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that path were verified ; corrected where correction was found to be needful; confirmed where it was not: but these irregularities still remained. Every probable supposition was put forward by which to account for them, as well as one or two very improbable ones: the resistance of the ethereal medium; the disturbing influence of a large satellite attending the planet; imperfection in the operation, or modification in the laws, of the sun's attractive force at such a distance; and, lastly, the action of an undiscovered mass beyond the orb whose irregularities were under discussion. The first would have explained only such a steadfast irregularity as that which it is believed to originate in Encke's comet; and was, from the value of its efficiency as in its case recognised, utterly inadequate for that of Uranus. The existence of the second was negatived by all observation; and the dimensions, the path, and all connected with the assumed satellite, it was found, must be such as were utterly at variance with all elsewhere exhibited. The third was a pure assumption, the extreme improbability of which forced itself at once on every mind. Who first thought of the fourth, it is difficult now to ascertain. It seems, however, to have been first definitely suggested in 1834, by the Rev. Mr. Hussey; but to this we shall hereafter have to refer. On the 23rd September, 1846, M. Leverrier,-a name till then unknown, or almost unknown, in England, but which has now become one for all nations and all times,-communicated to Dr. Galle, of Berlin, the final result of his calculations for this unknown mass; and on the evening of that day it was seen and recognised as a planet for the first time.
We cannot now follow the technical steps of the process which has issued in this noble discovery; a discovery certainly in itself of importance, as adding another denizen of no mean worth to our planetary system; but winning its highest value and beauty in our eyes, as having been one in which theory anticipated and guided direct observation, and the mind in its abstractions outstripped the eye in its far-seeing. The discovery of Uranus was an act of pure observation; that of the five asteroids the fruit of a conjecture as to probability; in the case of Neptune theory fixed the distance, weighed the mass, told the year, defined the place, and then, and not till then, appealed to observation, and demanded, rather than prayed, confirmation from it. Some of the preliminary steps of reasoning by which the sphere of inquiry was circumscribed, and the looser elements of the body determined, are of themselves striking examples of logical clearness and farseeing acumen. The unknown body acts on the mass of Uranus, but exerts no influence on that of Saturn; it is, therefore, to be sought without the orbit of the former, and not between those of the two. Did it lie near the path of the remoter, it would still, if large, disturb that of the nearer also; while, if small, it would act upon the first only when in conjunction with it, or for a short time before and after that phase of nearest mutual approach; whereas the observed perturbations indicate an influence exerted during lengthened and continuous periods: therefore a large mass, located at a considerable distance from the one on which it acts, is to be looked for and estimated. But, if this distance exceed a certain determinate amount, the mass capable, at such a degree of removal, of disturbing to such an extent the motions of Uranus, will also reveal its presence by a smaller amount of disturbance on those of Saturn; therefore it lies within certain limits, both as regards distance and mass. Finally, these disturbances are in the plane of the orbit, not from it, either by ascent or descent; therefore the disturbing orb lies nearly in the same plane, and not greatly either above or below it.
The way was thus so far clear ; the approximative elements were thus rudely ascertained ; and the rest was the work of patience, accuracy, zeal, and faith. And these labours ended in the discoverer's being able to give in minute detail the distance from the sun, the mass, the rate and period of revolution, and the inclination of orbit, of the planet in question, before it had yet been looked for; and to indicate a very small zone of the heavens within which at one particular time it must be found.
The orb thus nobly discovered, is one which has for itself claims on our attention of the most imposing kind. It ranks the third of the secondary bodies of our system, its diameter being estimated at about 50,000 miles, and its mass at 230 times that of the Earth. Its distance from the sun is nearly 2,900,000,000 miles, or rather more than thirty times that of the earth; and the proportion of light and heat it receives from the central orb one nine hundredth (200) that which falls on our world. Its period of revolution round the sun is estimated at 172 of our years; and its mean rate of movement will therefore be little more than 10,000 miles an hour,-in strange contrast to that of the
swift-winged Mercury,' which is 110,000. Whether or not it rotates upon its axis, is not yet definitely known, nor is it likely to be, unless some unwonted and peculiar phenomenon shall indicate the existence of such rotation, for years to come; and many other minor details with regard to the orb, the inclination of its axis, its possession of an atmosphere, &c., remain of course still to be investigated. One observer, Mr. Lassell, reports that he has seen what appears to him to be an attendant satellite ; and likewise a peculiarity in the apparent form of the planet's
disc, which he refers to its being encompassed with a ring or rings like those of Saturn. The latter of these observations has since been confirmed by other astronomers: and we are thus presented with a second example of a phenomenon hitherto regarded as most singular; and the purpose of which the more sordid utilitarianism of our age is wholly baffled to define.
There are two facts, however, connected with the newly-discovered planet,—the one certain, and the other all but certain, which merit particular attention. The first of these is its deviation to a far greater extent than any one of those bodies heretofore known, from what is known as Bode's law of the distances. According to this law—or rather rule, seeing it simply expresses a fact of which no explanation whatever can be given,—the various planets are placed at distances bearing a certain and uniform relation to each other: this proportion being that, the interval between Mercury and Venus being assumed as unity, the intervals between the successive orbs each double upon the one before it. Had the newly-discovered orb conformed to this rule, it would have been found at a distance of 3,600,000,000 miles from the sun. Its actual distance is about seven-ninths of this amount. And such a deviation, important and interesting in itself, as the first example of departure from a rule hitherto found universal, derives additional interest from the fact, that chiefly on it conjectures have already been founded relative to the possible existence of a second unknown orb, situated as much beyond the distance indicated by the law, as the present one falls within it. This conjecture, however, must be left to time to verify. It is more than probable that, if such an orb exist, the means which have guided our telescopes with such unerring aim toward this one, must again be cmployed for its discovery: its disturbing action be watched and waited for; and direct observation, almost powerless at such a distance, be guided and led out by theory toward a mind-seen result.
The second of the two facts we have referred to is one of yet higher interest and importance, and certainly one more unexpected still. It is believed that the planet is self-luminous. This inference has been deduced from its high degree of visibility and great clearness of light, not only as compared, or rather contrasted with Uranus, but beyond what is comprehensible in conformity with the known principles of optics." It is, indeed, conceivable, that the physical organisation of the orb may be such, as shall give to its surface a light-reflective power very far beyond all we have experience of, at least among the other orbs of the system; but it is very questionable whether any amount of this, within the limits of probability, would account for a
planet receiving little more than a third of the sunlight which Uranus receives, nearly equalling it in visibility, and far surpassing it in vividness of light. Here, too, at all events, we are called on to “stand still and see”: to rid the mind of every bias, and of all prejudgment, and to esteem the treasure-house of physical variety still unexhausted, and the phases of physical appearance still not all seen. And should this most unexpected and important fact be hereafter established, we shall then be sented with a startling and striking converse to the fact arrived at by the masterly induction of the lamented Bessel, with regard to the stars Sirius and Procyon—the first, one of the most majestic orbs which our firmament can claim,—that each is associated in binary combination with masses yet mightier than themselves, like our planets opaque and non-luminous; suns of darkness, whose light, if ever they shone, has waned and gone out for ever. And, on the supposition of the planet in question being self-luminous, it becomes an interesting object of inquiry whether, from any adjacent system, our sun can appear with it to constitute a double star. The same distinguished astronomer has succeeded not only in determining the distance from us, but in calculating very closely some of the elements, of a double star in the Swan,—the only one with regard to which we have as yet definite estimate of the mass of the constituent orbs, and their distance from each other. According to him, the radius of their orbit is about three times that of Uranus, and the aggregate of their masses half that of the sun. Their separate masses cannot yet be determined; but they certainly far more nearly approach each other than do those of our primary orb and this his self-luminous attendant, and each is in revolution round their common centre of force,-a phenomenon which cannot be presented by such a system as ours may exhibit. There are other cases, however, in which the two associated orbs are very much more disproportioned as to size-possibly as much so as our sun and his attendant; and in some at least of these, the apparent proximity of the orbs is much greater than that of the system investigated by Bessel. It is possible, then, that should the suspicion regarding Neptune be correct, our solar system may present itself to these as a double star, of which the smaller orb alone is in orbitual revolution; and possible, on the other hand, that some of those we see may be exactly analogous schemes. There are many of them, with regard to which double motion is still undemonstrated, and observation may yet reveal some in which it does not take place. Whenever such shall be made known to us, in which the larger orb is perfectly at rest with regard to the smaller, the probabilities will be very strong, that
in such a system the analogies with our own are yet more definite and extensive; and that a train of opaque planets, even as here, encircle at various distances, and with varying times, the primary orb. For it is due to the mutually counteracting and neutralising influence of these, that our sun exhibits no sensible revolution round their common centre of force; and were he alone with Neptune in the system, he would no longer hold his place of central repose.
The history of this beautiful discovery is, especially to the Englishman, in many respects a very painful record. Through the singular and obstinate faithlessness of an English astronomer-royal in mathematics generally, and specially it would seem in English mathematics and mathematicians, and his apparent ignorance of the true value and power of the tools which should have been very familiar to him; and the yet more extraordinary fact of a Cambridge professor of astronomy having again and again seen the object for which he was appointed to search, and was actually searching; seen it in the place where he was forewarned to expect it; seen it as a marked and conspicuous object; having on his own showing conducted his search for a new orb without a map of the stars at hand; having actually found that an orb not before seen had, in the interval between two examinations, wandered into the zone he was appointed to examine, without apparently a passing thought that this might be the one he was in search of;-by this unfortunate co-operation of singular faithlessness, and carelessness not often paralleled in the history of science, a young English mathematician has been deprived, to a great extent, of the honour of a discovery, which will deservedly immortalise the names connected with it; and with which too those of Mr. Airey and Professor Challis will, at least for a generation or two, be associated in a not very enviable notoriety. We say to a great extent; for the full right of independent, and at least simultaneous discovery, will by every ingenuous mind, French* or English, be awarded to Mr. Adams: of that of prior public announcement, he has been, without fault of his, permanently deprived. The long standing and resolute faithlessness of the astronomer-royal has had its fitting meed of contempt rendered to it by the result; and we can well conceive it is with no very pleasurable feelings the Cambridge professor now knows
In this class we do not include M. Arago, whose ingenuousness, at least where England and her science were concerned, has never been very brilliantly displayed. His dictum has already been pronounced,—that Mr. Adams has no right to be mentioned in connexion with the discovery. Unfortunately, however, for the realisation of this imperious judgment, the sway of M. Arago is limited, both as regards the present and the future; and it is quite possible that Mr. Adams's name may outlive his own.