gree; from which it is inferred that the period of its revolution may be about 450 years, and that the semi-major axis of its orbit is seen under an angle of more than 15". Were these and other correlative points accurately settled, we might soon determine to a near approximation the extent of its orbit, the space through which it moves in the course of a revolution, and, consequently, its rate of velocity; but as the motion of revolution of this star is so extremely slow, a considerable period of years may elapse till all the elements of its orbit be accurately ascertained.

A few years ago, a method was pointed out by M. Savory, a French astronomer, by which the dimensions of the orbit of a revolving star might be determined. This method depends upon the fact that light moves with a certain known rate of velocity. Suppose that one of the double stars moves round another in an orbit which is nearly parallel to our line of vision, it is evident that the one half of its orbit will be nearer us than the other, and that, at the most distant point of its course, the star will be removed from us to a distance equal, or nearly equal, to the whole diameter of its orbit farther than when at the point which is nearest the earth. As the light which proceeds from the star takes a certain time in moving across the interval which separates us from that body before it reach our eye, we must necessarily see the star in a point of its orbit different from that in which it is actually placed. Let S (fig. 33) represent the central star, E the earth, and HFK G the orbit of the revolving star. When the star is at H, it is nearest the earth; and when at K, it is farther distant by the whole diameter of its orbit. Now, when the star proceeds from H, the nearest point of its orbit, its light will take a longer period to reach the earth in proportion as it moves on in its course from H to G and from G to K, and, consequently, will appear to take a longer time than in reality it does in moving along that portion of its orbit; but in returning through the other half of its orbit, K F H, it will appear to pass through it in a less space of time than it actually does, since the light which proceeds from it takes less and less time to reach our eye as it approaches in its course towards F and H. If, therefore, we could accurately determine the difference of time between these two half revolutions of the star, we should have data sufficient for determining, to a near approximation, the dimensions of the orbit in miles or other known measures;

and, having found these dimensions, the distance of the star from the earth could likewise be found by an easy trigonomet rical calculation.

This method of finding the dimensions of binary systems is entitled to the praise of ingenuity; but it will be difficult, in many instances, to put it in practice. Its accuracy will depend upon our knowing the position of the orbit with regard to our eye, and our ascertaining exactly when the star is in H or at K, or the two opposite points of its orbit. Besides, a very long time must intervene before observations of this kind can be completed, since most of the periods that have been determined in regard to double stars extend to several hundreds of years, and the shortest period yet known of any of

these revolving bodies is above thirty years. It is generally taken for granted, by those who have adverted to this subject, that the distance between the revolving and the central star is as great, or nearly as great, as that which intervenes between us and the nearest star; and hence, in their illustrations of this point, they have supposed light to take at least one year in crossing the orbit of a revolving star, which, of course, would make the diameter of such an orbit above six billions of miles. But there appears no reason for forming such extravagant suppositions, as in such a case the binary stars could scarcely be supposed to have any intimate connexion. We might almost as soon suppose that the star Sirius might revolve around our sun, or the sun around Sirius. It is not likely that the double stars in general are much farther from each other than the distance I formerly supposed, namely, 200,000,000,000; and, consequently, the diameter of their orbits about 400,000,000,000 of miles. Through this space light would pass in the course of 24 days and 23 hours; and therefore it would require very accurate determinations indeed of the points H and K, or the nearest and remotest points of the orbits, before any precise conclusions could be deduced, if the stars be not farther distant than I have supposed, and it is perhaps as probable that they are considerably within that distance. It is not improbable, however, that the dimensions of the orbits of some of those stars whose periods are shortest may in this way be determined; but a considerable period must elapse before the requisite observations can be made.

CHAPTER IX.

ON TREBLE, QUADRUPLE, AND MULTIPLE STARS.

BESIDES the combinations of double stars described in the preceding chapter, treble, quadruple, and multiple stars have been discovered, many of which appear to be intimately connected, and to be formed into regular systems, whose motions and phenomena must, of course be more diversified and com

plicated than those of binary systems. Without entering into particular discussions on this subject, I shall present to the reader only two or three general remarks, with a short list of some of the treble and multiple stars to which I allude.

The more profound and minute our investigations are into the scenery of the heavens, the more do we discover of the endlessly diversified modes by which the system of universal nature is arranged and conducted, and the more clearly do we perceive a display of the infinite wisdom and intelligence of its almighty Author. Who could have previously conceived of one sun and system revolving round another, had not recent observations demonstrated the astonishing fact? As one discovery naturally leads to another, so the facts which have already been ascertained may lead to discoveries in future generations still more wonderful and sublime than those which have hitherto been brought to light. The discovery of binary systems leads to the conclusion that almost all the close groups or clustering stars visible to the naked eye or descried by telescopes are multiple systems, or suns and planetary worlds linked together by a universal law or principle, acting in different modes, and producing an immense variety of physical phenomena and effects. Guided by principles and facts recently brought to light, astronomers have only to direct their attention more particularly to such objects, to watch with care the slightest movements in the sidereal heavens, and take their measurements of distances and angular positions with the utmost precision; and then we may expect that succeeding generations will have unfolded to their view a more sublime and comprehensive prospect of the arrangements.of the universe.

In certain cases, it has already been ascertained that treble stars form one connected system. The star marked Cancri is a treble star of this description. Two of the stars are considerably unequal; the largest of these is larger than the single star, and the least of the two is less than the single star. The first and second largest, as described by Sir W. Herschel, are pretty unequal, and the second and third pretty unequal. The two nearest are pale red. They require very favourable circumstances to be distinctly seen; they are just separated by a power of 227, and with 460 their distance is 4th the diameter of the smaller one. This is considered a case in which three suns revolve around a common centre. Observation has not yet afforded sufficient data for determining the particular mo

M

tions or arrangements of such complex systems; but we may conceive them as arranged in a manner somewhat similar to what we have delineated in fig. 34, where the point C may represent the common centre of gravity around which the three bodies revolve. The circles A B, D E, F G, represent the

orbits of the revolving bodies, which may be conceived as lying in different planes oblique to each other, to prevent any occasional collision or too near an approach.

A quadruple system may be represented by fig. 35, where Cis the centre of gravity round which the four bodies revolve, and the circles aa aa, bb bb, &c., the respective orbits in which they move. The star & Lyre is probably a system of this kind. It is a star of the fifth magnitude, situated about two degrees northeast from the bright star Vega, or a Lyræ.