it may appear, in contrast with our enormous planet on the one side and our brief existence on the other, may in the course of time exercise a considerable influence upon the earth's mass. For its operation, this influence has timemillions of years, and the nebulous matter 'scattered,' as Laplace has said, 'with such profusion throughout the universe.'

SECTION IV.

SYSTEMS OF COMETS.

Comets which have or seem to have a common origin-Double comets-Systems of comets according to M. Hoek-Distribution of aphelia over the celestial vault; region of the heavens particularly rich in aphelia.

WHEN, in accordance with the actual facts of science, we endeavour to form an idea of the constitution of the visible universe, we see that the celestial bodies which compose this whole are everywhere distributed into groups and associations united by the common bond of universal gravitation.

There are the planetary systems. In the centre of each group is a star or central sun, whose preponderating mass retains near him, circulating in regular orbits, other stars or planets, to which this central sun distributes heat and light. Our planetary system is the type of associations of this kind.

There are the stellar systems, groups of two, three, or more suns gravitating about one another, probably in accordance with the same laws. These systems are themselves the elements of greater associations, which, like the resolvable nebulæ known under the name of stellar masses, are composed of myriads of suns. The Milky Way is one of the most splendid examples of these immense agglomerations.

In certain regions of the heavens the nebulæ are themselves to all appearance grouped into systems, so that the general plan of the universe is one vast synthesis of associations of

different orders encompassing each other without end. Nor can any individual star escape the necessity of forming a part of one of these groups.

Are there likewise systems of comets?

It is certain, in the first place, that there are some comets which belong to the solar system. Originally strangers, they have become drawn into it by the action of the planetary masses, and have since contributed to form an integral part of the group. We have seen that it is possible for comets, through the effect of perturbations, to escape from the power of the sun's attraction; others, on the contrary, owing to the insignificance of their masses, unable to resist the causes that tend to precipitate them into the focus of their movement, may possibly become blended with the central mass; or perhaps, shattered and scattered throughout the interplanetary spaces by the successive perturbations of the planets, they may constitute a sort of resisting medium, the elements of which in the course of time may be a source of increase to the planetary masses themselves.

Besides, we are already in a position to answer the question. We have seen Biela's comet divide into two; and the twin bodies into which it separated, performing their voyage in concert, may be said to constitute an embryo cometary system. The comet observed by M. Liais in 1860 was an example of another kind, since, if the two comets of which it is formed should withdraw from the sun, and, still maintaining their relative position, should leave the system, they would constitute in space a group of two independent comets.

But are all the other comets-I mean the non-periodical comets which describe parabolas or hyperbolas-are they to be regarded as independent voyagers journeying from one solar system to another, and never staying their vagrant course? Are there not amongst these some which move

in groups and make the circuit of their long orbits in company together?

This question appears capable of direct solution through the researches of a Dutch astronomer, M. Hoek. By comparing and studying the elements of different comets M. Hoek has discovered that several of their number appear to have had a common origin, and that before entering the sphere of the sun's attraction they formed groups or systems, in proof of which he shows that at some former epoch these bodies were near together, and had each an initial movement in the same direction and of the same velocity. Moreover, in his opinion, comets of elliptic or periodic orbits form the exception, the immense majority of comets moving in curves with endless branches. Arriving singly or in groups from the sidereal depths, they enter our system, sent thither by some star from which they have receded so far as to be beyond the preponderance of its attraction, and to fall temporarily under the attraction of our own sun. But in what manner has M. Hoek discovered that certain comets have emanated from the same focus and have probably a common origin?

To solve this difficult question the Dutch astronomer has compared the elements of the comets which are determined with sufficient accuracy to admit of comparison, those-for example, of the comets calculated since 1556. He has determined the positions of their aphelia, collecting first in a separate group the comets whose apparitions were not separated more than ten years, and whose aphelia were included within a circle of about ten degrees diameter. And further, he has investigated whether the orbits of comets thus grouped three and three or in greater numbers have not points of intersection in

common.

Let us, following M. Hoek, take an example, selecting in the first place the comets of 1672, 1677, and 1683, and in

the next place the comets 1860 III., 1863 I., and 1863 VI. The positions of the aphelia of these six comets are as follows:

Now ten degrees of longitude, at a latitude of 73°, represent an angular distance of 33°, so that the differences of longitude measured upon the arc of a great circle are equivalent in each group to a little more than three degrees. This of itself is a remarkable coincidence. But if we investigate the points of intersection of the different orbits a still more surprising coincidence appears, for we find that these points are grouped together in a region of the heavens the extent of which is not more than two degrees in diameter, and which has its centre at about 319° of longitude, and 78° of south latitude. By drawing a straight line joining the sun and y Hydræ we obtain nearly the common intersection of the orbits of the last five comets.

On calculating the distances between the comets and the sun at different epochs in past ages M. Hoek has obtained the results which are given in the following tables, the unit of distance being the mean radius of the terrestrial orbit:—