of 3300 of double and triple stars, from his own solitary observations, accompanied with all the micrometrical measurements. Struve, the celebrated astronomer of Dorpat, has arranged a catalogue of no less than 3000 double stars; and before he determined the characteristics of each of these, he examined about 120,000 stars; a laborious process, which none but an astronomical observer can duly appreciate. Mr. Dunlop has formed a catalogue of 250 double stars in the southern hemisphere; and Sir J. Herschel, during his late residence at the Cape of Good Hope, has added considerably to their number; so that we may now reckon about 6000 of these interesting objects as having already been discovered, even making allowance than many of these objects are common to the lists of the observers now specified.

It is not at all improbable that the phenomena of some of the double stars now alluded to may arise from accidental proximity, the one star, though far remote and unconnected with the other, lying nearly in the same visual line. Thus, the star a, figure 13, might appear nearly in contact with the star b, placed at an immense distance beyond it, when viewed nearly in the same straight line by the eye at c, so as to produce the phenomenon of a double star at d b. But,

reasoning à priori, it appears in the highest degree improbable that such coincidences should happen in the case of all, or even of the greater part of the double stars which have now been discovered; and therefore Mr. Michell, so early as the year 1783, in a paper inserted in the "Philosophical Transactions" for that year, states it as his opinion that they are binary systems intimately connected. "The very great number of stars," says he, "that have been discovered to be double, treble, &c., particularly by Mr. Herschel, if we apply the doctrine of chances, as I have done in my 'Inquiry into

the probable Parallax of the Fixed Stars,' published in the Philosophical Transactions for 1767, cannot leave a doubt with any one properly acquainted with the force of those arguments, that by far the greatest part, if not all of them, are systems of stars so near each other as probably to be liable to be affected sensibly by their mutual gravitation; and it is therefore not unlikely that the periods of the revolutions of some of these about their principals may some time or other be discovered."

The prediction here announced by this ingenious gentleman has now been fully realized by Sir William Herschel and other astronomers, and is no longer a subject of conjecture, but an ascertained fact. This is the discovery to which I have alluded above, one of the most important and interesting discoveries which astronomy has unfolded during the present age, and which opens to our view a new prospect of the plans and arrangements of Infinite Wisdom.

Having made these preliminary remarks, I shall now proceed to a more particular detail of the facts which have been ascertained respecting binary systems.

When Sir W. Herschel first directed his attention to this subject, in order, if possible, to determine the annual parallax, he was not a little surprised that, instead of finding, as he expected, a regular annual change of the two stars, by one alternately shifting its position with respect to the other, which a parallax would have produced, he observed in many instances "a regular progressive change, in some cases bearing chiefly on their distance, in others on their position, and advancing steadily in one direction, so as clearly to indicate either a real motion of the stars themselves, or a general rectilinear motion of the sun and whole solar system, producing a parallax of a higher order than would arise from the earth's orbital motion." In an elaborate paper on this subject, read before the Royal Society, June 9, 1803, he considers specifi cally all the motions and combinations of motion that can possibly be supposed, in order to account for the phenomena, particularly of the double star Castor, and satisfactorily demonstrates that nothing but the idea of the smaller star revolving around the larger in a circular or elliptical orbit will solve the phenomena in question; and this conclusion has been amply confirmed by all succeeding observations. Such stars, therefore, must be considered as physically connected by the law

of mutual gravitation, so that they describe orbits around each other and around their common centre of gravity, and bear a relation to each other similar to that which the planets bear to

our sun.

From the paper of Sir. W. Herschel now referred to, I shall select, as a specimen of the motions of double stars, some of his observations of Castor, or a Geminorum. It appears that Dr. Bradley, in the year 1759, had observed the position of the two stars which form this double star, and communicated it to Dr. Maskelyne, who made a memorandum of it, of which the following is a copy: "Double star Castor. No change of position of the two stars; the line joining them at all times of the year, parallel to the line joining Castor and Pollux in the heavens, seen by the naked eye." The object of Dr. Bradley in observing the exact position of these stars was to determine if any change happened in their position at opposite periods of the year so as to indicate an annual parallax. The angles of position observed by Sir W. Herschel are as follow:

From these observations, it appears that, from the year 1759, when Dr. Bradley observed the positions of these two stars, to the year 1803, there has been a portion of an orbit described by the smaller star around the greater equal to forty-five degrees and thirty-nine minutes; and from the time that Herschel commenced his observations in 1779 till 1803, an arc of twenty-four degrees and thirty-six minutes had been passed over. Hence Sir W. Herschel concludes: "The time of a periodical revolution may now be calculated from the arch 45° 39', which has been described in 43 years and 142 days. The regularity of the motion gives us great reason to conclude that the orbit in which the small star moves about Castor, or, rather, the orbits in which they both move round their com

mon centre of gravity, are nearly circular and at right angles to the line in which we see them. If this should be nearly true, it follows that the time of a whole apparent revolution of the small star round Castor will be about 342 years and two months." This subject may be illustrated to the general reader by the following diagram:

tion in a circle. Since 1803 its motion has been regularly traced by Struve, Sir J. Herschel, and Sir J. South; and in 1816 it was found about 57° from its first position, and in 1830 about 68°, still regularly progressing. In 1819 the distance of the small star from Castor was five seconds and a half, and in 1830 it was little more than four seconds and a half. Although Sir W. Herschel, as above stated, conjectured the period of revolution to be about 342 years, yet later astronomers, from a comparison of all the observations recently made, are disposed to conclude that its period is little more than 250 years.

More than fifty instances of changes in the angles of position of double stars were observed by Sir W. Herschel, besides those which have been more recently observed by his son and other astronomers, most of which indicate motions which are regularly progressive; but a considerable number of years must elapse before their periods can be determined with any degree of accuracy. The following double stars are considered as demonstrative instances of circular progressive motion: y Virginis, § Ursa Majoris, 70 Ophiuchi, σ and n Corona, Bootis, n Cassiopeia, y Leonis, Herculis, d Cygni, u Bootis, e 4 and e 5 Lyræ, 2 Ophiuchi, μ Draconis, e Bootis, and Aquarii. The periodic times of some of these have been determined to a near approximation. One of the stars of Gamma Virginis is reckoned to revolve about the other in the space of 629 years; the small star of Gamma Leonis in 1200 ysars; the star connected with Epsilon Bootis in 1600 years; that of 61 Cygni in 452 years; that of Sigma Coronæ in 287 years; that of 70 Ophiuchi, as ascertained by Professor Encke, in 80 years; that of Xi Ursæ in 58 years; that of Zeta Cancri in 55 years; and that of Eta Coronæ in 43 years.

A whole revolution of some of these stars has been nearly completed since observations began to be made on such objects. The motion of the small star of Xi Ursæ began to be traced about the year 1781; in 1819 it had moved 219° from its position in 1781; in 1830 it was 303° from that position, progressing in a circle; and about this time, or the beginning of 1840, it has probably finished its orbital revolution. The star Eta Coronæ, whose period is forty-three years, has not only accomplished a complete revolution, but is actually considerably advanced in its second period. Sir