would be found to be travelling in the same direction, is demonstrably minute. Lastly, we have the evidence derived from the equality of the motions of the five stars, and here again the antecedent probability of the coincidence is so minute as to force upon us the opinion that the actual coincidence is not accidental. The combination of the three lines of evidence leads to a feeling of absolute certainty that the five stars are associated into a single scheme or system.

Let us pause for a moment to contemplate the significance of this result. One of the stars of the set of five is the middle star in the tail, which I have already referred to as having a companion visible to the naked eye. Now, it had long since been proved that the bright star and its small companion are really connected. The evidence had been no other than that community of proper motion which I have been dealing with in the case of the five stars. Mizar and Alcor, then, were looked on as a wide double, and astronomers had contemplated with interest and amazement the wondrous cycle corresponding to the motions of stars separated in reality by an enormous interval. For the nearest of the stars in our northern skies is more than 720,000 times farther from us than we are from the Sun. Mizar is presumably much farther away. Now, whatever distance separates Mizar from us, cannot be more than about 240 times as great as that which separates Alcor from Mizar : so that Alcor must be at least 3,000 times farther from its primary orb than we are from the Sun. How enormous, then, must be the period required for the revolution of the two stars around their common centre of gravity!

But this is not all. Mizar has a close companion, as well as its distant companion, Alcor. This close companion has the same proper motion as Alcor and Mizar, and belongs, therefore, to the same scheme as the other four stars. Α sort of dignity is thus given to the star-system we are

considering, by the triplicity of one of its members. At present, however, we are dealing with another consideration -the magnitude, namely, of the cyclic period appertaining to the scheme. Now, the close companion of Mizar, though undoubtedly it is in reality travelling round that star, moves yet so slowly that no sign of a change of place has as yet been detected by astronomers. Therefore the period of revolution, even for this comparatively close pair, must be very large, and Alcor must have a very much longer period; so that we may accept without surprise Baron Mädler's estimate that Alcor occupies no less than 7,659 years in travelling around Mizar.

But what sort of periods can we assign to the cyclic revolutions of the five stars, when the comparatively close companions of one of the set occupy periods of revolution so enormous? Again, how can we resolve the questions which at once suggest themselves respecting the relations which prevail in such a system? That the whole system revolves around its centre of gravity is of course certain. But there are numberless ways in which the revolution may take place, depending on the relations between the weight and velocity of the different orbs forming the system. Any two of the five may really form a pair, any three may form a triplet. We cannot tell where the centre of gravity of the scheme may be. We have no knowledge of the true relative positions of the five orbs. We cannot guess what the real direction of their orbital motions may be. We are, in fact, altogether in doubt on every subject connected with the system, except the main fact that the whole system has a drift carrying it bodily forwards at the rate of many millions of miles per annum. It is in this connection that the appearance of such systems as these in the heavens, seems to me so interesting-I may almost say, so imposing a phenomenon. The life of man is a period too short to tell us

anything even of the subordinate motions of such a scheme, -the motion of Mizar's companion about its primary, or of Alcor about both; but the duration of the human race, nay, of the solar system itself, may be out-lasted by a single revolution of the great star-system placed out yonder in the celestial depths. From the far-off times of the Chaldæan shepherds the great Septentrion star-system has looked down with seemingly unchanging aspect on the rise and fall of many nations and races of men. When the human race has perished from this globe, when the earth has become what the moon now is, a scene of utter barrenness and desolation, the star-system will doubtless have exhibited many changes. But only when millions of æons have passed, and the earth is nearing the scene of its final absorption beneath the solar oceans, will the stately motions of the star-system have begun to work out the full series of cyclic changes appertaining to a scheme so extensive and so complicated.

But the star-drift in Ursa Major is only one instance out of many. Looking more closely than we have yet done into the sidereal scheme of which our Sun is a member, we see it breaking up into subordinate star-systems of greater or less extent. Our Sun himself may not be a solitary star as has been commonly supposed. From among the orbs which deck our skies, there may be some which are our Sun's companions on his path through space, though countless ages perhaps must pass before the signs of such companionship will be rendered discernible. On every side we see drifting star-schemes, and comparatively few stars are to be recognised as voyaging in solitary state through space. From the complexity of such systems as we see in Gemini and Cancer, to schemes such as the one in Ursa Major, and thence to solitary stars such as Arcturus and Sirius appear to be, we recognise a number of gradations, and it yet

remains to be determined to what class of these schemes our

Sun belongs.

Verily much remains to be learnt respecting our galaxy. Since the days of Sir W. Herschel, or rather since the younger Herschel completed the noble series of labours commenced by his father, a sort of rest has fallen upon astronomy, so far as the science deals with the relations of the great sidereal system. But there is room for much new labour in this wide field of research. The Herschels dealt with generalities. They discussed the galaxy as a whole, and it was no part of their work to examine into the details of the stellar scheme. The work they took in hand to do they accomplished with marvellous success, insomuch that they have left little for others to achieve in the same direction. But it would be a mistake to renew the Herschelian mode of inquiry-to continue to neglect details and consider only the grander features of the galaxy. The work of survey has been completed. In examining, part by part, the field which has been plotted out for us, we must adopt new principles for our guidance. To deal, now, with generalities alone, as the Herschels did, would be to destroy those scarcely recognisable indications which can alone guide us to new knowledge. We must in future examine the sidereal scheme detail by detail, feature by feature. The work will not be light, and many workers will be wanted. But the result will be worth the toil. Not in our day, perhaps not for many generations, may the fruits of such labours be reaped. But gradually astronomy will gather in her harvest, and when it is garnered, the rich reward of many years of toil will be found in a clear knowledge of the relations presented by the wondrous galaxy to which our Sun belongs.

From the Student for October 1870.

ARE THERE ANY FIXED STARS?

DURING the last few years astronomers have been attacking questions which seem, at first sight, far beyond the range of the human intellect, or of the instrumental appliances which human ingenuity can devise. A marked contrast, indeed, is to be distinguished between the inquiries which have been made within the last decade and the most valuable discoveries of all previous times. Not one of the results which had rewarded the labours of scientific men up to the middle of the present century would have seemed incredible to Francis Bacon had it been predicted to him: nay, there is scarcely one of them which is not more or less distinctly shadowed forth in that strange and little read work of his, the 'Sylva Sylvanum.' But even he, daring as were his conceptions and hopeful as were his views of the powers of that method of research which he inculcated, would probably have smiled with contempt had the idea of analysing the Sun or the fixed stars been mooted in his presence. The Frenchman who lately brought before the Imperial Academy at Paris the absurd proposition that our astronomers and physicists should make signals to the inhabitants of Mars and Jupiter scarcely appears a greater dreamer to us than any one would have appeared to Bacon who put forward a notion seemingly so preposterous.

At first sight it may seem to many that the subject I have now chiefly to deal with-the determination, namely,