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strong unassisted vision, but Dr Wollaston, who had a remarkably keen eye, often tried the experiment under the most favourable circumstances, but always without success. Captain Smyth, a good gazer also, relates that he gave great attention to the matter on the summit of Mount Etna, and in other fine-climate places, but could never obtain a sight of Jupiter's moons without a glass. It is certain that they were completely unknown before the time of Galileo and Harriot; and we may suspect some unintentional selfdeception in all accounts of their visibility to the unarmed eye. M. Mädler, indeed, mentions the curious fact of the postmaster Nernst seeing one satellite without a telescope, and drawing a diagram of its position; but upon the instrument being applied it was found that three of the satellites had nearly closed, and it was their united brightness that thus reached the piercing eye of Nernst. From Zach's memoir on Harriot, it appears that he was undoubtedly the first observer of the solar spots in 1610, followed by Galileo, Scheiner, and Fabricius, in Fabricius, in 1611.

Just as the civil wars were on the eve of breaking out, interrupting the peaceful occupations of science, and agitating the homes of England, the first transit of Venus ever known to have been seen by human eyes was witnessed. This was in December 1639. The time was calculated by Jeremiah Horrocks, born in an obscure village near Liverpool, who observed the phenomenon, in connection with his friend William Crabtree, a clothier at Broughton, near Manchester. The planet appeared on the sun, according to the calculation. The day of the event was Sunday, respecting which Horrocks writes: "I observed from sunrise till nine o'clock, again a little before ten, and, lastly, at noon, and from one to two o'clock, the rest of the day being devoted to higher duties, which might not be neglected for these pastimes." Horrocks died in the beginning of the year 1641, when only twenty-three years old, having advanced the lunar theory in a way which Newton has eulogized. Crabtree followed him to

the grave a few months afterwards, likewise at a very early age; and another mutual friend, the inventor of the micrometer, William Gascoygne, also young, was slain in the bloody fight on Marston Moor between Charles and Cromwell.

The true constitution of the Saturnian globe, known from remote antiquity as a dull, sluggish object in the heavens, first dawned upon Huyghens, a Hollander, worthy of remembrance for relinquishing French honours and emoluments, nobly refusing to be made a special exception to the edicts levelled against the Huguenots, among whom he was numbered. He had applied himself to the construction of large telescopes, and, on March 21, 1655, discovered a satellite of Saturn, succeeding at the same time in discerning the annular appendage of the planet. Galileo, astonished and perplexed with its appearance, had sent to Kepler, the formidable word,

Smaismrmilmepoetalevmibvnenvgttaviras, from which, by transposition of the letters, the following sentence is obtained, Altissimum Planetam tergeminum observavi.

"The most distant planet I have ob

served to be threefold."

Huyghens, adopting the style of the age, published the cipher, aaaaaaa ccccc deeeeeg h i ii ii ii i l l l l m m n n n n n n n n n o o o o p p q r r s tttttuu uu u, which contains the letters of the sentence in alphabetical order,

Annulo cingitur, tenui, plano, nusquam cohærente, ad eclipticam inclinatio.

"The planet is surrounded with a ring, thin, plane, nowhere adhering, and inclined to the ecliptic."

The elder Capini, between the years 1671 and 1684, discovered four of the planet's moons; and Herschel two more in 1789, enlarging the cortege to seven. The latter, with his mighty instrumental power, established the rotation of the rings, first seen double by the brothers Ball at Minehead in Devonshire on October 13, 1665, and discerned their shadow on the body of Saturn at the time when the edge, be

ing turned to the earth, was invisible. It still remains an unsettled point whether this wonderful apparatus may not be further divisible; but, rigidly examined by Struve, it is now known to be not mathematically concentric with the ball of the planet, which lies towards the west in the annulus.

We now come to the illustrious resident at Slough, Sir William Herschel, who pushed the bounds of the system over an extent of space equal to its former dimensions, added to the number of its constituents half as many again as those previously known, and was the first to garner in a new primary planet. Including Halley's comet, the solar universe consisted of eighteen bodies when Herschel commenced his labours, but he increased them to twenty-seven by the discovery of two moons to Saturn, with Uranus and six satellites. This far remote traveller had been observed before by Hamstead, Bradley, Mayer, and Lemonnier, but the length of its orbital route and the consequent apparent slowness of its pace, warded off suspicion of the planet's motion, and the object was registered as a star. Eleven times it had been noticed by Lemonnier, over whose misfortune Delambre sighs, for had he only arranged his observations methodically, he would have discovered its planetary character, and the honour of the event have belonged to France. It was on Tuesday the 13th of March 1781, between ten and eleven o'clock in the evening, while Herschel was examining some small stars near the feet of Gemini, that he observed one of them to have a sensible amount of diameter, though less bright than the others. This was at first taken to be a comet, and announced as such, but very speedily recognised as a superior planet, resembling the rest in every possible point of comparison. The stranger, still called the Georgian in the Nautical Almanack, but more commonly Uranus, expanded the system to the enormous linear extent of eighteen hundred millions of miles from the sun, and accounted for certain perturbations in the motions of Jupiter and Saturn, for which no perceptible cause

appeared, but which had been sagaciously referred to the action of some unknown outlying body, an idea which was now completely verified.

The opening of the present century was distinguished by the discovery of constituents within the ancient limits of the system. From the days of Kepler, the notion had floated in the minds of astronomers, chiefly of the German school, that some undiscovered body must exist in the vast interval between Mars and Jupiter, so disproportionate to the distances between Mars and the Earth, and the interior planets respectively. The notion was strengthened by the curious but purely empirical law of Bode, that the interval between the orbits of any two planets is about twice as great as the inferior interval, and only half the superior one. Accordingly, at a congress of practical observers held at Lilienthal in 1800, an association was formed, of which M. Schroeter was elected the president, and Baron de Zach the secretary, for the purpose of accurately examining the whole zodiac, each taking a compartment, in order to bring out the latent planet from its hiding place. The result was, within six years, the detection of the singular family of asteroids, occupying the immense gap between Mars and Jupiter, which, from their small size, the correspondence of their mean distances from the sun, and the intersection of their orbits, appear as if the fragments of a large original planet, disrupted by some tremendous cataclysm. Since these extraordinary bodies were found, no new planet enriched the astronomical catalogues till the close of the year 1845, when Herr Hencke of Berlin added a colleague to the asteroids, under the name of Astrea.

The course of remark brings us to M. Le Verrier's planet, the discovery of which is the great scientific event of 1846, the most memorable achievement of theoretical astronomy, in which observation has been outstripped by induction.

When Columbus commenced his enterprise in search of a western main across the Atlantic, its existence in that direction was not the romantic

speculation of an imaginative mind, but his judgment had dwelt upon significant data in favour of the attempt to reach a continent beyond the untraversed wastes of the ocean. Martin Vicenti, a Portuguese pilot, had assured him, that far to the west of Cape St Vincent he had taken a piece of carved wood from the sea, evidently not laboured with an iron instrument, which must have floated from some unknown western land, following the set of the current. Pedro Correo, his brother-in-law, had also informed him that he had seen a similar piece of wood off Porto Santo, which seemed to have come from the same quarter; and immense reeds were known to have been cast upon the Madeira Isles, dissimilar to those with which Europeans were acquainted. Trunks of huge pine trees had been thrown upon the shores of the Azores, of a species different to any that grew upon the islands; while the bodies of two dead men had been drifted to the island of Flores, in the fifteenth century, whose complexion and features proclaimed them to belong to an unknown race. These facts had not been lost upon Columbus. His sagacious mind perceived their bearing. They appeared as evidences giving to the conviction the force of demonstration, that by proceeding westerly over the Atlantic, he should arrive at a continent before unknown, a presumption which the undertaking confirmed. In a somewhat similar manner has Le Verrier's planet been found, though at the distance of more than three thousand millions of miles from our globe, the conviction of its existence, the calculation of its mass, of the dimensions of its orbit, of its periodic time, and its particular locale in the heavens, ante-dating its discovery and leading to it. The manner in which this result has been obtained, stripped of its technicalities, may be readily apprehended by the general reader.

Planetary perturbations are inequalities of motion, consequent upon their mutual gravitation towards each other. Such disturbances are mostly inappreciable, taking short intervals of time, but become sensible by accumulation

through longer periods. Sometimes, the effect produced, in the case of a particular planet, is an acceleration in its rate of movement; at other times, a retardation, and derangement ensues in the forin and magnitude of the planetary orbits, their dilation and contraction, and in their mutual inclinations. No permanent alteration, however, is occasioned in the system, but its stability is guaranteed, by the tendency of the disturbing forces ere long to go upon another tack, and work out a compensation for the inequalities induced.

Where disturbance and compensation transpire in a comparatively short period, the fluctuations are said to be periodic, while those which require an immense interval of ages for their completion, are discriminated as secular variations. Throughout the whole of the seventeenth century Saturn was observed to be a constant laggard with reference to his calculated post, while Jupiter showed his expertness in being always beforehand at

his.

Thus the one planet seemed to be lengthening, and the other to be shortening its period. But in the eighteenth century an inverse process was remarked. Saturn shewed acceleration, and Jupiter retardation. It is now well known that in a cycle of about 918 years, these two planets, owing to their action and reaction upon each other, go through a series of minute changes in their orbits which are compensated in that period. During half of it the major axis of Jupiter's orbit is increased, while that of Saturn's is diminished. During the other half, the reverse takes place, the entire cycle being known by the name of the "great inequality." Any competent geometer who has the elements of two planets in hand, their positions, masses, and periodic times, will be able to calculate the perturbations arising from their action upon each other, though the problem belongs to the highest branch of mathematics. But in the instance of Le Verrier's planet, a problem inverse to this, and one of far greater difficulty, never solved before, has been successfully worked, namely, to find the elements of an unknown disturbing body from a given amount

of perturbations to which one already known is subject, opening the way to the discovery of the concealed object.

Observations upon Uranus had shewn the motions of that planet to present great irregularities, which could not be explained by the action of Jupiter and Saturn; and after carefully examining the analytical theory of Uranus, Le Verrier, in the summer of 1846, published the following elements of an undiscovered planet, the cause of the perturbations:-Mean distance from the sun, 33 times that of the earth from the sun; greatest distance from the sun, 36 times that of the earth from the sun; period of revolution 217 years; mass 38 times that of the earth. At the close of August, in a paper presented to the French Academy, he pointed out the place in the heavens where it should be found, about 5' east of the star in Capricorn. Scarcely a month had elapsed when the following note from Dr Brünnow to Mr Hind of London announced the discovery of the stranger. "Berlin, Sept. 25. My dear Sir,-M. Le Verrier's planet was discovered here on the 23d of September, by M. Galle. It is a star of the eighth magnitude, but with a diameter of two or three seconds. (Then follow its places for Sept. 23, 24). The planet is now retrograde, its motion amounting daily to four seconds of time. Brünnow." M. Galle communicated immediately with M. Le Verrier by letter on the 25th, observing-" The planet, whose position you have described, really exists. On the same evening that I received your letter, I discovered a star of the eighth magnitude, which is not marked on the excellent chart of Dr Bremiker, and which forms part of the collection of celestial charts published by the Royal Academy of Berlin. The observations on the following night showed that this star is precisely the planet in question. M. Encke and I have, with the aid of Fraunhofer's large telescope, compared it with a star of the ninth magnitude."

The new member of the system was speedily picked up at Mr Bishop's observatory in the Regent's Park, and by Mr Lassell of Liverpool, to whom

it appeared to give promise of a ring and satellite. He writes "On the 3d instant (October), whilst viewing this object with my large equatorial, during bright moonlight, and through a muddy and tremulous sky, I suspected the existence of a ring round the planet, and on surveying it again for some time on Saturday evening last, in the absence of the moon, and under better, though not very favourable atmospherical circumstances, my suspicion was so strongly confirmed of the reality of the ring, as well as of the existence of an accompanying satellite, that I am induced to request you, as early as possible, to put the observations before the public. The telescope used is an equatorially mounted Newtonian reflector, of 20 feet focus, and 24 inches aperture, and the powers used were various-from 316 to 567. At 1 hours mean time, I observed the planet to have a very obliquely situated ring, the major axis being seven or eight times the length of the minor, and having a direction at right angles to a parallel of declination. At the distance of about three diameters from the disc of the planet northwards, and not far from the plane of the ring, but a little following it, was situate a minute star, having every appearance of a satellite. I observed the planet again about two hours later, and noticed the same appearances, but the altitude had then declined so much, that they were not so obvious. My impression certainly was, that the supposed satellite had somewhat approached, but I cannot positively assert it. With respect also to the existence of the ring, I am not able absolutely to declare it, but I received so many impressions of it, always in the same form and direction, and with all the different magnifying powers, that I feel a very strong persuasion that nothing but a finer state of the atmosphere is necessary to enable me to verify the discovery. the existence of the star having every aspect of a satellite, there is not the shadow of a doubt. Afterwards, I turned the telescope to the Georgium Sidus, and remarked that the brightest two of his satellites were both obvi

Of

ously brighter than this small star accompanying Le Verrier's planet."

A just tribute has been paid to the merits of Le Verrier by the principal European nations in complimentary honours, and these, with some more substantial tokens of reward, have also been conferred by his own country. The Minister of Public Instruction, M. Salvandy, in a report to the Crown, states" Sire, a great discovery-one of the rare efforts of the human mind -has conferred honour on France, and on your reign. A French youth, M. Le Verrier, of the Academy of Sciences, by the unaided power of profound thinking, using the mathematics as its instruments, has grasped, in the regions of space, beyond our solar system, a planet which, but for him, might have remained for ever undetected by observation. In general, observations have preceded science, but in this case science has only had to follow the steps of theory, and to look at the exact point indicated for the unknown star, which theory alone had as yet seen by the mind's eye. There has not, in the whole history of science, occurred a more striking event than this. In recommending M. Le Verrier for the Legion of Honour, I also recommend M. Galle of Berlin, who has been the first to discover the Planet Le Verrier. He thus united with M. Le Verrier in a discovery which has astonished the scientific world." It is little likely that astronomers will agree to this title of the planet, or to that of Gallia, which has been proposed, as they refused to elevate Herschel and George the Third to a place in the skies, and went to Pagan mythology for the name Uranus.

But we have not yet done with this discovery. It will be seen that only by accidental circumstances has France the honour of it. In fact, so far from M. Salvandy's statement being correct, that but for the young French academician's labours, great as are his merits, the planet might have remained for ever unknown, all its elements had been arrived at by a Cambridge student, and were in the hands of our Astronomer Royal months before their publication among our neigh

bours, while the object itself had been seen, though not recognised. It appears from a public communication of Sir John Herschel's, that when the late illustrious astronomer Mr Bessel visited him at Collingwood, on the 12th of July 1842, he remarked "that the motions of Uranus, as he had satisfied himself by careful examination of the recorded observations, could not be accounted for by the perturbations of the known planets, and that the deviations far exceeded any possible limits of error of observation. In reply to the question, whether the deviations might not be due to the action of an unknown planet? he stated that he considered it highly probable that such was the ease, being systematic, and such as might be produced by an exterior planet. I then enquired whether he had attempted, from the indications afforded by these perturbations, to discover the position of the unknown body, in order that a hue and cry might be raised for it. From his reply, the words of which I do not call to mind, I collected that he had not then gone into that enquiry, but proposed to do so, having now completed certain works which had occupied too much of his time. Accordingly, in a letter which I received from him after his return to Königsberg, dated November 14, 1842, he says, ' In reference to our conversation at Collingwood, I announce to you (melde ich Ihnen) that Uranus is not forgotten.' Doubtless, therefore, among his papers will be found some researches upon this subject."

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Upon retiring from the chair of the British Association, a fortnight before the discovery of M. Galle, Sir John Herschel, in remarkable words, referred to the astronomical events of the past year, observing that it had given a new planet (Astrea) to our list, and adding, It has done more, it has given us the probable prospect of the discovery of another. We see it as Columbus saw America from the shores of Spain. Its movements have been felt trembling along the far-reaching line of our analysis, with a certainty hardly inferior to that of ocular demonstration." This striking para

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