in light vanished after two years, and has never since been seen. In 1572, a star was discovered in Cassiopeia, which rapidly increased in brightness till it even surpassed that of Jupiter; it then gradually diminished in splendour, and after exhibiting all the variety of tints that indicates the changes of combustion, vanished sixteen months after its discovery without altering its position. It is impossible to imagine anything more tremendous than a conflagration that could be visible at such a distance. It is however suspected that this star may be periodical and identical with the stars which appeared in the years 945 and 1264. There are probably many stars which alternately vanish and reappear among the innumerable multitudes that spangle the heavens,. the periods of thirteen have already been pretty well ascertained. Of these the most remarkable is the star Omicron in the constellation Cetus. It appears about twelve times in eleven years, and is of variable brightness, sometimes appearing like a star of the second magnitude; but it neither always arrives at the same lustre, nor does it increase or diminish by the same degrees. According to Hevelius, it did not appear at all for four years. y Hydræ also vanishes and reappears every 494 days, and a very singular instance of periodicity is given by Sir John Herschel in the

star Algol or 6 Persei, which is described as retaining the size of a star of the second magnitude for two days and fourteen seconds; it then suddenly begins to diminish in splendour, and in about three hours and a half is reduced to the size of a star of the fourth magnitude: it then begins again to increase, and in three hours and a half more regains its usual brightness, going through all these vicissitudes in two days, twenty hours, and forty-eight minutes. The cause of the variations in most of the periodical stars is unknown, but, from the changes of Algol, M. Goodricke has conjectured that they may be occasioned by the revolution of some opaque body, coming between us and the star, obstructing part of its light. Sir John Herschel is struck with the high degree of activity evinced by these changes in regions where, but for such evidences, we might conclude all to be lifeless.' He observes that our own sun requires nine times the period of Algol to perform a revolution on its own axis; while, on the other hand, the periodic time of an opaque revolving body sufficiently large to produce a similar temporary obscuration of the sun, seen from a fixed star, would be less than fourteen hours.

Many thousands of stars that seem to be only brilliant points, when carefully examined are found

to be in reality systems of two or more suns, some revolving about a common centre. These binary and multiple stars are extremely remote, requiring the most powerful telescopes to show them separately. The first catalogue of double stars, in which their places and relative positions are determined, was accomplished by the talents and industry of Sir William Herschel, to whom astronomy is indebted for so many brilliant discoveries, and with whom the idea of their combination in binary and multiple systems originated—an idea completely established by his own observations, recently confirmed by those of his son. The motions of revolution of many round a common centre have been ascertained, and their periods determined with considerable accuracy. Some have, since their first discovery, already accomplished nearly a whole revolution, and one, » Coronæ, is actually considerably advanced in its second period. These interesting systems thus present a species of sidereal chronometer, by which the chronology of the heavens will be marked out to future ages by epochs of their own, liable to no fluctuations from planetary disturbances, such as obtain in our system.

In observing the relative position of the stars of a binary system, the distance between them, and also the angle of position, that is, the angle

which the meridian or a parallel to the equator makes with the line joining the two stars are measured. The accuracy of each result depends upon taking the mean of a great number of the best observations, and eliminating error by mutual comparison. The distances between the stars are so minute that they cannot be measured with the same accuracy as the angles of position; therefore, to determine the orbit of a star independently of the distance, it is necessary to assume, as the most probable hypothesis, that the stars are subject to the law of gravitation, and consequently, that one of the two stars revolves in an ellipse about the other, supposed to be at rest, though not necessarily in the focus. A curve is thus constructed graphically by means of the angles of position and the corresponding times of observation. The angular velocities of the stars are obtained by drawing tangents to this curve at stated intervals, whence the apparent distances, or radii vectores, of the revolving star become known for each angle of position; because, by the laws of elliptical motion, they are equal to the square roots of the apparent angular velocities. Now that the angles of position estimated from a given line, and the corresponding distances of the two stars, are known, another curve may be drawn, which will represent on paper the actual orbit of

the star projected on the visible surface of the heavens; so that the elliptical elements of the true orbit and its position in space may be determined by a combined system of measurements and computation. But as this orbit has been obtained on the hypothesis that gravitation prevails in these distant regions, which could not be known à priori, it must be compared with as many observations as can be obtained, to ascertain how far the computed ellipse agrees with the curve actually described by the star.

By this process Sir John Herschel has discovered that several of these systems of stars are subject to the same laws of motion with our system of planets: he has determined the elements of their elliptical orbits, and computed the periods of their revolution. One of the stars of y Virginis revolves about the other in 629 years; the periodic time of

Coronæ is 287 years; that of Castor is 253 years; that of & Bootes is 1600; that of 70 Ophiuci is ascertained by M. Savary to be 80 years; and Professor Encke has shown that the revolution of

Ursæ is completed in 58 years. The two first of these stars are approaching their perihelia,y Virginis will arrive at it on the 18th of August, 1834, and Castor some time in 1855. The actual proximity of the two component stars in each case will then be extreme, and the apparent angular