In 1704, Maraldi observed a variable star in the constellation Hydra. This star had been described by Montanari in 1670, but was not visible in April, 1702. Maraldi saw it for the first time in the beginning of March, 1704, in the same place where it had been seen thirty-four years before. It appeared of the fourth magnitude, and continued nearly in the same state till the beginning of April. It then gradually diminished till the end of May, when it could no longer be seen by the naked eye, but was visible through the telescope for a month longer. It could not be seen again till the end of November, 1705, when that part of the heavens began to emerge from the sun's rays. It was then very faint, and grew less and less till the end of February, 1706, and could then be scarcely perceived even with a telescope. It did not reappear till the 18th April, 1708, when it was larger than a star of the sixth magnitude, and increasing in lustre. It was seen by the same observer afterward, in the years 1709 and 1712. From the observations of Maraldi, Mr. Pigot concludes that its period was then 494 days; but from observations made by himself, he thinks that now it is only 487 days; so that from the time of Maraldi it has shortened seven days. The following are the more prominent particulars relating to this star: 1. When at its full brightness it is of the fourth magnitude, and does not perceptibly change for the space of fourteen days. 2. It is about six months in increasing from the tenth magnitude and returning to the same; so that it may be considered as invisible during that time. 3. It is considerably more quick, perhaps one half more so, in its increase than in its decrease. 4. Though, when at its full, it may always be styled a star of the fourth magnitude, it does not constantly attain the same degree of brightness, but the differences are very small. 5. Its right ascension for 1786 was 13h 18' 4"; and its south declination 22° 9′ 38′′. It is marked No. 30 in Hevelius's Catalogue of the Stars; from which data, its place may easily be found on a planisphere or on the celestial globe.

In the year 1600, G. Jansonius discovered a variable star in the breast of the Swan, which was afterward observed by different astronomers, and supposed to have a period of about ten years. The results of Mr. Pigot's calculations from the observations of former astronomers are, 1. That it continues in full lustre for five years. 2. It decreases rapidly for two

years. 3. It is invisible to the naked eye for four years. 4. It increases slowly during seven years. 5. All these changes are completed in eighteen years. 6. It was at its minimum at the end of the year 1663. 7. It does not always increase to the same degree of brightness, being sometimes of the third, and at others only of the sixth magnitude. "I am entirely ignorant," says Mr. Pigot, "whether it is subject to the same changes in this century, having not met with any series of observations upon it; but if the above conjectures are right, it will be at its minimum in a very few years. Since November, 1781, to the year 1786, I have constantly seen it of the sixth magnitude, though I suspect that in 1785-6 it had rather decreased." This star is near Gamma in the Swan's breast: it varies from the third to the sixth, seventh, &c., magnitudes. Its right ascension is 20h 9′ 54′′; north declination 37° 22′ 37′′.

One of the most remarkable of these changeable stars is that called Algol, in the head of Medusa, in the constellation Perseus. It had long since been known to appear of different magnitudes at different times; but its period was first ascertained by John Goodricke, Esq., of York, who began to observe it in the beginning of the year 1783. It changes continually from the first or second to the fourth magnitude; and the time which elapses from one greatest diminution to the other was found in 1783 to be, at a mean, 2 days, 20 hours, 49 minutes. The change is thus: during four hours it gradually diminishes in lustre; during the succeeding four hours it recovers its first magnitude by a like gradual increase; and during the remaining part of the period, namely, 2 days, 12 hours, 42 minutes, it invariably preserves its greatest lustre ; after the expiration of which its diminution again commences. According to Mr. Pigot, who has made many observations on such stars, and paid particular attention to the subject, the degree of brightness of this star when at its minimum is variable at different periods; and he is of the same opinion in regard to its brightness when at its full; but whether these differences return regularly or not has not been determined. The right ascension of Algol, or B Persei, for 1786, was 2h 54' 19"; and its north declination 40° 6' 58". It is situated 12° east of Almaach, in the foot of Andromeda, and may be known by means of three stars of the fourth magnitude lying a few degrees southwest of it, and forming a small triangle. It

comes to the meridian on the 21st December, about nine o'clock in the evening; but as it continues above the horizon at least twenty hours out of the twenty-four, it may be seen every evening from August to May.

Another variable star is to be found in the neck of the Swan. The period of this star had been settled by Maraldi and Cassini at 405 days; but from a mean of the observations of Mr. Pigot, it appears to be only 392, or, at most, 396 7-8 days. The particulars relating to it are, 1. When at its full brightness, it undergoes no perceptible change for a fortnight. 2. It is about three and a half months in increasing from the eleventh magnitude to its full brightness, and the same in decreasing; for which reason it may be considered as invisible during six months. 3. It does not always attain the same degree of lustre, being sometimes of the fifth and sometimes of the seventh magnitude. The right ascension of this star is 19h 42′ 21′′; and its north declination 42° 22′ 58". It is situated in the neck, nearly equidistant from Beta and Gamma, and south by west from Deneb Cygni, at the distance of about twelve degrees, and is marked Chi.

The star Eta Antinoi is another star of this description, whose variation and period were discovered by Mr. Pigot in 1785. From his corrected observations, he concludes that it continues at its greatest brightness forty hours without decreasing; it is sixty-six hours after it begins to decrease before comes to its full diminution; after which it continues stationary for thirty hours more; and then increases for thirtysix hours. In every period it seems to acquire its full brightness, and to be equally decreased. Its period, therefore, is seven days, four hours; and its greatest and least variation is from the third to the fifth magnitude. Its right ascension is 19h 41' 34"; and its north declination 0° 28′ 14". It is about eight degrees south from Altair, the principal star in the constellation Aquila.

The above descriptions may suffice as specimens of the phenomena of variable stars. There are about seven or eight other stars which have been observed to be certainly variable, among which are the following: A star in the Northern Crown, whose right ascension is 15h 40' 11"; north declination 28° 49′ 30′′; and period 10 months. A star in Hercules, whose right ascension is 17h 4' 54"; north declination 14° 38'; and period of variation 60 days. A star in So

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bieski's Shield, whose right ascension is 18h 36′ 38′′; south declination 5° 56'; and period 62 days. The star Beta Lyra, right ascension 18h 42' 11'; north declination 33° 7′ 46"; greatest and last variation, 3, 4, 5; supposed period, 6 days, 9 hours. The star Delta Cephei, whose period is 5 days, 8 hours; right ascension 22h 21'; and north declination 57° 50'. With several others.

Besides these, whose variations and periods have been determined, there are about thirty-seven other stars, which are, with good reason, suspected to be variable, but whose periods of change have not yet been ascertained, on account of the want of a sufficient number of observers, who might devote their attention more particularly to this department of astronomical observation. For example, the star Pollux, or Beta Gemini, is suspected to change from the first to the third magnitude.

When contemplating such changes among bodies so immensely distant and of so vast magnitude, we are naturally led to inquire into the causes which produce those phenomena. Our ignorance, however, of the precise nature and constitution of those remote bodies, and of the scenes and circumstances in which they may be placed, prevent us from forming any definite or satisfactory conclusions. The following are some of the opinions which have been thrown out on this subject. It has been supposed that portions of the surfaces of these stars are covered with large black spots, which, during the diurnal rotation of the star, present themselves under various angles, and thus produce a gradual variation in its brilliancy. Sir W. Herschel says, "Such a motion may be as evidently proved as the diurnal motion of the earth. Dark spots, or large portions of the surface less luminous than the rest, turned alternately in certain directions, either towards or from us, will account for all the phenomena of periodical changes in the lustre of the stars so satisfactorily, that we certainly need not look for any other cause." Sir Isaac Newton thought that the sudden blaze of some stars may have been occasioned by "the falling of a comet into them, by which means they would be enabled to cast a prodigious light for a little time, after which they would gradually return to their former state." But we know too little about the nature of comets to be able to determine what effect they would produce in such a case, nor are we certain that such bodies

are connected with other systems. If the fixed stars be nearly of the same nature as the sun, it is highly improbable that any such effect would be produced, even although a comet were to fall into its luminous atmosphere, as that atmosphere appears to have nothing in it that would take fire by the approach of any extraneous body, or that would "blaze" like combustible substances on the earth. The blaze, if such an effect were to take place, would scarcely be distinguishable from our globe, and much less from a distant system. Maupertius, in a "Dissertation on the Figures of the Celestial Bodies," is of opinion that some stars, by their prodigious quick rotation on their axes, may not only assume the figures of oblate spheroids, but that, by the great centrifugal force arising from such rotations, they may become of the figures of millstones, or be reduced to flat circular planes, so thin as to be quite invisible when their edges are turned towards us, as Saturn's ring is in such positions. And when any eccentric planets or comets go round any fixed star, in orbits much inclined to its equator, the attraction of the planets or comets in their perihelions must alter the inclination of the axis of that star; on which account it will appear more or less large and luminous, as its broad side is turned more or less towards us. This opinion, at best, I consider as having a very small degree of probability, and almost quite untenable. Mr. Dunn, in a paper in vol. 52 of the "Philosophical Transactions," supposes that the interposition of some gross atmosphere may solve the ́phenomena under consideration. "The appearance of new stars," says he, "and the disappearance of others, possibly may be occasioned by the interposition of such an ethereal medium within their respective orbs as either admits light to pass freely or wholly absorbs it at certain times, while light is constantly pursuing its journey through the vast regions of space."

Whatever opinions we may adopt on this subject, it is evident that the regular succession of the variations of periodical stars preclude the idea of their being destroyed. It is likewise evident that motion of some kind or other, either in the stars themselves or in some bodies either directly or remotely connected with them, must be one of the causes of the phenomena in question; and it is not improbable that different causes in different instances may opperate in producing the effects. It does not appear to me probable that the cause