In drawing to a close this chapter on the most difficult and important problem of astronomy-a chapter which, long as it is, has barely sufficed to convey so much of the history of investigation as seemed essential to my purpose-I would remark that there is no branch of research with which astronomers have better reason to feel satisfied. I cannot consent to speak of recent successes as though they had removed a reproach from astronomy. There is no single part of the history of this problem of which astronomers have not abundant reason to be proud. By long and patient labours they have been able to overcome difficulties which might fairly have been thought insuperable. Availing themselves at every step of the best means they could secure for approximating more and more closely to the truth, they have necessarily had during their advance to pass from ground which they had formerly occupied. But every such change of ground has been an advance towards more complete success. Sir John Herschel has said that the recent correction in the value of the Sun's parallax, corresponds to the apparent breadth of a human hair at 125 feet, or of a sovereign at eight miles off; and that, moreover, the error has been detected and the correction applied; and that the detection and correction have originated with the friends and not with the enemies of science.' But I would go even farther than this, since in place of regarding the recent change as involving the detection and correction of error, I would speak of it rather as

a new approximation, more successful indeed than former ones, but not therefore changing those former successes into defeats. The astronomy of half a century since had as good reason to be proud of Encke's work as the astronomy of the present day has to rejoice at the successes of Hansen, Foucault, Leverrier, and Stone; or as the astronomy of future ages will have to boast of those labours by which the results now accepted will inevitably be improved

upon.

CHAPTER II.

THE SUN AS RULER.

ONE of the most important of the Sun's functions is that by virtue of which he rules the motions of his family of planets. By the exercise of his mighty attractive influence he continually controls the tendency which they have to rush tangentially far out into space beyond the influence of his illuminating, heating, and actinic rays. Their swift orbital motions, combined with the relative stability of their axial pose, result in producing the orderly succession of the seasons. But this succession would come to an end, were it not for the stability of their orbital motion; and this stability is due to the Sun's overmastering attraction. To this it is due that the paths of the planets though undergoing continual processes of variation, yet suffer no sudden changes as respects their distance from him, or, therefore, as respects the period necessary for a complete revolution. Nay, so perfect is the whole scheme of governance that even the processes of slow change take place within limits, and those limits not very wide. Not merely can the orbits of the planets suffer no sudden change, but they can neither suffer a great change nor a permanent change.

We might dwell much farther on the importance of the Sun's influence as the most massive portion of the scheme of which he is the centre. On the one hand we might point to the possibly even vital importance of the action which causes the terrestrial equinoxes to circuit the ecliptic in their grand precessional year of 25,868 solar years, or of the slowly-exerted influence which changes the eccentricity of the Earth's orbit. It is very far from unlikely that but for these influences the Earth would long since have been rendered unfit, through a species of exhaustion, for being the abode of living creatures. But as it is, continents become oceans, and oceans continents, one hemisphere interchanges with another the office of supplying the chief proportion of land surface; activity is followed by rest and rest by activity; and so through countless cycles this globe has been and will continue to be a fit abode for innumerable races. On the other hand, we might dwell on the influence which the Sun's mighty attractive influences exert in gathering in from all sides abundant supplies of motive energy, to recruit it may well be, his seemingly exhaustless stores of heat and light and chemical activity.

But without dwelling further in this place on themes of which some will find a place in other chapters, while others-interesting and fascinating though they be— must yet be regarded as lying outside the range of our subject, let us proceed at once to consider what the Sun's true influence is, by virtue of that principle of gravitation which causes every particle of his mass to

aid in attracting all bodies within the sphere of his influence.

In one sense it may be said that the sphere of the Sun's influence is all space. According to the present conceptions of the power of gravitation, there is no particle of matter throughout the whole universe which does not feel the attractive influence of the Sun (as indeed of every particle of matter). But to all intents and purposes the Sun's reign may be regarded as limited. His influence on the stars is not merely minute so far as the amount of motion it is capable of producing in any given interval is concerned, but it is to be regarded as the influence of a peer among peers, not of a king over his subjects. The results of the mutual attractions of the stars may be, and doubtless are, of the utmost importance, but they do not belong to the history of the Sun as a ruler. On every side, then, the Sun's rule is limited-for in all directions there are stars, and the sphere over which each star rules, is as definite as that governed by the sun, so that in each direction we come upon regions where his influence is subordinate to the influence of some other orb.*

It is sometimes said that a body like a comet can pass from the sway of one star to come permanently under the dominion of our Sun or of another star, and vice versâ; but setting aside the case of interference with such a body through the action of a planet, or by reason of atmospheric resistance near the Sun, or the like, this can never happen. For let us suppose that a comet is passing from the sphere of one star's influence to that of the Sun's. Then it cannot be moving on a closed orbit around the Sun, at this time, for if so it is already subject to the solar dominion, contrary to our supposition. It must then be travelling on a hyperbolic or parabolic orbit around the Sun and so must eventually pass