heavenly bodies and systems of bodies after the equilibration of their energies with their surroundings-after, for instance, each sun has exhausted its superior heat, and therefore no longer ceases to part with heat to surrounding matter-will undergo a process of dissolution, thus completing the cycle of its transformations. It is so, we see, on the minor scale in every case with which we can deal. It is so with the individual members of animal and vegetable races, with families of animals and vegetables, with groups of these families, with nations, with social organisations. In every case we see how the life of each aggregate is limited in time, and tends to death, but how, also, after death the parts of the aggregate are dissolved, and become ready to take part in the formation of other aggregates. Hence

14. This rhythm of evolution and dissolution, completing itself during short periods in small aggregates, and in the vast aggregates distributed through space completing itself in periods which are immeasurable by human thought, is—so far as we can see, universal and eternal—each alternating phase of the process predominating now in this region of space and now in that, as local conditions determine.

From the cases considered in the last paragraph, we can proceed with a certain degree of confidence to cases more extended, until we recognise in the solar system (for instance) the evidence of youth, and life, and old age, as stages of evolution,-though our processes and the range of our observation are too limited to enable us to judge (otherwise than from analogy) that after old age and death in these vast physical aggregations there comes a stage of dissolution completing the cycle of transformations. All we can say on that point is that, as in every case we can deal with to the end, we have found dissolution following the state of equilibrium which we call death in the case of the individual and compare to death in other cases, so also it is with those cases which (because of the limited range of our vision) we can only deal with in minute parts. We judge then that planets after this stage of death have a stage also of disso

lution, though no physical experience enables us to say what that stage is like. We pass also to higher orders of being. We see suns of various ages throughout stellar space, and learn to recognise in their case also progression and evolution, up to and beyond the fulness and prime of stellar life. In their case also must come death, with equilibration between their energies and the receptive capacities of matter around them; and after this physical death must come, though in ways we cannot perceive, a process of dissolution, completing the cycle of transformations. So also with higher orders, with systems of suns, with systems of such suns, and so on, absolutely without end.

We come then to the final statements respecting the operations of nature and their significance. After what has been already explained, these need no words of mine to make them clearer. Nor could this paper be better closed than in the very words of this great teacher of our age :—

15. All these phenomena, from their great features even to their minutest details, are necessary results of the persistence of force, under its forms of matter and motion. Given these as distributed through space, and their quantities being unchangeable either by increase or decrease, there inevitably result the continuous redistributions distinguishable as evolution and dissolution, as well as all those special traits above enumerated.

16. That which persists, unchanging in quantity, but ever changing in form under these sensible appearances which the universe presents to us, transcends human knowledge and conception, IS AN UNKNOWN AND UNKNOWABLE POWER, WHICH WE ARE OBLIGED TO RECOGNISE AS WITHOUT LIMIT IN SPACE, AND WITHOUT BEGINNING OR END IN TIME.

A SURVEY OF THE NORTHERN HEAVENS.

THE ideas entertained by the non-scientific public respecting the extent and nature of the researches made by astronomers into the constitution of the heavens, are, for the most part, singularly inaccurate. Many suppose, for instance, that the astronomer knows the distances of at least those stars which are visible to the naked eye, and they hear with surprise that there are not six stars of the millions which astronomy really has to deal with whose distances can be regarded as even approximately determined. It is commonly supposed, again, that the whole surface of the celestial sphere has been so surveyed with powerful telescopes that astronomers know according to what laws the stars are distributed over the heavens. Few, indeed, even among those who may be called students of astronomy are aware how very far this idea is from the truth. We hear so often of the star-gaugings of the Herschels, and the results inferred from those star-gaugings are so confidently insisted upon in our text-books of astronomy, that when the actual extent of the Herschelian gauges is mentioned, the student is apt, in his sense of surprise and disappointment, to undervalue the labours of the Herschels as unduly as he had before exaggerated their extent and importance. Yet another mistake is commonly made. The range of the telescopes employed by astronomers is compared with the range of unaided vision, and men are apt to suppose that

the astronomer obtains the same insight into the constitution of the heavens as though his powers of vision were correspondingly increased. The fact is forgotten that the telescope can only show the astronomer a minute portion of the heavens at a single view; that the information it is capable of supplying is in a sense piecemeal; and that the real lessons taught by the telescope, so far as the distribution of the heavenly bodies is concerned, can only be learned by combining together a number of large-scale views of separate portions of the heavens, into a single comparatively small-scale picture. It is not known that our materials for this work are for the most part incomplete, and that in those few instances where we have complete materials little has been done to utilise them.

I propose in this paper to consider, briefly, the extent of the researches hitherto made into the subject of the constitution of the star-depths; and then to describe some of the conclusions which seem deducible from an inquiry I have recently instituted, with the object of presenting in a single picture the results of one of the noblest series of labours yet undertaken by astronomers: Argelander's complete survey of the northern heavens with a telescope showing stars down to the tenth magnitude. Setting aside surveys limited. to small regions of the heavens, it may be said that the only observational labours yet directed to the solution of the noblest physical problem man can study, are the stargaugings of the Herschels. These star-gaugings constitute in reality but a minute proportion of the work achieved. by these great astronomers. Yet, we may justly say, with Struve, that not one of the feats undertaken by the Herschels surpassed in daring that of attempting to gauge the stardepths with a telescope a foot and a half in aperture. If an astronomer devoted all his observing hours to such work, he would need, on a moderate computation, full eighty years to complete the survey of the heavens.' It is,

1 6

'Supposons que l'on puisse faire, pendant 100 nuits de l'année,

therefore, with no thought of undervaluing the credit due to the Herschels for their star-gaugings, but solely because it is important that true ideas should be held on the subject, that I now refer to the relatively small extent of the heavens actually included in their survey.

Sir William Herschel published, in 1785, the results of no less than 3,400 star-gauges. Each star-gauge gave the number of stars visible in the field of view of Herschel's telescope, this field being equal in extent to almost exactly one-fourth of the apparent dimensions of the moon's disc; and in the course of these surveys-setting aside rough estimates on which Herschel himself laid no stress,—about 90,000 stars were counted.

At first sight we seem to have here a widely extended survey. The portion of the heavens actually gauged was equal to an extent which it would require 800 moons to cover; and thirty times as many stars as can be seen with the naked eye in the darkest and clearest night were actually counted. But when we leave the region of ordinary notions -whether as respects the moon's apparent size or the multitude of the stars and enter into the real particulars, the startling nature of the disproportion between the extent of the heavens and the portion surveyed by Herschel is at once recognised. It is calculable that each field of view surveyed by Sir William Herschel amounted to but the 832,979th part of the celestial sphere. So that, in fact, he had gauged but about the 250th part of the area of the heavens. To obtain a clear idea of the minuteness of this proportion, suppose for a moment that the whole surface of the heavens is represented by an ordinary chess-board; then the combined extent of all the gauge-fields of Sir William Herschel would amount to barely the fourth part of one of the black or white squares of such a board.

chaque nuit 100 jauges, et il ne faudra pas moins de 83 ans pour le jaugeage du ciel entier. En effet, les 3,400 jauges de Herschel forment un de ses travaux les plus hardis.' Struve's Études d'Astronomie stellaire, note 74.