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that the principal one of the stars observed by Mayer was itself double; which observation was again confirmed, in 1825, by Sir James South. The three stars are nearly equal in brightness, and are ranked as between the fifth and sixth magnitudes; but whether the masses of the three bodies are really nearly the same can only be determined by their own movements, as deduced from such observations as have been made by William and Otto Struve. The former astronomer observed them first in 1826, since which time they have been closely followed by himself and son. The remarkably accurate observations of Baron Dembowski at Gallarate, near Milan, in Italy, together with those made by Dawes in England, have been by Otto Struve combined with his own; and from the entire assemblage of all the appropriate observations he finds that the apparent orbit of the star B about the star A is completed in about 62.4 years, under the assumption that the apparent orbit is circular, which appears to be very nearly the case (the real orbit has an eccentricity of 0.35). The third star of the group, indicated by the letter C, is apparently about ten times as far from A as is the star B. Its angular movement relative to the former star is therefore correspondingly slow, it having described only 47° in ninety years, while the star B has entirely completed its revolution and described a portion of its second orbit. An interesting peculiarity of the motions of the star C consists, however, in this, that its movements are by no means regular. It is in fact subject to repeated alternations within periods of about ten years, within which time it moves sometimes forward rapidly, and at other times backward, and at other times it is stationary. Its apparent orbit around the star A is therefore essentially an epicycloid; but unhappily the present state of mathematical analysis does not enable us to say whether these irregularities in its movement are due to the perturbing attractions of the stars A and B, or whether we must assume that the star C describes an elliptical orbit about an invisible point central between it and a fourth invisible star, D, while the central point itself describes a much larger orbit about the stars A and B. Otto Struve states that if we refer the positions of the star C to a point half-way between the stars A and B, we can closely represent all our observations by assuming C to move in a small

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ellipse, having a diameter of a third of a second, which latter is, at a distance of 5.5 seconds, carried uniformly about the point central between A and B.-Bullet. Hebdomadaire Assoc. Scientifique, 1875, 217.

ON THE CHEMISTRY OF THE SOLAR SYSTEM.

Observation and theory have led Lockyer to the conclusion that the various elements constituting the sun are arranged in layers according to the atomic weight of their vapors. Thus outside of all is hydrogen, with an atomic. weight of one. Then follow, in regular order, magnesium, calcium, sodium, chromium, manganese, iron, nickel, etc. At the centre the nobler and rarer metals must be found, constituting the substance of the sun. In this same order should the nebulous mass have been arranged from which the solar system was developed, according to La Place's hypothesis, consequently the exterior planets of the solar system should be principally formed by the condensation of the metalloids, and the inferior planets be composed of the metallic elements; thus Lockyer explains the feeble specific gravity of the former, and the greater mass of the latter planets. The composition of the atmospheres of the planets, which give only a few rays of absorption in the spectrum, seem to confirm this view of Lockyer. Even the composition of the outer shell of the earth would seem to accord therewith, since it is formed in the following proportion: Oxygen, 500; silicium, 250; the other metalloids, 227; and of other simple. bodies, 23 parts out of a thousand. If, on the other hand, we add the liquid portion of water, it will be found that hydrogen enters in a still larger proportion, and, with oxygen, acquires a predominance; so that it may be said our earth is. composed principally of oxygen and hydrogen, with a small percentage of metals and metalloids.-La Nature, III., 206.

A FAMOUS SOLAR ECLIPSE.

The total solar eclipse of the 3d of June, 1239, was a memorable event in Central Europe, and has formed the subject of an interesting memoir by Celoria, one of the astronomers at the Observatory of Milan. This gentleman has collected together all the accessible notes with reference to observations made at that time upon the eclipse, from

which it appears that the line of central totality passed through Northern Italy and Southern Spain. Our author has sought, by a very careful discussion of the observations at command, to determine the precise northern limit of visibility of the total eclipse; and, by comparing the observations with the tables of Hansen, to deduce some positive addition to our knowledge of the secular changes in the orbit of the earth and moon. He expresses his results by an equation which shows that we need but one more similar eclipse in order to arrive by means of a second such equation at more correct elements of the lunar orbit.-Publication of the Royal Observatory, Milan, No. 10, 1875.

STUDIES UPON THE DIAMETER OF THE SUN.

As the extensive work of Father Rosa upon the solar diameter is likely to provoke much further investigation of this subject, notwithstanding all that has been done by Auwers, Wagner, Newcomb, and many others, we quote the following conclusions to which he has been led, as published in the posthumous work recently edited by Father Secchi. First, the body of the sun must be considered as composed of two masses nearly independent of each other, viz., of a solid nucleus enveloped by a gaseous matter. The expression "solid nucleus" can even be applied to the central portion of a gaseous mass whose condensation is such that it is necessary that it should be nearly independent of its envelope. Second, the deformations of the photosphere are not due directly to the force of gravity. Third, the continuous force, that which especially deforms the photosphere, is connected with that which produces actual secular movements of the centre of gravity of the sun as demonstrated by Le Verrier. Fourth, the vertical diameter of the sun experiences an annual variation or a semi-annual period, such that it is greater when the sun is north of the equator. Fifth, the mechanical theory of the sun's motions demands that its centre of gravity should describe, in its apparent movement, a great circle of the celestial sphere. The centre of figure, according to Airy, describes a parallel circle, lying northward therefrom. Sixth, it results evidently from the two preceding sentences that the plane of the ecliptic is not parallel to that which cuts the photosphere into two symmet

rical parts. Seventh, it also results that the mean accumulation which is produced at the extremities of the diameter of the photosphere takes place instantly by preference in the northern hemisphere. Eighth, it seems, then, probable that a larger quantity of the gaseous mass may be suspended in the northern than in the southern hemisphere. It must be that this accumulation occasions the excess of temperature in the northern hemisphere, and opposes therein some resistance to the manifestation of the interior activity of the

Father Rosa finally concludes that the secular variations of the photosphere and of terrestrial magnetism are simultaneous, and subjected to an oscillation of 663 years, similar to that which the apogee of the apparent orbit of the sun is subject to. We can thus consider our sun as making a part of a triple stellar system, in which the interior star combines with our sun in a movement about the same centre, whose period is 663 years.-Biblioth. Univers., 1874, 259.

SOLAR RADIATION IN EGYPT.

M. Picte has communicated several series of observations which he has had occasion to make during a prolonged sojourn at Cairo, as director of the Physical Cabinet of that city, and among them he has made some measurements of solar radiation with a large actinometer. His apparatus consisted of a sort of boiler, of two thin plates of parallel sheet iron, filled with water, placed in a chest full of black cotton, and closed over its face, which was turned toward the sun, by a comparable number of plates of glass. As a result he finds that, in Egypt, a surface of one square meter exposed normally to the rays of the sun in the middle of the day absorbs very nearly twelve calories or units of heat per minute. Other observ ers, also operating in Egypt, have found about ten calories. The diurnal evaporation produced by the action of the sun is one sixth of an inch of water. The evaporation which is due to the dryness of the air and to the wind is one third of an inch.-Biblioth. Univers., 1874, 484.

THERMOGRAPHS OF THE ISOTHERMAL LINES OF THE SOLAR DISK.

Professor Mayer announces that he has devised a method for obtaining the isothermals on the solar disk, by which in

vention he thinks it possible that an entirely new branch of solar physics may be created. He causes the image of the sun to fall upon the smoked surface of thin paper, while the other side of the paper is coated with a film of Meissel's double iodide of copper and mercury. When the wave of heat, passing through the carbon and the paper, has warmed the thin film of iodide to the temperature of 70° Centigrade, this substance is blackened; and if, beginning with a very small aperture of the telescope, we gradually increase it until we obtain the smallest area of blackened iodide that can be produced with a well-defined contour, we thus obtain a determination of the area of maximum temperature on the solar disk. On using a larger aperture of the object-glass of the telescope, a larger surface of blackened iodide is formed, the new area being bounded as before by a well-defined isothermal line; and by repeating this process maps are obtained of the isothermals of the solar disk. An exposure of about twenty minutes is required to obtain these thermographs, which are sufficiently permanent to allow one to trace accurately their isothermal contours. But other substances exist which are more suitable than the iodides for producing permanent thermographs. Professor Mayer states that, as far as he has at present applied this method, he concludes that there exists on the solar disk an area of sensibly uniform temperature and of maximum intensity. This area of maximum temperature is of variable size. It is in motion on the solar image. It is surrounded by well-defined isothermals. The general motions of these isothermals follow the motions of the central maximum area, but they have also their independent motions.-American Journal of Science, July, 1875, 50.

A NEW METHOD OF COMPUTING PLANETARY PERTURBATIONS. The immense amount of labor that has, for a hundred years past, been spent upon the computation of the mutual perturbations of the planets, and the great amount of time and ingenuity employed by mathematicians in devising the methods of special perturbations and mechanical quadratures, would justify a prolonged notice of the new method of computing special perturbations developed in a recent memoir by Professor Gylden, of Stockholm, and which has been

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