FOOTNOTES:

FOOTNOTES:[85]Keeler,Lick Publications, vol. iii., p. 214.[86]Rosse,Transactions Royal Dublin Society, vol. ii., p. 93.[87]First detected as such by Holden and Schaeberle in 1888,Monthly Notices, vol. xlviii., p. 388.[88]Deslandres,Bulletin Astronomique, February, 1900.[89]Astronomical Journal, Nos. 539, 547.[90]Maunder,Knowledge, vol. xix., p. 39.[91]Dr. Max Wolf places the point of nebular concentration in R.A. 12h53m, D. +61° 20´, that assigned to the galactic pole being in R.A. 12h49m, D. +62°.Königstuhl Publ., Bd. I., p. 174.[92]T. J. J. See, 'Repulsive Forces in Nature,'Popular Astronomy, No. 100, December, 1902.[93]The Earth's Beginnings, pp. 243-247.[94]Cf.Moulton,Astrophysical Journal, vol. xxii., p. 165.[95]Monthly Notices, vol. lx., p. 259.[96]SeeKnowledge, vol. xiv., p. 50.[97]Harvard Annals, vol. xxvi., p. 206.[98]Astronomical Journal, vol. ii., p. 100, 1852.[99]Astrophysical Journal, vol. xii., p. 158.

[85]Keeler,Lick Publications, vol. iii., p. 214.

[86]Rosse,Transactions Royal Dublin Society, vol. ii., p. 93.

[87]First detected as such by Holden and Schaeberle in 1888,Monthly Notices, vol. xlviii., p. 388.

[88]Deslandres,Bulletin Astronomique, February, 1900.

[89]Astronomical Journal, Nos. 539, 547.

[90]Maunder,Knowledge, vol. xix., p. 39.

[91]Dr. Max Wolf places the point of nebular concentration in R.A. 12h53m, D. +61° 20´, that assigned to the galactic pole being in R.A. 12h49m, D. +62°.Königstuhl Publ., Bd. I., p. 174.

[92]T. J. J. See, 'Repulsive Forces in Nature,'Popular Astronomy, No. 100, December, 1902.

[93]The Earth's Beginnings, pp. 243-247.

[94]Cf.Moulton,Astrophysical Journal, vol. xxii., p. 165.

[95]Monthly Notices, vol. lx., p. 259.

[96]SeeKnowledge, vol. xiv., p. 50.

[97]Harvard Annals, vol. xxvi., p. 206.

[98]Astronomical Journal, vol. ii., p. 100, 1852.

[99]Astrophysical Journal, vol. xii., p. 158.

THE PROCESSION OF SUNS

Phenomenaare functions of time, and the form of the function has to be determined in each particular case. That is what the historical method comes to, and its use is prevalent and almost compulsory. We can no longer be satisfied with a simple bird's-eye view of the universe; our thoughts are irresistibly driven to grope into its past, and to divine its future. Statical conceptions sufficed for our intellectual forefathers. They aimed at establishing the equilibrium of things, while we see them in a never-ending flux. One aspect of them calls up the next, and that another, and so onad infinitum; we cannot, if we would, balance our ideas on the pivot of the transient present.

The immutable heavens of the ancientsstrike us to-day as the invention of a strange race of beings. We see them, on the contrary, with Shelley as a 'frail and fading sphere,' a 'brief expanse,' the seat and scene of change. The 'fixed' stars long ago broke away from their moorings, and began to flit at large through space. Of late a less obvious, more intimate kind of mobility has been attributed to them. Grooves of individual development have been assigned to them, along which they appear to shift as the tardy ages go by; and since everything that grows must decay, the orbs of heaven, too, incur the doom of mortality. But modern science has done much more than extend to them the dismal philosophy of the phrase, 'Tout passe, tout casse, tout lasse.' The grandiose enterprise has been not unsuccessfully essayed of tracing in detail the progress of sidereal evolution, and of marshalling the vast stellar battalions in order of seniority. This has been rendered feasible by the disclosures of the spectroscope. Apart from their guidance, the track might have been seen by elusive glimpses, but could never have been laid down with any approach to definiteness. Herschel found for it aterminus à quoin nebulæ of various forms, but attempted to pursue it no further. We do not hesitate to run it on, from station to station, right down to theterminus ad quem. Not, it is true, without the perception of outstanding difficulties and insecurities, which yet seem to be outweighed by a certain inevitableness of self-arrangement in the related facts.

The argument from continuity is that mainly relied upon. An unbroken succession of instances is strongly persuasive of actual transition, provided only that a principle of development (so to call it) may reasonably be assumed as influential. A series of mineralogical specimens, however finely differenced, does not suggest the progressive enrichment of one original mass of ore. In the stars, on the other hand, a species of vitality may be said to reside. They are not finished-off products, but spontaneously-acting machines. They are centres of energy, which they dispense gratis, supplying the cost out of their own funds. And the process is not only obviously terminable, but must be accompanied by constitutional alterations, which might be traceable by subtle methods of inquiry. Theyaretraceable, unless we are deceived by illusory appearances.

Secchi's classification of the stars was unwarped by any speculative fancy. It was purely formal; it aimed only at providing distinct compartments for the convenient arrangement of a multitude of differently characterized items of information. Then, by degrees, the closeness of the gradations between one class and the next came to be noticed; partitions melted away; the methodized array showed itself to be in movement; and the bare framework took shape, under the auspices of Zöllner and Vogel, as a cosmic pedigree. The white stars were set forth as the progenitors of yellow, yellow of red stars; and the insensibly progressive reinforcement of the traits of relationship between the successive types went far towards demonstrating some partial, if not a complete, correspondence of the indicated order with the truth of things. It has since been found necessary to divide the first stellar class into helium and Sirian stars; and here, too, essential diversity shades off imperceptibly into likeness approximating to identity. All the groups hang together; the entire scheme ison an inclined plane of change. Helium stars, as they condense, pass into Sirian, these into solar stars, which finally, reddening through the increase of absorption, exhibit the badge of post-meridional existence in fluted spectra. The finality of the red stage is, indeed, very far from being absolute, but what lies beyond is matter of conjecture.

There are several good reasons for taking helium stars to be the 'youngest' or most primitive of the amazing assemblage that sparkle in the vault of heaven. The first is their affinity with nebulæ. Every star, perceived to be involved in folds or effusions of shining haze, has yielded—if bright enough for profitable examination—a spectrum of helium quality. Further, they are remarkably tenuous bodies. It has been ascertained with approximate certainty, from the investigation of stellar eclipses, that helium stars are commonly, perhaps invariably, of far slighter consistence than the sun. Radiation, however, is maintained by contraction; hence, orbs at the outset of their course must be, on the whole, the most diffuse. A third note of youth is membership of embryo systems, and this isaffixed very markedly to helium stars. One-third certainly, probably one-half of those lately submitted to trial by Professors Frost and Adams proved to have spectroscopic companions. They are pairs believed to have been recently divided by the fission of a single parent-globe. And this is an operation which must, we should suppose, be undergone early, or not at all.

The spectra of helium stars are peculiar and suggestive. Those belonging to Miss Maury's earliest groups—many of them visibly nebulous—bear next to no traces of metallic absorption, showing instead lines of oxygen, of nitrogen, and of hydrogen in all its three series. The conditions, accordingly, needed to produce the 'cosmic' modification of hydrogen are realized in these inchoate bodies. What those conditions actually are we cannot tell, yet it may be confidently surmised that they will prove to be of an electrical nature. Hydrogen resembles the metals in being electro-positive; it collects at the negative pole during the electrolytic decomposition of water. There is, however, an unmistakable tendency in primitive sidereal objects to display absorption rays ofelectro-negative rather than of electro-positive elements. It is conceivable that hydrogen may be capable of altering its behaviour in this respect, and that the molecules radiating the Pickering and Rydberg series, in addition to the more familiar Huggins series, have, in fact, through some corpuscular re-arrangement, assumed the electro-negative quality properly characterizing a non-metallic substance. The association of this form of hydrogen with oxygen and nitrogen in early helium stars would thus be naturally related to the simultaneous quasi-disappearance from them of the spectral badges of metals.

The helium-line most distinctive of this stellar family is situated well up in the blue. It appertains to the same vibrational sequence with D3, which is also represented in Rigel, one of the more 'advanced' Orion stars. In Rigel, too, we meet a fairly prominent magnesium ray, lying below the blue helium emanation, while as yet iron is unapparent. Numerous fine, faint streaks, due to its absorption, only emerge when the Sirian type is fully reached, and they are mostly of the'enhanced' kind. When the spark discharge is substituted for the arc as the source of illumination, certain lines in the resulting spectrum brighten relatively to the others, and these have been distinguished by Sir Norman Lockyer as 'enhanced.' Now, the rule is strikingly prevalent that the absorption rays in white stars are of this class; yet it can no longer be interpreted as indicating for them an excessively high temperature. Rather, it would seem that electrical conditions still imperfectly defined are in question, and their gradual removal or subsidence is, beyond doubt, largely instrumental in bringing about the transition to the solar stage. The effacement of helium-absorption is even more perplexing. No sooner does iron begin to show than it vanishes. There is still a faint trace of its 'blue' line in Vega; none survives in Sirius.

In spectra of the solar type two great bars of violet light are stopped out by calcium; otherwise metallic arc-lines predominate, while those of hydrogen are no longer so powerfully emphasized as in white stars. Moreover, the whiteness of the unveiled Sirian photosphereshas become tinged with yellow owing to the development of a shallow envelope partly impermeable to blue rays. For this reason the comparative extension of their ultra-violet spectra affords, for stars of different types, no secure criterion of relative temperature. Sound in principle, it becomes inapplicable when the unknown factor of general absorption comes into play. The energy-curve of the solar spectrum, as it is, can be determined; the energy-curve of the solar spectrum, as it would be if unaffected by general absorption, has to be constructed from inference. But only photospheres bare to space give comparable results. Hence, there are no valid grounds for asserting that Sirius is hotter than the sun, or the sun than Betelgeux. It may be so, but the evidence at present available is inconclusive. The appearances expounded in this sense may bear quite different meanings.

The reasons for holding that solar mature into Antarian stars are of the same character, and of equal cogency with those tending to prove their own development from luminaries of Sirian type. There is a similar continuityof specimens. They can be ranged one after another in an unbroken series, in which, as we run down the line, primrose shades into orange, and orange into red; general absorption arrests an increasing percentage of the blue radiations, while specific absorption becomes strengthened by dusky channellings of titanium. Carbon stars are less easily located. Dr. Vogel regards them as co-ordinate with the Antarian class. The two varieties of red stars with banded spectra descend, in his opinion, from the common stock exemplified by our sun. Professor Hale also favours this view, some attendant difficulties notwithstanding. His photographs have certainly established for carbon stars links of relationship both with the Antarian and the solar families; yet the fact remains indisputable that the carbon type is, to a great extent, isolated from all the rest. Tokens of a genuine migration towards it are few and obscure.

The ultimate fate of both tribes of red stars can only be conjectured. Most of the objects constituting them vary in brightness, some to the verge of periodical extinction; and variability may be a symptom of interior dilapidation. But the organization of such bodies is profoundly enigmatical. They are exceptionally remote, and offer slight holding-ground for inquiry. No indications have been gathered as to their density or intrinsic light-power. Very little is known about their movements. They rarely form binary combinations, and those that they do form are almost always relatively fixed. No red star travels in a computed orbit; only one, η Geminorum, occurs on the long list of spectroscopic binaries. The revolutions of this curious system ought to prove, when thoroughly investigated, replete with interest and instruction.

Coupled stars present special opportunities to students of cosmogony. They are obviously contemporaries; they have started fair in the evolutionary race; identical influences have acted upon them; hence, differences in their standing can only result from dissimilarities in mass or composition. It is commonly taken for granted that a body containing less matter than its fellow must develop faster, and incur the final quenching sooner. But Sir William and Lady Huggins have adverted to the probability of the very opposite being the case.Powerful surface-gravity may, they consider, serve to hasten the transition from a Sirian to a solar spectrum; and we should then have giant suns like Capella, advanced in type while at a very early stage of condensation. This perhaps explains the remarkable spectral relations of contrasted stellar pairs. Always, so far as we yet know, the Sirian spectrum is yielded by the lesser star, the mass of which, judging by analogy, must be even smaller than would be indicated by the proportion of its faintness. It is true that the distribution of mass in binary systems is often widely different from what might have been anticipated. Certain purplish satellites, for instance, of undetermined spectral quality exercise a gravitative sway of surprising force. Some results of this kind lately obtained by Mr. Lewis and others are likely to prove of fundamental importance to theories of stellar evolution.

What we know of 'dark stars' has been mainly derived from the observation of stellar systems. They are assumed to be the denizens of a stellar Hades, dim wanderers amid the shades, who 'have had their day, and ceased tobe' as suns. In the 'cold obstruction' of these viewless orbs the grand cosmical procession is held to terminate. Their presence attests the downward progress of decay, and gives logical completeness to the argument for development. Yet there are circumstances warning us against too full an assurance that their status is really that of skeletons at the feast of light. They are very frequently found to be in close attendance upon brilliant white stars. Thus intimately, if incongruously, coupled, they circulate and compel circulation in brief periods, as members of systems just, it might be said, out of the shell. What are we to think, for instance, of the obscure body spectroscopically discovered to control the revolutions of the chief star in the Orion trapezium? It is evidently comparable in mass with that imperfectly condensed luminary. Is it credible that it has already traversed all the stages of stellar existence, and cooled down to planetary rank? So violent an assumption should at least not be made without due consideration; and we may more prudently hold our judgment in suspense as to whether globes so circumstanced—and they abound—should be regarded as effete, or as abortive suns.[100]

Speculations on the exhaustion of stellar vitality have lately become inextricably involved with the complex problem of elemental evolution. A dim inkling has been acquired of the activity in the universe of obscure forces, availing, we can just see, to falsify many forecasts. The theory, among others, of the dissipation of energy needs to be revised or qualified. Nor was it propounded by Lord Kelvin with dogmatic certainty. He carefully noted the possibility that in 'the great storehouses of creation' reserves of energy might be provided by which the losses incurred through radiation could be, wholly or in part, made good.[101]The anticipated possibility is perhaps realized in the phenomena of radio-activity. But if we inquire how, we are met at the threshold by difficulties connected with the origin of helium. Heliumappears to result from the disintegration of radium, its generation being accompanied by the setting free of enormous quantities of energy. Its copious presence, then, argues long-continued and lavish expenditure of heat and light. Yet it is as a constituent of highly primitive orbs that it is chiefly conspicuous. Gaseous nebulæ, too, include immeasurable supplies of it, while it is incompatible with whatever we seem to know about them to suppose that radium at any time entered into their composition.

The genesis of the elements has, in truth, not yet been made the subject of coherent speculation. Current ideas regarding it imply a double course of change, by aggregation first, and subsequently by disintegration. And this should give us a twofold series of elements. On one side there should be fixed survivals from the advancing process, on the other, products of decomposition, continuously evolved, and even now accumulating. If the claim of helium to take rank among these last should be finally established, our conceptions of the nature and history of nebulæ might have to undergo a strange inversion; but the outcomeof the researches in progress is still uncertain, and may be far off.

It is, nevertheless, quite clear that the electronic theory of matter supplies no genuine explanation of the source of energy in the universe. What is given out when the atoms go to pieces must have been stored up when they were put together. Whence was it derived? This is the fundamental question which underlies every discussion concerning the maintenance of the life of suns. It is unanswered, and probably unanswerable.

FOOTNOTES:[100]It must at the same time be borne in mind that their total darkness is not proved. All that is certain is that their spectra are not bright enough to leave any impression on the exposed plates.[101]Thomson and Tait,Natural Philosophy, Appendix E, p. 494, edition 1890.

[100]It must at the same time be borne in mind that their total darkness is not proved. All that is certain is that their spectra are not bright enough to leave any impression on the exposed plates.

[101]Thomson and Tait,Natural Philosophy, Appendix E, p. 494, edition 1890.

OUR OWN SYSTEM

Oursun is clearly middle-aged. It bears none of the marks associated with juvenility in stars, while its decrepitude is in the distant future. It is crossing, most likely, a level tract where recuperation so nearly balances expenditure that radiation can be maintained for an indefinite time at a high and fairly uniform standard. Stars of the solar type pursue the even tenor of their way with particularly few interruptions. They show little tendency to intrinsic variability. Their periodicity, when it exists, is due to the presence of a companion. Variables, in other words, belonging to the spectral family of our sun, are binary systems; and they are usually, if not always, non-eclipsing binaries, on the pattern of δ Cephei. Light changes can thus be impressedupon sunlike stars by external influence; they do not conspicuously arise through native instability.

Our planet, accordingly, is attached to a safe and steady luminary, one subject, not to destructive spasms, but to vicissitudes so mild as to evade distinct meteorological recognition.[102]It is, moreover, governed by a polity settled on a broad basis of tranquillity and permanence. All this is as it should be. The conditions specified were a pre-requisite to the unfolding of human destinies. Nor can it be confidently asserted that they have been realized anywhere else. Our system may be unique; while, on the other hand, replicas of it might, imperceptibly to us, be profusely scattered through the wide realms of space. It is certain that a telescopic observer on Sirius or α Centauri would see our sun unattended; not even Jupiter could be brought into view byoptical appliances in any degree comparable to those at our disposal.

There are, nevertheless, strict limitations to the possible diffusion of planetary worlds like those that wander amid the zodiacal constellations. We have become aware of incapacitating circumstances, by which a multitude of stars are precluded from maintaining retinues of subordinate globes. Spectroscopic discoveries have compelled a revision of ideas as to cosmical arrangements. Especially the large proportion established by them of binary to single stars makes it impossible any longer to regard the solar system as a pattern copied at large throughout the sidereal domain. We cannot, then, compare it with any other; the mechanism of which the earth forms part must, perforce, be studied in itself and by itself, and it may, for aught that appears, be the outcome of special and peculiar design.

The machine in question is self-sustaining and self-regulating; no extraneous power noticeably affects its working. This immunity from disturbance is the fortunate consequence of its isolation. A great void surrounds it.The span of Neptune's orbit is but a hand-breadth compared with the tremendous unoccupied gulf outside—unoccupied, that is to say, by bodies of substantial mass. The feebleness of starlight relatively to sunlight affords some kind of measure of the impotence of stellar attractions to compete with the over-ruling gravitational power that sways the planetary circulation. This it is which gives to it such remarkable stability. The incomparable superiority of the sun over his dependent orbs not only safeguards them against foreign interference, but reduces to insignificance their mutual perturbations. Hence the strong concentration of force exemplified in our system—the absolutely despotic nature of the authority exercised—makes for a settled order by excluding subversive change.

The organization of the solar kingdom, as disclosed by modern research, is greatly more varied and complex than Laplace took it to be. His genetic scheme was, indeed, no sooner promulgated than deviations from the regularity and unanimity of movement upon which it was based began to assert their inconvenient reality. They have since multiplied; and,emerging to notice under the most unlikely aspects, they occasion incongruities which tax, for their explanation, all the resources and audacities of the most inventive cosmogonists. Let us briefly consider their nature.

The swarm of asteroids that bridge the gap between Mars and Jupiter revolve, it is true, with the general swirl of planetary movement; but they use a large license as regards the shape and lie of their orbits. And their partial exemption from the rules of the road becomes entire for comets and meteors, which have nevertheless proved themselves to be aboriginal in our system by their full participation in its proper motion. Finally, several of the major planets set convention at defiance in the arrangement of their several households, and thereby intimate departures from the supposed normal course of development so frequent and so considerable as to shake belief even in its qualified prevalence. Thus, the anomalously short period of the inner satellite of Mars, besides throwing doubt over its own mode of origin, tends to obscure the history of its more sedately circulating associate. Deimos cannot have been thrown off from its primaryunder conditions materially different from those attending the birth of Phobos.

The sub-systems of Uranus and Neptune exhibit, moreover, eddies of retrograde movement suggesting primitive disturbances of a fundamental kind; while the surprising disclosures connected with Saturn's firstborn and furthest satellite, photographically detected by Professor W. H. Pickering in 1898, have added one more knotted thread to the tangled skein we would fain unravel. Until acquaintance was made with Phœbe, counterflows of revolution within the same satellite-family were unknown, and, if contemplated at all, would have been scouted as impossible. One ternary star, to be sure—ξ Scorpii—had been recognised as probably owning an immediate and a more remote attendant, in oppositely directed orbital movement;[103]but the cases are in many ways disparate, and the analogy, though instructive, is imperfect.

If the ninth Saturnian moon is to be regarded as sprung from the condensing mass of the planet, a total change in the state of the parent body must have supervened during thelong interval between its separation and that of its successor, Iapetus. The change, in Professor W. H. Pickering's opinion,[104]was nothing less than a reversal of axial movement. The nebulous spheroid destined to develop into the wonderful Saturnian system had, presumably, when Phœbe became detached from it, a diameter of sixteen million miles, and gyrated tranquilly from east to west, in a period of about a year and a half. But the action of sun-raised tides availed first to destroy and finally to invert this movement; for the natural outcome of tidal friction is synchronism, and this implies agreement, both in period and direction, between the rotation and revolution of the body acted upon. Acceleration through contraction did the rest; and by the time another satellite was ready to separate, the originating globe span normally in seventy-nine days, the actual revolutionary period of Iapetus. The view that such was the course of events is plausible at first sight; yet the doubt remains whether the cause alleged wasadequate to the effect produced. At the distance of Saturn, solar tidal friction exerts only about 1/20000 its power on the earth;[105]its efficacy would, on the other hand, be greatly enhanced by the distension of the mass subjected to it; but approximately to what extent, our powers of calculation are impotent to determine.

This is not all. Exhaustive photographic research promises to unfold intricacies of construction in secondary systems demanding the patient industry of many generations for their complete unravelment. The families of the great planets will perhaps be found to include crowds of inferior members which pay slight heed in their circulatory arrangements to the trammels of convention. In those of both Jupiter and Saturn the phenomenon has lately been brought to light of 'asteroidal' satellites, as they may be termed, minute bodies travelling round their primaries at nearly the same mean distances, each group evidently representing the unagglomerated materials of a single full-sized satellite. The pigmy components of such groups doubtless exist in multitudes; each great planet, most likely, is encompassed by at least one zone of moonlets; but so far only specimen-objects have been picked up. The tenth Saturnian satellite, discovered, like its predecessor, by Professor W. H. Pickering, is thus associated, by its period and locality, with Hyperion, the seventh and least prominent of Saturn's visual train, the apparent insignificance of which suggested to Sir John Herschel that it might have many co-occupants of the wide gap between Titan and Iapetus.[106]But the surmise had to await verification until methods were intensified beyond what seemed possible in the middle of the nineteenth century.

The corresponding Jovian pair found by Professor Perrine circulate far outside the boundaries of the original Galilean realm, in orbits which interlock as a consequence of their marked difference in eccentricity.[107]They are mutually inclined at an angle of 27 degrees, nor are they supposed actually to intersect, sothat collisions are apparently out of the question. Direct movement is indicated, but cannot yet be claimed to belong quite certainly to both objects. We are only beginning to make acquaintance with the submerged populations of the Saturnian and Jovian kingdoms; they are perhaps multitudinous; they are certainly peculiar, and we await impatiently and curiously the further developments of their remarkable behaviour.

The one certain inference derivable from the diversity of facts ascertained within the last hundred years is that our world is not (so to speak) machine-made. Themodus operandiemployed to disengage the planets from their nebulous matrix was not of cast-iron rigidity; it was adaptable to circumstances; it left room for the display of boundless inventiveness in details. This was made, nevertheless, to consist with the perfect preservation of the main order, both in design and operation. The general plan is broadly laid down and unmistakable; the springs of the machine are undisturbed in their free play, and for the primary reason that departures from regularity, which might, in any way, prove a menace tostability, affect bodies of negligible mass. The great swing of settled movement goes on irrespectively of them. 'De minimis non curat lex.'

So the erratic procedure of comets is harmless only because of their insignificance. If imitated by substantially attractive masses, it could not fail to jeopardize the planetary adjustments. Even the asteroids would be unsafe neighbours but for their impotence; and it is remarkable that Mercury, by far the smallest of the major planets, circulates along a track of the asteroidal type. It would seem as if an important size carried with it an obligation to revolve in an orbit of small eccentricity, inclined at a low angle to the principal plane of the system. The reason why this should be so is not obvious; but were it otherwise the equilibrium, now so firmly established, would subsist precariously, or not at all.

The assertion, indeed, that it is firmly established can only be made under reserve. We are ignorant of any causes tending towards its overthrow; yet they may supervene, or be already imperceptibly active. One such lurking possibility is the presence of a resisting mediumin interplanetary space. Waifs and strays of matter must, beyond doubt, be encountered there—outlawed molecules, self expelled from the gaseous envelopes of feeble globes; thin remnants of cometary paraphernalia, driven off amid the fugitive splendours of perihelion; products of ionic dissociation set flying by the impact of ultra-violet light—and all disseminated through an ethereal ocean, which 'is cut away before and closes from behind' as moving bodies traverse it. That its indifference is shared by ordinary material substances, when in the last stage of attenuation, is a plausible but unverified conjecture. It is only safe to say that retardation of velocity in what may pass for empty space is insensible or null.

There may, nevertheless, be springs of decadence in the solar system. Some of them have been discussed by M. Poincaré,[108]whose confidence in the reassuring demonstrations of Laplace and Lagrange is inversely proportional to the magnitude of the terms they were forced to neglect. They dealt with fictitious globes, devoid of appreciable dimensions, andswayed by the strict Newtonian law. But the real planets and their satellites are acted on by other forces as well, frictional, magnetic, radio-repulsive, the joint effects of which may not be wholly evanescent. The tidal drag on rotation undoubtedly occasions a small but irretrievable loss of energy. The moon, for instance, as M. Poincaré states, now gains, by the reactive consequences of tidal friction in widening its orbit, no more than 1/28 thevis vivaof which the earth is deprived by the infinitesimal slowing down of its rotation; and the remaining 27/28, being dissipated abroad as heat, are finally abstracted from the system.

The ultimate state, we are told, towards which the planetary mechanism tends is that of the synchronous revolution, in a period of about twelve years, of all its members. This might, apart from the possibility of a resisting medium, have indefinite permanence; otherwise precipitation to the centre would gradually ensue, and one solitary sphere, cold, stark, and unilluminated, would replace the radiant orb of our cerulean skies, with its diversified and exquisitely poised cortège. Unsecured drafts upon futurity, however, are not among themost valuable assets of science, and a consummation so immeasurably remote may be anticipated by a score of unforeseen contingencies. What can be and has been ascertained is the relative durability of the scheme with which the visible destinies of the human race are so closely connected. It will unquestionably last long enough for their accomplishment. Curiosity that would seek to pierce the ulterior darkness is likely to remain ungratified.

But there is a further outlook. Other and incalculable items remain to be taken into account. The sun, although an autocrat within his own dominion, is himself subject to external influences. As a star, he is compelled to follow whithersoever the combined attractions of his fellow-stars draw him; nor can we thoroughly interpret the summons which he obeys. The immediate upshot in the transport of the solar system towards the constellation Lyra has, it is true, been determined, but the eventual scope and purpose of the journey remain profoundly obscure. The pace is to be reckoned as leisurely: twelve miles a second is little more than half the average stellar speed.We should, however, probably suffer no inconvenience from being whirled through the ether in the train of such a stellar thunderbolt as Arcturus. Only the excessive velocities of any adventitious bodies we might happen to pick up would betray to ordinary experience the fact of our own swift progress. As it is, our sweepings from space appear to be scanty.

If shreds from inchoate worlds, or dust of crumbled worlds, strewed the path of our system, they should be annexed by it in its passage, temporarily or completely, and we should then expect to find the apex of the sun's way marked, if no otherwise, by the predominant inflow from that quarter of comets and meteors. Yet there is no trace of such a preference in the distribution of their orbits. Hence the enforced conclusion that the sun has attached to him, besides the members of his immediate household, an indefinite crowd of distant retainers, which, by their attendance upon his march, claim with him original corporate unity. To this rule there may be a few exceptions. An occasional aerolite probably enters the earth's atmosphere with hyperbolic velocity, and takes rank accordingly as,in the strictest sense, a foreign intruder; but the broad truth can scarcely be challenged that the sun travels through a virtual void.

We can, however, see no necessity why he should for ever continue to do so. Widely different conditions seem to prevail near the centre and out towards the circumference of the sidereal world. What may be designated the interior vacuity of the Milky Way is occupied mainly by stars of the solar type, including one to our apprehension super-eminent over the rest; they are separated by vast, apparently clear intervals; they are non-nebulous, and of stable constitution. This secure habitat is ours for the present, although it may at some future time be exchanged for one less exempt from disturbance. The shape and size of the sun's orbit are utterly unknown; the changes of environment, accordingly, that will accompany the description of it defy conjecture. Our actual course is inclined at a small angle to the plane of the Milky Way. It will presumably become deflected, but perhaps not sufficiently to keep our system clear of entanglement with the galactic star-throngs. In our ignorance of their composition no forecast of the results can be attempted: they are uncertain and exorbitantly remote. Moreover, the comparative slowness of the sun's motion in a manner guarantees the permanence of his subsisting cosmical relations. For anything that science can tell, they may ultimately be subverted by some preordained catastrophe; but the possibility lies outside the sphere of rational forecast.

The universe, as reflected in the mind of man, gains extent as the mirror acquires polish. Early astronomers conceived of but one solar system and one 'dædal earth,' upon which the 'pale populace of heaven' rained influences sinister or propitious. Later, human egotism took another form. The whole universe was assimilated to our particular little settlement in it. Terrestrial conditions were universalized. None divergent from them were counted admissible or profitable. But one answer seemed possible to the perpetualCui bono?with which restless thought assailed the heavens. But one purpose was regarded as worthy of fulfilment, that of multiplying, in distant sidereal climes, copies of our own planet, and of providing suitable locations for myriads ofintellectual beings, as little alien to ourselves as might be compatible with the minimum of diversity in their material surroundings.

The spread of this astral philanthropy has been in some measure checked by the advance of knowledge. Our position and circumstances have been shown by it to be, if not quite peculiar, at any rate very far from inevitable. It has reduced, by a process of exclusions, to a relatively limited number the class of stars that can fairly be regarded as possible centres of vitality; it has immensely widened the scope of discernible variety in cosmical arrangements, and held out warnings against errors of exposition due to inborn prejudices. And we shall surely not wander from the truth by recognising our inability to penetrate all the depths and complexities of Infinite Design.

FOOTNOTES:[102]The uncertainty affecting the best attainable results in weather-cycle investigation is rendered strikingly apparent by a comparison of the able and laborious papers by H. W. Clough (Astrophysical Journal, vol. xxii., p. 42), and C. Easton (Petermann'sGeogr. Mittheilungen, 1905, Heft VIII., andProceedings Amsterdam Academy of Sciences, June 24, 1905).[103]R. T. A. Innes,Reference Catalogue, p. 155A.[104]Harvard Annals, vol. liii., p. 61, where, however, the reversal is explained by a shifting of the plane of rotation.[105]G. H. Darwin,Philosophical Transactions, vol. clxxii., p. 526; Moulton,Astrophysical Journal, vol. xi., p. 110.[106]Monthly Notices, vol. ix., p. 91.[107]F. E. Ross,Lick Bulletin, No. 82.[108]Annuaire du Bureau des Longitudes, 1898.

[102]The uncertainty affecting the best attainable results in weather-cycle investigation is rendered strikingly apparent by a comparison of the able and laborious papers by H. W. Clough (Astrophysical Journal, vol. xxii., p. 42), and C. Easton (Petermann'sGeogr. Mittheilungen, 1905, Heft VIII., andProceedings Amsterdam Academy of Sciences, June 24, 1905).

[103]R. T. A. Innes,Reference Catalogue, p. 155A.

[104]Harvard Annals, vol. liii., p. 61, where, however, the reversal is explained by a shifting of the plane of rotation.

[105]G. H. Darwin,Philosophical Transactions, vol. clxxii., p. 526; Moulton,Astrophysical Journal, vol. xi., p. 110.

[106]Monthly Notices, vol. ix., p. 91.

[107]F. E. Ross,Lick Bulletin, No. 82.

[108]Annuaire du Bureau des Longitudes, 1898.

REMNANTS AND SURVIVALS

Ifthe sun and planets were, in sober truth, wrought into their present shape out of a primordial nebula, the comparatively void surrounding space should naturally be strewn with fragments of unappropriated material. For the process of englobement could hardly, one would think, be carried out with such neatness and precision as to leave no shreds or shavings lying about the great atelier. Residual stuff there must be, unless our preconceived ideas are grossly erroneous; nor have we far to look in order to find it. We find it, apparently, under two forms presenting curious dissimilarities, yet belonging fundamentally, we can scarcely doubt, to the same order of things. These two kinds of waste product may be identified in the innumerable army ofcomets and in the strange, pale cone of the zodiacal light.

One of the most important and secure additions to knowledge in the department of cosmogony made during the nineteenth century was the establishment of comets in a position of entire, perennial, and aboriginal dependence upon the sun. That is to say, a vast majority, if not the whole of them, attend him on his sidereal journey. They are, accordingly, and have immemorially been his clients, and they can lose that status only through the effects of violent disturbance compelling them to depart irrevocably from their closed orbits along hyperbolic tracks. A trifling leakage of comets from our system is thus possible, which may or may not be compensated by annexations of adventitious members of the class, similarly banished from the precincts of remote stars. But this is a secondary consideration; the essential point to be borne in mind is that comets are native-born subjects of the sun, that they make an integral part of his cortège, that they own the same substantial origin, are dominated by his power, and must share his fortunes. Their study should then provestrongly illuminative as to the pre-history of our system, and for this especial reason, that they seemingly belong by right to that vanished world which it is the chosen task of cosmogony to reconstruct. They are, we can infer, the genuine primitives of the solar company; they retain something of prairie wildness, not having been broken in by steadily enforced gravitational discipline. Each perihelion passage is an adventure; between it and the next, fateful incidents may occur. Forces negligible on dense planetary globes act sensibly on their tenuous materials; they in part strikingly illustrate, and in part fantastically invert, the common modes of natural procedure. But it is their antiquarian significance that mainly concerns us here.

Admitting for the inchoate solar nebula such a constitution as that devised by Kant, and adopted with amendments by M. du Ligondès, we find ourselves confronted with the almost inevitable consequence of symptomatic survivals. Wisps of crude matter, in other words, which escaped being drawn into the vortices of embryo planets should continue to circulate, as they had from the first circulated, in all possible planes, and with no partiality for either a right-handed or a left-handed direction. These waifs and wastrels should, in fact, be indistinguishable from comets—'les seuls témoins,' according to the French cosmogonist, 'qui nous restent sur le mode de la circulation première.'[109]The identification is seductive to the imagination, and does not fall far short of convincing the reason.

There is clear evidence that what we may venture to call the native mode of cometary circulation is absolutely exempt from the rules which impress the movements of the planets with an unequivocal stamp of congruity. The few comets showing some degree of compliance with the general plan are those which have been subjected to manifold perturbations, and can hence no longer be called as unbiassed witnesses; while their untrained associates, left relatively free to follow the impulsion of their start, betray no geometrical preferences in their manner of travelling. They revolve indifferently with or against the courseof the signs; their paths are inclined at every possible angle to the ecliptic; they approach the sun in sensibly equal numbers from all quarters of the sky; they agree only in pursuing ellipses so elongated as to verge towards the parabolic limit. But just in this way, and no otherwise, we should expect to find bodies circulating which, having been aggregated at random (as Kant supposed) in the beginning, had departed to the least possible extent from the initial conditions of their systemic union. A goodprimâ faciecase can, then, be made out for regarding comets as samples of the used-up nebula, as superannuated constituents of an inconceivable chaos, which, evading the operation of laws of change, have floated down the stream of ages virtually intact and undisturbed.

Yet the question has other aspects besides this purely mechanical one. They should all be harmonized by truth, which cannot be more securely guaranteed than by consilient testimony; nevertheless, there are difficulties in effecting the accommodation. Comets are not, in a chemical sense, closely related to nebulæ. They are fundamentally of carbonaceous composition—free hydrogen makes no spectroscopic show in them—while they include metallic ingredients occasionally rendered glowing by the powerful excitement of a perihelion rush-past. But gaseous nebulæ shine mainly with the light of certain unknown substances, reinforced by rays of hydrogen and helium. Carbon flutings and metallic lines are alike alien to their spectra. Nor is there any community that we yet know of between the chemistry of white nebulæ and that of comets. The nebular hypothesis of cometary origin is thus discountenanced by the results of light-analysis. Still, there are possibilities of reconcilement. Spectral conditions must be subject to change. The quality of light emitted by a body of mixed composition cannot fail to alter with the inevitable alteration of physical state brought about by external influences or internal change.

Selective illumination is beyond doubt largely concerned in modifying the information we are able to obtain as to the composition of remote masses, and its modes of action seem capricious because they are very imperfectly understood. Hence, spectral modifications maytake place merely through the substitution of some elements for others in carrying (let us suppose) an electric discharge, though all were from the first simultaneously present in unvaried proportions. Moreover, chemical immutability can no longer be taken for granted. We have learned of late that even elementary individuality breaks down under the battering-ram of time. Sooner or later the stamp, however apparently inviolable, will be defaced, transformations of species will ensue, and novel combinations of material will subtly accommodate themselves to the needs of a growing world. These things, it is true, are involved in much obscurity, but we have caught glimpses of instability clear enough to convey an emphatic warning against dogmatic interpretations of spectral characters. Physical science may then license M. du Ligondès' theory of comets with a provisionalNihil obstat.

The zodiacal light suggests a different set of considerations. Comets being of pre-planetary origin, the ecliptical glow must be supposed post-planetary. It belongs to a later epoch, being composed, according to an acceptedopinion,[110]of superfluous materials left over from the construction of the train of globes to which our own belongs. It might be compared to whey from which the curd has been separated. All the good has been got out of it; we might be tempted to throw it aside upon the rising rubbish-heap of the skies, with the importunate asteroidal throng, a few dozen undistinguished comets, and some hundreds of ill-defined meteoric systems. But celestial refuse is always worth sifting, above all, for tokens of genealogical descent, and we should be unwise to neglect the chance of finding them in the peculiar relations of the zodiacal light.

A triple phenomenon, it consists, when completely seen, of a cone, a band, and a counterglow. The connection of these parts into a whole is obvious, though enigmatical. Usually, however, only the cone is visible. It appears about the time of the spring and autumn equinoxes, after sunset and before sunrise respectively, as a faint lenticular illumination, tapering upward from the sun's place belowthe horizon to an apex high up near the meridian. Under the name of the 'False Dawn' it was familiar, probably from an early age, to Oriental peoples. But they looked for it at the opposite end of the night from that favoured by European observers; nor did the phenomenon attract any particular notice here in England until 1660, when Joshua Childrey published a description of it in hisBritannia Baconica. Yet it had been specifically observed about seventy years previously by Christoph Rothmann of Hesse, and must have been less intelligently perceived by numberless spectators, who most likely included it, with such miscellaneous objects as comets' tails, auroral beams, and meteor-trails, in the undefined class of appearances known from of old astrabes.

The light is ordinarily much feebler than the Milky Way, which it nevertheless on occasions unmistakably outshines.[111]Real fluctuations of brightness seem implicated in thesechanges; yet they follow no traceable law of periodicity, and are certainly independent of the sunspot cycle.

The counterglow, first remarked by Pezénas in 1730,[112]soon fell into oblivion, and had to be rediscovered after six-score years by Brorsen, who bestowed upon it its current title of the 'Gegenschein.' Of late it has been pretty constantly observed, particularly by Professor Barnard, to whom it presented itself, owing to the scantiness of the available records, as a surprising novelty.[113]Surprising it certainly is. The appearance of the Gegenschein is that of a large elliptical patch of diffuse light, measuring about 12 by 9 degrees, and situated diametrically opposite to the sun.[114]Now and again, though somewhat rarely, it is perceived to be united to the cone by the 'zodiacal band,' a strip of evanescent luminosity nearly following the line of the ecliptic. We cannot, then, be mistaken in recognising the great pyramidal beamcentred on the sun, with the counterglow and its linking band, as sections of a single formation, constituting in a manner the substratum of the solar system. A recent observation made by Professor Newcomb under unique conditions proves it to be much less exclusively 'zodiacal' than had been supposed. Looking north from the summit of the Rothhorn, at midnight, on July 29, 1905, he perceived a well-marked glow spreading 35 degrees from the sun's place.[115]It was the light in its thwartwise aspect, which had never before been seen, or even looked for; and we learn from it the remarkable fact that the sun is enclosed in a vast, dimly luminous sphere, with a girth not much smaller than the orbit of Venus, and indefinitely diffused along the equatorial plane.

Notwithstanding its dim indefiniteness, neither the spectroscope nor the camera is wholly ineffective for the scrutiny of this extraordinary appurtenance. We have learned positively that its radiance is of the continuous sort, the origin of which through the reflection of sunlight from small solidbodies seems more than probable. The whole structure must accordingly be of a pulverulent or meteoric nature; it consists of independently moving particles. But to the further question, Under what regimen do these particles circulate? no decisive answer is as yet forthcoming. M. Hansky[116]and others hold the light to be a true solar appendage, an extension of the corona, in which case it would have a formal, but no material permanence. It would represent the continually changing aggregate of multitudinous minute bodies issuing from or repelled by the sun, and in large proportion falling back towards his surface. Yet some difficulty is raised to this view by the vast dimensions of the problematical glow. That it extends far beyond the earth's orbit is rendered patent by the phenomena of the Gegenschein and the band. True, the scope of the sun's repulsive action cannot be limited; still, we might naturally expect its products to become too attenuated for recognition beyond a radius of perhaps fifty million miles.

Admitting, on the other hand, the residual character of the zodiacal light, we should attribute to it a constitution analogous to that of Saturn's rings. Each one of the cosmic atoms collected in it would revolve round the sun on its own account, scarcely disturbed by its neighbours. Nor need we despair of determining with reasonable certainty which way the truth lies in this matter. The rival hypotheses may be tried by a criterion the application of which is by no means remotely feasible. It is furnished by the geometrical relations of the zodiacal light. Evidently, if the sun can claim organic connection with it, its axis should coincide with the plane of the solar equator; while, if it represent wastage from the Kantian nebula, it should stretch along the principal plane of our system—the plane of maximum moment of momentum—the plane towards which the primitive agglomeration of revolving particles collapsed as it condensed. The question of planes is, then, crucial. Is the zodiacal effluence placed symmetrically as regards the solar equator, or does it appertain properly to the ecliptic, which deviates very slightly fromthe fundamental plane of the solar system? The evidence is, unfortunately, contradictory. Most observers have located the dim equinoctial cone right along the pathway of the sun; some, under exceptionally favourable circumstances, have perceived in it a marked departure from the track of the Signs.

M. Marchand's determinations from the Pic du Midi, for instance, indicated a probable coincidence between the solar equatorial plane and the axis of the light;[117]and Dr. Max Wolf succeeded, in 1889, in getting a photographic impression which, though partial and imperfect, tended to corroborate Marchand's inference.[118]Again, on November 16, 1904, when the cone showed a remarkable lustre, it was distinctly perceived at Königstuhl to sheer off and separate from the ecliptic as it mounted the sky. Now, however, that a beginning has been made in photographing this enigmatical tenant of the sphere (the feat has been performed at Flagstaff as well as at Heidelberg), we may confidently expect a speedyreconcilement of inconsistent statements regarding its whereabouts. Until then we cannot venture to assert that it is in actual reality what it appears to be, a nebulous survival.

Back to Index Next