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being kept in place between slides of microscope glass. The effect of the heat from the electric discharge is to vaporize the metal, which is instantly condensed in a transparent layer upon the cold glass, which can then be studied by the microscope, and can be used in various ways to determine the character of the metal and the peculiarities of the discharge.-12 A, X., 190.

STEAM FOG-WHISTLES.

It has been found by General Duane, of the United States Engineers, in his experiments made to determine the best form of boilers for steam fog-signals, that as the steam used is at a high pressure, and is drawn off at intervals, there is a constant tendency to foam and throw out water with the steam. To counteract this, a horizontal tubular boiler, like those used in locomotives, is recommended by him. The steam-dome must be very large, and surmounted by a steampipe 12 inches in diameter. The steam should be drawn off at a point ten feet above the water level in the boiler. The diameter of the boiler whistle should be two thirds of its length, and the vertical distance of its lower edge above the coping, for a steam pressure of 50 pounds, should be from one third to one fourth of the diameter.-Elliot's European Light-house System, p. 25.

THE GAS GUN FOR FOG-SIGNALS.

A very ingenious application has been made, by Mr. Wigham, of the explosive nature of a mixture of ordinary gas in air. He establishes at any point on the coast where a fog-signal is desired a gas gun. It is simply a tube of iron, connected with the gas-holder by the proper pipe; the holder, of course, may be at any convenient distance. The gas-holder is filled with a mixture of one fourth air, and the remainder coal-gas and oxygen, and this mixture is allowed to flow into the gas gun, when it may be fired off by touching a match to the proper orifice, taking care, of course, to close all communication with the holder. By using an electric spark, instead of the match, the service of the gun may be made still easier. The flash from this gun is said to illuminate the fog much better than that from a discharge of gunpowder.-Elliot's European Light-house System, p. 74.

A NEW FOG-SIGNAL.

Experiments made in England with gun-cotton in the open air are said to have demonstrated that a mass of ten ounces of compressed gun-cotton, fired by means of two ounces of dry gun-cotton, as a primer, the whole being detonated with fulminate of mercury, produced a discharge which could be heard very distinctly at a distance of ten miles in all directions. These results were so satisfactory that it has been determined to build a parabolic reflector of cast iron, by which the intensity of the sound of the explosion of a charge of compressed cotton placed in its focus will be greatly intensified in one direction. The trials of the adaptability of this device as a fog-signal will be made at the Royal Arsenal, Woolwich.-The Engineer.

NEW METHOD OF OBSERVING THE VIBRATIONS OF A TUNINGFORK.

A new method of determining the absolute number of vibrations corresponding to any musical note is described by Poske, and has a high value in comparison with those that have hitherto been employed, which may be classified as graphic, acoustic, and optic: the first of these three is comparatively rough; the use of the siren is a good example of the acoustic method, although its practicable employment is found troublesome; and of the optical, that of Lissajous is in high esteem. The new method proposed by Poske consists, first, in replacing the clock or chronometer by the electro-magnetic rotation apparatus of Helmholtz, whose velocity of rotation is extremely constant, and can be deter mined accurately to its ten-thousandth part. The essential portion of this apparatus consists in a centrifugal regu lator, which diminishes the strength of the electric current by the diminution of the number of contacts, as soon as the velocity of rotation exceeds a certain limit. The observer examines, through a microscope, a minute bright point upon the vibrating rod or cord, which point by its vibrations appears as a bright line; and between the eye and the vibrating point there also rotates a disk perforated with a known number of slits. The combination of the revolving slits and the vibrating point causes the latter to appear to move

slowly to and fro, in periods similar to the acoustic phenomena known as "beats." It is evident that the number of vibrations of the point is determined by the velocity of rotation, the number of slits, and the duration of the beats, the accuracy of the method being very great.

In the application of his method of determining the time of vibration of a tuning-fork, Poske has also been able to show that the vibrations of the latter vary with the amplitude of the arc of vibration; that the durations diminish in a geometrical series as the amplitudes diminish; and that, in general, the change in duration is proportional to the first power of the amplitude, and not, as in the pendulum, in proportion to the square of the amplitude.-Poggendorff Annalen, CLII., 463.

THE ACTION OF ORGAN-PIPES.

Mr. Hermann Smith states as the result of experimental studies that within an organ-pipe the "air reed" vibrates in arcs whose extent diminishes as we increase the speed of the reed, or that the times vary with the amplitude; and to this he adds the remarkable feature that the motion of vibration is an activity tempered by rests, and that the note of every open organ-pipe is not single, but a concord of two tones.-12 A, X., 162.

EFFECT ON SOUND AND LIGHT OF THE MOVEMENT OF THE OBSERVER.

The long-vexed question as to the effect, upon observations, of the movement of the observer, and the source of light or sound, has been elucidated by Baron Eotvos, of Pesth, who, in a recent communication, extends his former investigations, and offers a satisfactory refutation of several objections that have been raised. According to him, in case the source of sound or light be moving, the intensity must be defined as the living force that would fall, in a unit of time, upon a unit of surface, parallel to the wave surface, if all vibrations were like those which are imparted to the surface at that instant in which the intensity is to be determined. The formula for the intensity in question, as deduced by Eotvos, shows that the movement of the observer has a decided effect upon the result; and by applying this

to the case of an observer moving upon a locomotive, with a velocity of a hundred feet per second, and listening to a sound whose origin moves at the same rate toward him, he finds the observed sound 0.8 times as loud as when both are at rest. A method is also explained by him, showing the possibility of testing his conclusions by experiments on the heat received and sent by moving bodies.-Poggendorff Annalen, CLII, 535.

THE THEORY OF RESONATORS.

Lord Rayleigh contributes an extract from a forthcoming work by himself on acoustics, in which he submits a new theory of the action of resonators, and opposes emphatically the general statement that a resonator augments the body of sound by offering a column of air which is capable of vibrating in unison with the original sounding body. The exceptions to this rule, he thinks, are very important in a theoretical point of view; and he prefers to reverse the statement, and to say that the neighborhood of a resonator in unison with a sounding body diminishes the loudness of the latter. The resonator, in fact, instead of augmenting the effect of a source of sound, annuls it altogether, so far as external space is concerned, by absorbing the condensations and rarefactions into itself.-Phil. Magazine, p. 419.

VIBRATION OF MEMBRANES.

In a paper read before the London Mathematical Society, Lord Rayleigh demonstrates the theorem that an increase in the dimensions of a vibrating system is attended by a rise in pitch. For instance, if the system consists of a uniformly stretched membrane, with a fixed edge, it follows that any contraction of the boundaries must cause an elevation of pitch. If the membrane be uniform, of given density and given tension, the frequency of vibration is a function of the size and form; and if the form is invariable, the frequency varies as the linear dimension. The pitch of the vibrations of a regular polygon is intermediate between those of the inscribed and circumscribed circles. When the area of the membrane is given, it is easy to see that any projecting corners tend to raise the pitch, thus among rectangles of a given

area the square gives the gravest tone, and any membrane not a circle is higher in pitch than the circle of equal area. In estimating therefore the lower limit to the pitch of a regular polygon, it is best to substitute for it a circle of equal area.-Proc. London Mathematical Society, V., 9.

THE SPECTRA OF THE LEAST FUSIBLE METALS.

Messrs. Lockyer and Roberts have attempted to investigate the nature of the absorption spectra of the least fusible metals, for which purpose they employed the oxyhydrogen blowpipe to volatilize the substances. Their experiments, conducted at these high temperatures upon more than twenty metals, go far, they think, to support the conclusions which they had previously drawn from experiments at a lower temperature on more easily volatilized metals, viz., that in passing from a liquid to the most perfect gaseous state, vapors are composed of molecules of different orders of complexity, and that this complexity is diminished by the disassociating action of heat, each molecular simplification being marked by a distinct spectrum.

THE CAUSE OF THE VARIATION OF GASEOUS SPECTRA.

The variations in the spectra of gases have been usually supposed to depend to a certain extent upon the temperature at which the light is produced. Wüllner has advanced the theory that the spectrum depends upon the nature of the electric spark; but Goldstein has recently advanced opposite views, to the effect that the different order of spectra are entirely independent of the form of the electric discharge by which the light is produced. He states that he has been able to secure a notable increase in the width of the lines of the spectrum of hydrogen when the pressure has been less. than one one-hundredth part of a millimeter. His experiments lead him to think that any given order of spectrum can be produced, if we only have a sufficiently high temperature.— 19 C, VII., 444.

A SIMPLE SPECTROSCOPE FOR STARS.

For the purpose of observing the spectrum of stars or other points of light, Zöllner describes a very compendious instrument to be used in combination with the eye-piece of

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