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1684 that have come down to us were made during two weeks at Rome by Bianchini; and in order to derive from these the best possible results, Neugebauer has reduced them all anew, by using the best materials available, he having access to original letters and drawings still in existence. The observations made by Bianchini were of the simplest kind; for instance, he would hold a stretched thread in such a position that, while it covered some one of the known fixed stars, it nearly covered the comet itself, whose position, relatively to the ends of the thread, was then estimated by the eye. Other and more exact observations were made with the help of rude instruments. The relative positions of the comet, as deduced by great labor, in general seem to be trustworthy to within a few minutes of arc, and Neugebauer's elements of its orbit are not greatly different from those given by Halley. It follows, as the most interesting result of the investigation, that on the 18th of June, 1684, the distance of the comet from the earth was only the hundredth part of the distance of the earth from the sun, being, in fact, only the thirtieth part of the distance of the earth from Coggia’s comet on the 21st of July, 1874, when it was nearest the earth. It seems, therefore, that the orbit of the comet of 1684 approaches more nearly to the orbit of the earth than almost any other known comet, and that under favorable circumstances we shall be justified in expecting some meteoric display yearly about the 18th of June, at which time the earth annually comes into the plane of this comet's orbit.- Inaugural Dissertation, Breslau, 1873.
THE PHENOMENA OF COMETS. As the result of a suggestive paper by Faye on the forms of comets, he states that he has been led to conclude with perfect certainty that cometary phenomena reveal to us in the heavens the existence of a second force totally different from attraction, and capable of playing an important part, and producing before our eyes gigantic phenomena; that, with great probability, this force is nothing less than the repulsion due to heat. In order to demonstrate experimentally the existence of such a repulsion (which is mathematically deducible from the dynamic theory of gases), he advises the following arrangement: A jar of very rarefied air is illuminated by means of the spark of an induction apparatus; the glass bell-jar in which the vacuum is to be made is traversed by the two wire conductors of the apparatus, the one vertical and the other horizontal, and the induction spark itself appears in the form of feebly luminous rays, whose colored stratifications surround the horizontal conductor with a luminous sheath of a well-marked blue color. The hori. zontal wire having been made of a thin plate of platinum, an independent electric current is passed through it, so as to render it red-hot, and immediately the blue-colored sheath of rarefied air is repelled from the red-hot platinum plate. After baving made all possible variations of this experiment, he concludes that it demonstrates a repulsion between the heated platinum and air.-12 A, X., 289.
THE CONSTITUTION OF COMETS. Mr. Lockyer briefly reviews the state of our knowledge as to the spectroscopic observations of comets. In general, observers seem to agree that these bodies consist, in part at least, of not very dense incandescent vapor, while in some cases very dense or possibly very complex vapors, or even glowing solid substances, seem to have been present. Huggins first suggested the idea that the rarer cometary vapors might be composed of nitrogen, but subsequently suggested the theory that a comet is composed of carbon, and that a temperature prevails high enough to volatilize a portion of this substance, giving rise to the three bands coinciding with those of olefiant gas. Mr. Vogel has, however, shown that this is a very questionable theory, and that we are only justified in concluding that a portion of the light emitted by the comet is its own light, and very probably comes from glowing gas. Mr. Lockyer, moreover, found the nucleus of Coggia's comet deficient iņ blue rays, whence its temperature must have been low, which conclusion is further justified by the fact that cometary light gives channeled space spectra, which latter are peculiar to low temperatures.-12 A, X., 180.
THE FORMATION OF THE TAILS OF COMETS. The distinguished Italian astronomer, Schiaparelli, has communicated to the journal of the Italian Spectroscopic Society some studies upon the nature of the repulsive force which contributes to the formation of the tails of comets. The actual existence of this force is established by the accurate observations that have been made. He examines successively the various theories that have been put forth--the electrical theory as proposed by Zöllner, the molecular theory of Zenker, and the theory of Faye that repulsion is exerted by all incandescent surfaces, and that thereby the gaseous matters attending the comets are repelled from the surface of the solid nucleus. The latter does not seem to Schiaparelli to afford any material explanation of the phenomena. The electric theory was first suggested by Olbers, but has been in general terms objected to by Herschel, Lamont, etc. Zöllner's views have been elaborated more minutely, and he has endeavored to show that the free electricity existing upon the surface of the earth is sufficient to produce an effect similar to that found in comets; but his conclusions are in too many respects at variance with actual observations to allow the author to consider them as affording a reasonable explanation as to the nature of comets. Zenker's views are rejected by him for the reason that the evaporation of fluids from the surface of the nucleus should give rise to several phenomena not seen in comets, while, on the other hand, it does not explain the multiple tails which have frequently been observed. In conclusion, Schiaparelli thinks that we are forced to believe that the repulsive force acting upon comets is a force exterior to the comet itself, and since this force evidently operates in the direction of the radius drawn from the comet to the sun, therefore we must regard it as baving for its origin the sun or some medium surrounding the sun, and lie adds that this is about all we at present know upon the subject.—3 B, XXXV., 263.
WINNECKE'S COMET, The comet known as Winnecke's comet is a very faint telescopic comet, whose recent appearance was observable by only a few of the largest telescopes in the world. It was first observed in 1819, but attracted no notice until in 1858 it was again observed by Winnecke. From the observations made at that time, Winnecke showed that it accomplished its revolution in its orbit in about 2400 days, and that it was identical with the comet of 1819, having made seven
revolutions about the sun since that time without having been observed. Its short period of revolution gives it an additional interest; its movement seems to have experienced no disturbance in the course of the two revolutions that it has made during the past fifteen years.—13 B, III., 174.
ON THE REPULSIVE FORCES OF COMETS. Schiaparelli has recently published his views upon the cause of the peculiar phenomena exhibited by comets, views which he has entertained and in part published since 1862, during which time, on the other hand, he has taken so prominent a part in the development of our knowledge of shooting-stars. According to Schiaparelli, there actually exists a repulsive force, not only between the sun and the tail, but also between the particles composing the nucleus and the tail of a comet; and the phenomena exhibited by these bodies can not be explained without assuming these repulsive forces. In fact, if we consider, first, the formation of the tail itself, we shall find that the solar gravitation and the movement of the comet do not suffice to explain it, as has been abundantly testified to by all who have minutely examined the observations that have been made. Again, too, if we consider the tail itself, and its path in space, we shall find, as has been shown by Bessel in reference to Halley's comet, and by Pape and Bond in reference to Donati's comet, the existence of a repulsive force repelling the comet from the sun results with as niuch certainty as the movements of the earth prove the existence of an attracting force toward the suu. The third comet of 1862, on the other hand, whose tail was not projected in a direction opposite to the sun, but continually approached this direction as the comet moved away from the sun, shows that the repulsive force acts upon the particles of the tail as well as upon the nucleus of the comet; that, in fact, the tail was first sensibly brought under the inAuence of the sun after it had been projected from the nucleus of the comet by a force residing therein. Under a minute examination, the head of a comet sometimes shows beams of light projected from the nucleus, but subsequently turning back and forming portions of the tail, so that here, also, the existence of the repulsive force is apparent. Finally, the increase in the breadth of the tail as it separates from the
nucleus shows that the repulsive force is exerted, not only between the sun and the tail, but also between the separate atoms of which the tail is formed. Since, thus, repulsion is proved, and even apparent as we may say, we must attach some value to the determinations that have been made by Bessel, Pape, and Bond, who were obliged to make certain hypotheses as to the development of the tail, while, on the other hand, the third comet of 1862, like many others, seems to have developed its tail in such a manner that we have at present no safe foundation for computing the amount of the repulsive force existing therein.-19 C, VIII., 109.
PECULIAR AURORAL PRENOMENA. The nature of the auroral light is the subject of a communication by Lemström, who concludes that the white flaming appearances occasionally seen on the summits of mountains in Spitzbergen and in Lapland are of the same nature as the northern lights. Similar appearances have also been seen in other parts of the world. Electrical currents that develop themselves in the earth when the anroras are present are not induction phenomena caused by the atmospheric auroral currents, at least not in northern regions.--19 C, VII., 383,
THE SPECTRUM OF THE AURORA BOREALIS. The late Professor Angström, who at no time accepted the theory that the spectra of gases varied with the pressure and temperature and chemical process, advanced the opinion that the spectrum of the aurora borealis is composed of two different superposed spectra, the one consisting of extremely feeble bands of light, belonging to the spectrum of the negative pole, the other consisting of a single strong yellow line, which is characteristic of the aurora, and which, Angström believed, owes its origin to fluorescence or phosphorescence. - 12 A, X., 211.
THE GEOGRAPHICAL DISTRILU IION OF AURORAS. In a recent paper in Petermann's geographical notices, Professor Fritz, of Berne, gives the results of his researches into the geographical distribution of auroras. Having as far as possible eliminated the sources of error arising from the