nebula itself, and the probable position of the solar system within it. But while this idea impresses us with its grandeur, it at the same time furnishes us with a scale for estimating the immensity of nature. If all the separate stars which the most powerful telescope can descry, are only part of our own nebula, what must we think of the millions of nebulæ, some of which exhibit, by their proximity, the individual stars of which they are composed? while others, as they recede from our failing sight, display only in the best instruments a continuous and unbroken light, in which the spaces between the stars can no longer be seen. From the systems which roll within these groups of worlds, a new firmament of stars will be seen, and each system will have its milky way, exhibiting the projection of its nebula, varying in form and in lustre with its locality within the group. It is in vain to pursue ideas so vast and overwhelming: it is enough that the mind tries its strength, and stands self-convicted of its weakness.

Let us, therefore, turn our attention to nearer objects to our own nebula, and the stars which compose it. Not content with determining the probable position of the solar system within the nebula of the milky way, Sir William Herschel conceived the idea of ascertaining whether that system was stationary or moveable. By a comparison of the proper motions of the fixed stars, he determined that the solar system was advancing towards the constellation Hercules, and that, if it were viewed from one of the nearest of the fixed stars, the sun would appear to describe an arch of about one second. In reasoning respecting the insulated stars, which belong to what we may now call the solar nebula, he justly conceived that those which were double must form binary systems, or systems in which the two stars revolve round their common centre of gravity. We have said in many cases, because there can be no doubt that two stars may often form a double star, when they have no connexion with each other but that of similarity of direction. The same conception is applicable to more complicated systems, and he has shown how three or more stars may be permanently connected, by revolving in proper orbits

round a common centre.

These views, at first entirely speculative, received from subsequent and long-continued observation a very remarkable confirmation. If we suppose a line to join the centres of the two stars which compose a double star, then if the two stars have no relative motion, this line must form an invariable angle with the line or direction of their daily motion. By means of an ingenious position-micrometer, Sir William Herschel determined this angle (called the angle of position) for seven hundred and two stars, between 1778 and 1784. After a lapse of twenty years, he repeated his observations on the same stars, between 1800 and 1805,

and

and he had the satisfaction of finding that, in more than fifty double stars, there had been a decided change, either in their distance or in their angle of position. In this way he discovered that one of the stars of Castor revolved round the other in three hundred and forty-two years; that the small star of y Leonis performed its circuit in 1200 years, that of & Bootis in 1681 years, that of ♪ Serpentis in 375, and that of y Virginis in 708 years.

By this great discovery, the greatest, unquestionably, in the history of astronomy, the existence of systems among the fixed stars was completely established; but so far did Sir William Herschel's labours transcend those of the age in which he lived, that no attempt was made to repeat and to extend them. They were scarcely admitted into any astronomical work; they were ridiculed by men whose reputation had been eclipsed by his own; and they were received with a sort of incredulous wonder, even by the most ardent lovers of astronomy. The progress of knowledge and of discovery had paved the way not only for the highest achievements of Newton and Laplace, but also for their immediate reception among philosophers; and had these great men never lived, science would, in a few years, have received from other minds the same splendid accessions. The discoveries of Herschel, on the contrary, exhibited no continuity with those of his predecessors. Before his day sidereal astronomy had no existence; nor had the wildness of speculation ventured even to foreshadow its wonders. Entrenched in the remoteness of space, and among spheres which no telescope but his could descry, her walls were unscaled, and her outworks even unapproached, His genius, however, enabled him to surmount barriers hitherto impregnable, and conducted him in triumph into the very stronghold of her mysteries. The cessation of such gigantic labours would have been afflicting to science, had not that same wisdom which provided for the continuity of his name, provided also for the continuity of his labours.

In the year 1816, four years before the death of his venerable father, Mr. J. Herschel had begun a re-examination of the double stars, and had made some progress in it. The same idea had occurred to Mr. South, one of the most able and enterprising astronomers of the present day, and it was agreed that they should undertake the work in concert. They accordingly began in March, 1821, and continuing their observations in 1822 and 1823, they were able to communicate to the Royal Society in January, 1824, the position and apparent distances of 380 double and triple stars, the result of above 10,000 individual measurements.* The instru

This Memoir was honoured with the astronomical prize of the French Academy of Sciences.

ments

:

ments which they employed were two achromatic telescopes mounted equatorially the object-glass of the smallest had an aperture of three inches and three quarters, and a focal length of five feet, and was made by the late P. and J. Dollond. The power usually employed was 133, but powers of 68, 116, 240, 303, and 381 were sometimes used.. The largest telescope was seven feet in focal length, with an aperture of five inches, and is supposed to be the best that Mr. Tulley ever executed. The power commonly employed was 179, though 105, 273, and sometimes 600 were used.

No sooner had Mr. South completed his share in this great work, than he began another series of observations of equal difficulty and importance. They were made principally at Passy, near Paris, with the instruments abovementioned; and in November, 1825, he communicated to the Royal Society the apparent distances and positions of four hundred and fifty-eight double stars, of which one hundred and sixty had never before been observed.

While these observations were going on in England, an able continental astronomer, M. Struve, director of the Imperial Observatory of Dorpat, in Livonia, had occupied himself with the same subject; and such was his assiduity and zeal, that in four years he completed his Catalogus Novus Stellarum Duplicium et Multiplicium, containing no fewer than three thousand and sixtythree stars.* These observations were chiefly made with a telescope by Frauenhofer, which the Emperor of Russia had presented to the observatory of Dorpat. This magnificent instrument has a focal length of thirteen feet, and an aperture of nine inches, and cost thirteen hundred pounds. The King of Bavaria followed this noble example by ordering a still finer instrument for the same purpose; and the King of France, with a liberality still more patriotic, has had executed in his own capital an achromatic telescope, surpassing them all in magnitude and power. What a misfortune is it to English science, that the name of the most accomplished prince who has as yet occupied the throne of Charles I. does not appear in the list of sovereigns who have been thus rivalling each other in the patronage of astronomy! What a mortification to English feeling, that the subject of sidereal astronomy created by the munificence of George III, should thus be transferred to the patronage of foreign monarchs!

In taking a general view of the labours of Mr. Herschel and Mr. South, it appears that there are sixteen binary systems of stars perfectly established, and at least fourteen, of which the annual motion is not exactly determined.

The labours of this indefatigable astronomer have been rewarded by the Royal Society of London with one of their gold medals.

The

The established binary systems, with their periods and annual motions, are given in the following table. The signs + and — indicate the different directions of the motion.

Of these stars, Ursa Majoris possesses a very peculiar character, as the two stars revolve round their common centre of gravity with a motion so rapid as to admit of being traced and measured from month to month. After comparing all the observations with the latest by Mr. South, Mr. Herschel observes,

'Nothing can be more satisfactory than the confirmation these observations afford of the rapid motion ascribed to this remarkable star. In the interval of 1.97 years, since the epoch 1823.29, the motion has amounted to no less than 13° 55', in the direction n p, s f, or -70.025 per annum. The sudden diminution of velocity is, however, not confirmed. Indeed it rested on too short an interval, and on too few observations, to deserve great confidence. We cannot do better than recommend this star for the next ten or twenty years to the constant and careful measurement of astronomers; nor can we too strongly inculcate here the indispensable necessity of multiplying extremely their measures of position, to eliminate those errors of judgment to which the most experienced observers are liable in measures of this sort. This done, there is no doubt of our arriving at a precise knowledge of the elements and position of the orbit described by each about their common centre of gravity; and the question of the extension or non-extension of the Newtonian law of gravity to the sidereal heavens, the next great step which physical astronomy has yet to make, will be effectually decided.'

Another object of very peculiar interest to astronomers is 2 Herculis, which both Mr. Herschel and Mr. South have found to be single, with the best telescopes. In July, 1782, however, it was a distinct double star, the greater being of a beautiful bluish

white, and the lesser of a fine ash colour. In 1782, Sir William Herschel found the interval between the two stars to be one-half the diameter of the smaller one. In 1795, he could with difficulty perceive the small star. In 1802, he could no longer perceive it; but, in a very clear night, the apparent disc of 2 Herculis seemed to be lengthened in one direction. In 1803, with a power of 2140, he found the disc a little' distorted, but he was convinced that about three-eighths of the apparent diameter of the small star was wanting to make the occultation of it complete. If these two stars have not yet begun to separate, which we think the French achromatic telescope will determine, the separation will certainly take place in a few years, and astronomers will, no doubt, devote to it much of their attention.*

It is scarcely possible, we think, to peruse the preceding details concerning the history and present advanced state of astronomy, brief and imperfect as they are, without looking forward with the most intense interest to the future progress of the science. Even within our own system much remains to be investigated. The nature of the sun, and the constitution of its surface in relation to the more or less copious discharge of light and heat; the physical condition of the moon, which may yet exhibit among her mountains the works of living agents; the theory of the four new planetary fragments, which hold out to physical astronomy some of its most perplexing problems; the forms, the rotations, and the densities of most of the secondary planets, are all subjects fraught with the deepest interest to astronomers. The comets, too, those illusory bodies of which we scarcely know whence they come, or whither they go, have now been brought within the grasp of regular observation. The discovery of two comets with short periods, one of three and one-third years, revolving within the orbit of Jupiter, and the other with a period of five years, revolving within the orbit of Saturn, enables us to observe them period after period, and to study their motions as well as their physical constitution. But how shall we describe the future prospects of sidereal astronomy! In our own nebula we may trace the motion of the solar system round some distant centre; we may discover the causes which produce the phenomena of variable stars; and we may witness the extension of the law of gravity to the movements of binary, and even of more complicated, systems. Among the nebulæ beyond our own, discoveries still more extraordinary await us. May we not see even the operations of those powerful agents by which whole systems are formed; and of those still more tremendous forces by which other systems are destroyed? In the changes of particular nebulæ, and in the condensation of *Since the above was written, we find that the Dorpat telescope has separated the two stars of Herculis.

nebulous