(844.) Of the stars in the above list, that which has been most assiduously watched, and has offered phænomenon of the greatest interest, is y Virginis. It is a star of the vulgar 3rd magnitude (3.08 Photom. 3-494), and its component individuals are very nearly equal, and as it would seem in some slight degree variable, since, according to the observations of M. Struve, the one is alternately a little greater and a little less than the other, and occasionally exactly equal to it. It has been known to consist of two stars since the beginning of the eighteenth century; the distance being then between six and seven seconds, so that any tolerably good telescope would resolve it. When observed by Herschel in 1780, it was 5"-66, and continued to decrease gradually and regularly till at length, in 1836, the two stars had approached so closely as to appear perfectly round and single under the highest magnifying power which could be applied to most excellent instruments · the great refractor at Pulkowa alone, with a magnifying power of 1000, continuing to indicate, by the wedge-shaped form of the disc of the star its composite nature. By estimating the ratio of its length to its breadth and measuring the former, M. Struve concludes that, at this epoch (1836-41), the distance of the two stars, centre from centre, might be stated at 0"-22. From that time the star again opened, and at present (1849) the individuals are more than 2" asunder. This very remarkable diminution and subsequent increase of distance has been accompanied by a corresponding and equally remarkable increase and subsequent diminution of relative angular motion. Thus, in the year 1783 the apparent angular motion hardly amounted to half a degree per annum, while in 1830 it had increased to 5°, in 1834 to 20°, in 1835 to 40°, and about the middle of 1836 to upwards of 70° per annum, or at the rate of a degree in five days. This is in entire confermity with the principles of dynamics, which establish a necessary connexion between the angular velocity and the distance, as well in the apparent as in the real orbit of one body revolving about another under the influence of mutual attraction; the former varying inversely as the square of the latter, whatever be the curve described and whatever the law of the attractive force. It fortunately happens that Bradley, in 1718, had noticed and recorded in the margin of one of his observation books, the apparent direction of the line of junction of the two stars, as seen on the meridian in his transit telescope, viz., parallel te the line joining two conspicuous stars a and 8 of the same constellation, as seen by the naked eye. This note, rescued from oblivion by the late Professor Rigaud, has proved of singular service in the verification of the elements above assigned to the orbit, which represent the whole series of recorded observations that date up to the end of 1846 (comprising an angular movement of nearly nine-tenths of a complete circuit), both in angle and distance, with a degree of exactness fully equal to that of observation itself. No doubt can, therefore, remain as to the prevalence in this remote system of the Newtonian law of gravitation. (845.) The observations of Ursa Majoris are equally well represented by M. Mädler's elements (4 c of our table,) thus fully justifying the assumption of the Newtonian law as that which regulates the motions of their binary systems. And even should it be the case, as M. Mädler appears to consider, that in one instance at least (that of p Ophiuchi,) deviations from elliptic motion, too considerable to arise from mere error of observation, exist (a position we are by no means prepared to grant,)1 we should rather be disposed to look for the cause of such deviations in perturbations arising (as Bessel has suggested) from the large or central star itself being actually a close and hitherto unrecognized double star than in any defect of generality in the Newtonian law. (846.) If the great length of the periods of some of these bodies be remarkable, the shortness of those of others is hardly less so. Herculis has already completed two revolutions since the epoch of its first discovery, exhibiting in its course the extraordinary spectacle of a sidereal occultation, the small star having twice been completely hidden behind the large one. 7 Coronæ, Cancri, and § Ursæ have each performed more than one entire circuit, and 70 Ophiuchi and y Virginis have accomplished by far the larger portion of one in angular motion. If any doubt, therefore, could remain as to the reality of their orbitual motions, or any idea of explaining them by mere parallactic changes, or by any other hypothesis than the agency of centripetal force, these facts must suffice for their complete dissipation. We have the same evidence, indeed, of their rotations about each other, that we have of those of Uranus and Neptune about the sun; and the correspondence between their calculated and observed places in such very elongated ellipses, must be admitted to carry with it proof of the prevalence of the Newtonian law of gravity in their systems, of the very same nature and cogency as that of the calculated and observed places of comets round the central body of our own. 1 (847.) But it is not with the revolutions of bodies of a planetary or p Ophiuchi belongs to the class of very unequal double stars, the magnitudes of the individuals being 4 and 7. Such stars present difficulties in the exact measurement of their angles of position which even yet continue to embarrass the observer, though, owing to later improvements in the art of executing such measurements, their influence is confined within much narrower limits than in the earlier history of the subject. in simply placing a fine single wire parallel to the line of junction of two such stars it is easily possible to commit an error of 3° or 4°. By placing them between two parallel 'hick wires such errors are in great measure obviated. cometary nature round a solar centre that we are now concerned; it is with that of sun round sun - each, perhaps, at least in some binary systems where the individuals are very remote and their period of revolution very long, accompanied with its train of planets and their satellites, closely shrouded from our view by the splendour of their respective suns, and crowded into a space bearing hardly a greater proportion to the enormous interval which separates them, than the distances of the satellites of our planets from their primaries bear to their distances from the sun itself. A less distinctly characterized subordination would be incompatible with the stability of their systems, and with the planetary nature of their orbits. Unless closely nestled under the protecting wing of their immediate superior, the sweep of their other sun in its perihelion passage round their own might carry them off, or whirl them into orbits utterly incompatible with the conditions necessary for the existence of their inhabitants. must be confessed, that we have here a strangely wide and novel field for speculative excursions, and one which it is not easy to avoid luxuriating in. It (848.) The discovery of the parallaxes of a Centauri and 61 Cygni, both which are above enumerated among the "conspicuous" double stars of the 6th class (a distinction fully merited in the case of the former by the brilliancy of both its constituents), enables us to speak with an approach to certainty as to the absolute dimensions of both their orbits, and thence to form a probable opinion as to the general scale on which these astonishing systems are constructed. The distance of the two stars of 61 Cygni subtends at the earth an angle which, since the earliest micrometrical measures in 1781, has varied hardly half a second from a mean value 15"-5. On the other hand, the angle of position has altered since the same epoch by nearly 50°, so that it would appear probable that the true form of the orbit is not far from circular, its situation at right angles to the visual line, and its periodic time probably not short of 500 years. Now, as the ascertained parallax of this star is 0"-348, which is, therefore, the angle the radius of the earth's orbit would subtend if equally remote, follows that the mean distance between the stars is to that radius, as 15′′-5:0′′-348, or as 44.54 : 1. The orbit described by these two stars about each other undoubtedly, therefore, greatly exceeds in dimensions that described by Neptune about the sun. Moreover, supposing the period to be five centuries (and the distance being actually on the increase, it can hardly be less) the general propositions laid down by Newton', taken in conjunction with Kepler's third law, enable us to calculate the sum of the masses of the two stars, which, on these data we find 1 Principia, l. i. Prop. 57, 58, 59. to be 0.353, the mass of our sun being 1. The sun, therefore, is neither vastly greater nor vastly less than the stars composing 61 Cygni. (849.) The data in the case of a Centauri are more uncertain. Since the year 1822, the distance has been steadily and pretty rapidly decreasing at the rate of about half a second per annum, and that with and that with very little change in the angle of position. Hence, it follows evidently that the plane of its orbit passes nearly through the earth, and (the distance about the middle of 1834 having been 17") it is very probable that either an occultation, like that observed in Herculis, on a close appulse of the two stars, will take place about the year 1867. As the observations we possess afford no sufficient grounds for a satisfactory calculation of elliptic elements' we must be content to assume what, at all events, they fully justify, viz., that the major semiaxis must exceed 12′′, and is very probably considerably greater. Now this with a parallax of 0"-913 would give for the real value of the semiaxis 13.15 radii of the earth's orbit, as a minimum. The real dimensions of their ellipse, therefore, cannot be so small as the orbit of Saturn; in all probability exceeds that of Uranus ; and may possibly be much greater than either. (850.) The parallel between these two double stars is a remarkable one. Owing no doubt to their comparative proximity to our system, their apparent proper motions are both unusually great, and for the same reason probably rather than owing to unusually large dimensions, their orbits appear to us under what, for binary double stars, we must call unusually large angles. Each consists, moreover, of stars, not very unequal in brightness, and in each both the stars are of a high yellow approaching to orange colour, the smaller individual, in each case, being also of a deeper tint. Whatever the diversity, therefore, which may obtain among other sidereal objects, these would appear to belong to the same family or genus.2 (851.) Many of the double stars exhibit the curious and beautiful phænomenon of contrasted or complementary colours. In such instances, 3 1 Elements have been recently computed by Captain Jacob, for which see the table, p. 483. 2 Similar combinations are very numerous. Many remarkable instances occur among the double stars catalogued by the author in the 2nd, 3rd, 4th, 6th and 9th volumes of Trans. Roy. Ast. Soc. and in the volume of Southern observations already cited. See Nos. 121, 375, 1066, 1907, 2030, 2146, 2244, 2772, 3853, 3395, 3998, 4000, 4055, 4196, 4210, 4615, 4649, 4765, 5003, 5012, of these catalogues. The fine binary star, B. A. C. No. 4923, has its constituents 15" apart, the one 6m. yellow, the other, 7m. orange. other suns, perhaps, With their attendant moons, thou wilt descry, |