« ÎnapoiContinuați »
eous states, says that these investigations have occupied him continuously since 1869. In these he has experimented with gases under a pressure of 500 atmospheres. Of course, great difficulties have been experienced by him in measuring such pressures with accuracy; but the previous difficulties that he has experienced have been, or shortly will be, entirely overcome. His recent experiments fully confirm the conclusions published by him six years ago with reference to carbonic-acid gas, viz., that its contraction under great pressure is greater than it would be if the law of Boyle holds strictly good. Under a pressure of 223 atmospheres, this gas is reduced to of its volume under one atmosphere, being slightly less than one half the volume it ought to occupy if it were a perfect gas, and contracted in accordance with Boyle's law. He infers, by analogy, that the critical points of the greater number of gases not hitherto liquefied are probably far below the lowest temperatures yet attained; and these substances are not likely to be seen, either as liquids or solids, until we can obtain much lower temperatures than those produced by liquid nitrous oxide. Again, the law of Gay-Lussac, like that of Boyle, is true only within certain limits and conditions of gaseous matter; in fact the co-efficient of expansion changes rapidly with the pressure, and if the pressure remains constant the co-efficient changes with the temperature. In reference to the law of Dalton, which is that the particles of one gas possess no repulsive nor contractive power with regard to the particles of another, Dr. Andrews's experiments show conclusively that this is not true; and that the so-called critical point is, for instance, lowered by the admixture of carbonic-acid gas with a non-condensible gas. The law also entirely fails when one of the gases is at a temperature not greatly above its critical point; it only holds good when these gases are at feeble pressures, and at temperatures greatly above their critical points.
ON THE INFLUENCE UPON THE MOVEMENT OF A PENDULUM
OF A FLUID CONTAINED IN ITS SPHERICAL BOB. The illustrious Bessel, in prosecuting his investigations into the force with which the earth attracts various bodies, employed a pendulum having a hollow cylinder of brass as its bulb, in which he placed the various bodies to be experimented upon. His observations gave him the result that the attraction of the earth was the same for all the bodies upon which he experimented; and bis determination of the length of the simple seconds pendulum at Königsberg is one of the most correct we possess. He, however, found that when his cylinder was filled with water, the length of the seconds pendulum as computed for that substance was too great. The origin of this deviation Bessel attributed to the fact that the inclosed fluid was by the swinging of the pendulum set into vibrations of its own, whereby its moments of inertia in reference to the axis of vibration of the pendulum was different from what it would have been in the case of a uniform solid body. He accordingly found that the experiments made with long pendulums filled with water showed no such anomaly as in the case of shorter pendulums. Professor 0. E. Meyer, well known for his investigations into the friction of gases and fluids, having sug. gested a somewhat different explanation, his student, Lubeck, has made this matter the subject of an inaugural dissertation, in which he considers the movement of the fluid contained within a pendulum, whose bulb is, for simplicity's sake, a hollow sphere instead of a hollow cylinder, Lubeck shows that the Muid contained in the hollow sphere is not set in motion by its rectilinear movement, but only by its oscillations about the diameter at right angles to the plane of the pendulum's vibration. This oscillation takes place with velocities which are constant for each spherical surface concentric with the hollow sphere, and any initial oscillatory motion is, in a certain time, destroyed by the inner friction, provided that it is originally of the same order as the velocity of the pendulum itself. After this time had elapsed, the motion of the pendulum is quite periodical. The extent of the arc through which the pendulum swings diminishes in a geometrical ratio when the time increases in an arithmetical ratio. The duration of the vibration of the pendulum is greater than if the same pendulum contained within itself a perfect fluid, instead of one having internal friction. The duration of the vibrations is smaller than if the fluid sliould be replaced by a perfectly solid body. When the length of the pendulum becomes very great the inner friction of the fluid has no perceptible influence on the time of vibration.Lubeck's Inaugural Dissertation, Berlin, 1873.
THE CAUSE OF WOLF IN THE VIOLINCELLO. Mr. Kingsley, in a communication to the Cambridge Philosophical Society, states that the wolf, a name given to a well-known defect in the violin, occurs somewhere about low E or E flat, and has been attributed to the finger-board having the same pitch, so that it becomes, as it were, a portion of the string stopped down on it, and vibrates with it. Another explanation is given by Savart, viz., that the violincello is constructed of such dimensions that the mass of air included within the instrument resonates to a note making 85.33 vibrations in a second, a number which formerly represented the lowest F on the C string; but which now, owing to the rise of pitch since the beginning of the eighteenth century, nearly represents the note E immediately below it.— Nature, XII., 40.
THE PYROPHONE. In 1873 Mr. Kastner brought forward his new invention, the pyrophone, which consists essentially of a flame of hydrogen gas, burning within a tube in such a way as to produce the well-known singing sounds on a large scale. If in the tube of glass or any other material, we introduce two or more isolated flames of proper size, and if we place them at a distance from each other one third of the length of the tube, these flames will vibrate in unison. This phenomenon is produced as long as the flames remain separated, but ceases as soon as the flames are brought into contact. It is upon this principle that his pyrophone is based; and the principal objection to the original instrument, which consisted in the necessity of employing hydrogen gas, he has recently overcome, and states that he is now able to employ ordinary illuminating gas; but to do this he is obliged to eliminate the carbon, whereas at first it was impossible to make the tube vibrate with illuminating gas, although the flames were placed in the proper position. According to him, sonorous flames of illuminating gas are in fact enveloped by a photosphere which does not exist when the flame is simply lumi
This photosphere contains a detonating mixture of
hydrogen and oxygen, which determines the vibration of the air in the tube. In order that the sound be produced in all its intensity, it is necessary and sufficient that the number of detonations produced by the molecules of oxygen and hydrogen in a given time shall be in accord with the number of vibrations corresponding to the sound produced by the tube. He finds it sufficient then to increase the number of his flames, substituting four, five, six, or more jets of illuminating gas for his two jets of hydrogen, and diminishing the height of these flames correspondingly, until the sum total of the surfaces of the photospheres suffices to produce the vibrations of the air in the tube.—Bull. Hebd., 1875, 266.
RELATIVE EFFICIENCY OF VARIOUS FOG-SIGNALS. The principal instruments employed on the American coast as fog-signals are the Daboll reed trumpet, the locomotive whistle, and the siren. In a report on the relative efficiency of these instruments, General Duane states in reference to all of them that, while they are frequently heard at distances of twenty miles, yet as frequently they can not be heard a distance of two miles, and this with no perceptible difference in the state of the atmosphere. It is therefore very difficult to determine the relative powers of fog-signals, unless they are placed side by side, under exceptionally favorable atmospheric circumstances. The sound from the whistle is equally distributed in all horizontal directions, and is most powerful in a horizontal plane passing through the whistle. The sound from the siren is most distinct in the axis of the trumpet with which it is provided. The sound given by the Daboll reed trumpet is usually strongest in a plane perpendicular to its axis. In the average of a great number of experiments, General Duane concludes that the powers of the first-class siren, the 12-inch whistle, and the first-class Daboll trumpet may be expressed by the numbers 9,7, and 4. The extreme limit of the audibility of the sound of the trumpet is twelve miles; that of the 12-inch whistle about twenty miles. That of the siren has not been ascertained. The relative erpenditure of fuel by the steam-engines working these instruments at their full capacity is, for the siren, 9; the whistle, 3; and the trumpet, 1. As regards the skill and attention required in the management of these signals, the
siren seems to require the most, while the steam-whistle gives the least trouble. As to the anomalies observed in relation to the penetration and direction of sound from fogsignals, General Duane holds that they are to be attributed mainly to the want of uniformity in the surrounding atmosphere, and that snow, rain, fog, and wind have much less influence than has generally been supposed.- Rep. Light-house Board, 1874, Appendix.
FOG-SIGNALS. In the appendix to the recent report of the Light-house Board, Professor Henry gives the first account that has, as yet, appeared of the experiments and observations made by him in reference to fog-signals, and especially in reference to the acoustic phenomena exhibited on a large scale in the atmosphere. Among other matters, he states that Professor Bache adopted a very ingenious plan for an automatic fog-signal, which consisted in taking advantage of a conical opening in the rocky coast, generally designated as a blow- hole. On the apex of this hole he erected a chimney, which was terminated by a tube surmounted by a whistle. By this arrangement a loud sound was produced as often as a wave entered the mouth of the indentation. The penetrating power of the sound was, under favorable circumstances, due to the pressure of a column of water twenty feet high, giving a pressure of about ten pounds to the square inch. The effect of the percussion, however, sometimes added considerably to this. In practice it was found that this arrangement, which continued in operation for several years, did not entirely supersede the necessity of occasionally producing sounds of greater power. It is stated that Professor J. H. Alexander, of Baltimore, in his investigations on the use of the locomotive steam-whistles, experimentally demonstrated that the power of the sound depends upon the pressure of the steam in the boiler, and the pitch of the sound depends upon the distance between the edge of the whistle and the circular orifice through which the steam issues. Among the various steam fog-signals, one consisting of a double whistle, improperly called a steam gong, seems of interest. This consists of two bells of the ordinary steam-whistle upon the same hollow axes, mouth to mouth; the upper bell has a