". Bell. Mean 0'542′′ The interval given by the hammer, and by the bell, appeared to us absolutely the same, without any sensible difference. For this reason we have united them in the same Their tones however, were very different. Thus in solid bodies, as in the air, the tone makes no difference in velocity of the sound. mean. The temperature of the air during the experiment was 11 [51.8° F.]. The barometer was about 0.76 [29.9 in.]. In similar circumstances the velocity of sound in the air is 340-84 met. [372.487 yds] according to the experiments of the academy, which give 334′02 met. [365-034 yds] for the velocity under the same pressure, and at the temperature of melting ice. For the distance of 197-27 [215.587 yds] therefore, that at which the experiment was made, the time of transmission of the sound by the air was 0.579" The interval observed between the two sounds was.. 0'542′′ Difference, or time of its transmis. thro' the metal.. 0·037" ...... We do not pretend to give this small difference as exact, since the slightest errour would have a considerable influence on it, but it proves, that the transmission was not absolutely instantaneous. The second set of experiments was made by Messrs. Bouvard and Malus on twice the former number of cylinders, or a length |
