Part 9 (2/2)
erected by Mr. La.s.sell at his suggestively named residence of Starfield, near Liverpool. William La.s.sell was a brewer by profession, but by inclination an astronomer. Born at Bolton in Lancas.h.i.+re, June 18, 1799, he closed a life of eminent usefulness to science, October 5, 1818, thus spanning with his well-spent years four-fifths of the momentous period which we have undertaken to traverse. At the age of twenty-one, being without the means to purchase, he undertook to construct telescopes, and naturally turned his attention to the reflecting sort, as favouring amateur efforts by the comparative simplicity of its structure. His native ingenuity was remarkable, and was developed by the hourly exigencies of his successive enterprises. Their uniform success encouraged him to enlarge his aims, and in 1844 he visited Birr Castle for the purpose of inspecting the machine used in polis.h.i.+ng the giant speculum of Parsonstown. In the construction of his new instrument, however, he eventually discarded the model there obtained, and worked on a method of his own, a.s.sisted by the supreme mechanical skill of James Nasmyth. The result was a Newtonian of exquisite definition, with an aperture of two, and a focal length of twenty feet, provided by a novel artifice with the equatoreal mounting, previously regarded as available only for refractors.
This beautiful instrument afforded to its maker, October 10, 1846, a cursory view of a Neptunian attendant. But the planet was then approaching the sun, and it was not until the following July that the observation could be verified, which it was completely, first by La.s.sell himself, and somewhat later by Otto Stuve and Bond of Cambridge (U.S.).
When it is considered that this remote object s.h.i.+nes by reflecting sunlight reduced by distance to 1/900th of the intensity with which it illuminates our moon, the fact of its visibility, even in the most perfect telescopes, is a somewhat surprising one. It can only, indeed, be accounted for by attributing to it dimensions very considerable for a body of the secondary order. It shares with the moons of Ura.n.u.s the peculiarity of retrograde motion; that is to say, its revolutions, running counter to the grand current of movement in the solar system, are performed from east to west, in a plane inclined at an angle of 35 to that of the ecliptic. Their swiftness serves to measure the ma.s.s of the globe round which they are performed. For while our moon takes twenty-seven days and nearly eight hours to complete its circuit of the earth, the satellite of Neptune, at a distance not greatly inferior, sweeps round its primary in five days and twenty-one hours, showing (according to a very simple principle of computation) that it is urged by a force seventeen times greater than the terrestrial pull upon the lunar orb. Combining this result with those of Professor Barnard's[225]
and Dr. See's[226] recent measurements of the small telescopic disc of this farthest known planet, it is found that while in _ma.s.s_ Neptune equals seventeen, in _bulk_ it is equivalent to forty-nine earths. This is as much as to say that it is composed of relatively very light materials, or more probably of materials distended by internal heat, as yet unwasted by radiation into s.p.a.ce, to about five times the volume they would occupy in the interior of our globe. The fact, at any rate, is fairly well ascertained, that the average density of Neptune is about twice that of water.
We must now turn from this late-recognised member of our system to bestow some brief attention upon the still fruitful field of discovery offered by one of the immemorial five. The family of Saturn, unlike that of its brilliant neighbour, has been gradually introduced to the notice of astronomers. t.i.tan, the sixth Saturnian moon in order of distance, led the way, being detected by Huygens, March 25, 1655; Ca.s.sini made the acquaintance of four more between 1671 and 1684; while Mimas and Enceladus, the two innermost, were caught by Herschel in 1789, as they threaded their lucid way along the edge of the almost vanished ring. In the distances of these seven revolving bodies from their primary, an order of progression a.n.a.logous to that pointed out by t.i.tius in the planetary intervals was found to prevail; but with one conspicuous interruption, similar to that which had first suggested the search for new members of the solar system. Between t.i.tan and j.a.petus--the sixth and seventh reckoning outwards--there was obviously room for another satellite. It was discovered on both sides of the Atlantic simultaneously, on the 19th of September, 1848. Mr. W. C. Bond, employing the splendid 15-inch refractor of the Harvard Observatory, noticed, September 16, a minute star situated in the plane of Saturn's rings. The same object was discerned by Mr. La.s.sell on the 18th. On the following evening, both observers perceived that the problematical speck of light kept up with, instead of being left behind by the planet as it moved, and hence inferred its true character.[227] Hyperion, the seventh by distance and eighth by recognition of Saturn's attendant train, is of so insignificant a size when compared with some of its fellow-moons (t.i.tan is but little inferior to the planet Mars), as to have suggested to Sir John Herschel[228] the idea that it might be only one of several bodies revolving very close together--in fact, an _asteroidal satellite_; but the conjecture has, so far, not been verified.
The coincidence of its duplicate discovery was singularly paralleled two years later. Galileo's amazement when his ”optic gla.s.s” revealed to him the ”triple” form of Saturn--_planeta tergeminus_--has proved to be, like the laughter of the G.o.ds, ”inextinguishable.” It must revive in every one who contemplates anew the unique arrangements of that world apart known to us as the Saturnian system. The resolution of the so-called _ansae_, or ”handles,” into one encircling ring by Huygens in 1655, the discovery by Ca.s.sini in 1675 of the division of that ring into two concentric ones, together with Laplace's investigation of the conditions of stability of such a formation, const.i.tuted, with some minor observations, the sum of the knowledge obtained, up to the middle of the last century, on the subject of this remarkable formation. The first place in the discovery now about to be related belongs to an American astronomer.
William Cranch Bond, born in 1789 at Portland, in the State of Maine, was a watchmaker, whom the solar eclipse of 1806 attracted to study the wonders of the heavens. When, in 1815, the erection of an observatory in connection with Harvard College, Cambridge, was first contemplated, he undertook a mission to England for the purpose of studying the working of similar inst.i.tutions there, and on his return erected a private observatory at Dorchester, where he worked diligently for many years.
Then at last, in 1843, the long-postponed design of the Harvard authorities was resumed, and on the completion of the new establishment, Bond, who had been from 1838 officially connected with the College and had carried on his scientific labours within its precincts, was offered and accepted the post of its director. Placed in 1847 in possession of one of the finest instruments in the world--a masterpiece of Merz and Mahler--he headed the now long list of distinguished Transatlantic observers. Like the elder Struve, he left an heir to his office and to his eminence, but George Bond unfortunately died in 1865, at the early age of thirty-nine, having survived his father but six years.
On the night of November 15, 1850--the air, remarkably enough, being so hazy that only the brightest stars could be perceived with the naked eye--William Bond discerned a dusky ring, extending about halfway between the inner brighter one and the globe of Saturn. A fortnight later, but before the observation had been announced in England, the same appearance was seen by the Rev. W. R. Dawes with the comparatively small refractor of his observatory at Wateringbury, and on December 3 was described by Mr. La.s.sell (then on a visit to him) as ”something like a c.r.a.pe veil covering a part of the sky within the inner ring.”[229]
Next morning the _Times_ containing the report of Bond's discovery reached Wateringbury. The most surprising circ.u.mstance in the matter was that the novel appendage had remained so long unrecognised. As the rings opened out to their full extent, it became obvious with very moderate optical a.s.sistance; yet some of the most acute observers who have ever lived, using instruments of vast power, had heretofore failed to detect its presence. It soon appeared, however, that Galle of Berlin[230] had noticed, June 10, 1838, a veil-like extension of the lucid ring across half the dark s.p.a.ce separating it from the planet; but the observation, although communicated at the time to the Berlin Academy of Sciences, had remained barren. Traces of the dark ring, moreover, were found in drawings executed by Campani in 1664[231] and by Hooke in 1666;[232]
while Picard (June 15, 1673),[233] Hadley (spring of 1720),[234] and Herschel,[235] had all undoubtedly seen it under the aspect of a dark bar or belt crossing the Saturnian globe. It was, then, of no recent origin; but there seemed reason to think that it had lately gained considerably in brightness. The full meaning of this suspected change it was reserved for later investigations to develop.
What we may, in a certain sense, call the closing result of the race for discovery, in which several observers seemed at that time to be engaged, was the establishment, on a satisfactory footing, of our acquaintance with the dependent system of Ura.n.u.s. Sir William Herschel, whose researches formed, in so many distinct lines of astronomical inquiry, the starting-points of future knowledge, detected, January 11, 1787,[236] two Uranian moons, since called Oberon and t.i.tania, and ascertained the curious circ.u.mstance of their motion in a plane almost at right angles to the ecliptic, in a direction contrary to that of all previously known denizens (other than cometary) of the solar kingdom. He believed that he caught occasional glimpses of four more, but never succeeded in a.s.suring himself of their substantial existence. Even the two first remained unseen save by himself until 1828, when his son re-observed them with a 20-foot reflector, similar to that with which they had been originally discovered. Thenceforward they were kept fairly within view, but their four questionable companions, in spite of some false alarms of detection, remained in the dubious condition in which Herschel had left them. At last, on October 24, 1851,[237] after some years of fruitless watching, La.s.sell espied ”Ariel” and ”Umbriel,” two Uranian attendants, interior to Oberon and t.i.tania, and of about half their brightness; so that their disclosure is still reckoned amongst the very highest proofs of instrumental power and perfection. In all probability they were then for the first time seen; for although Professor Holden[238] made out a plausible case in favour of the fitful visibility to Herschel of each of them in turn, La.s.sell's argument[239]
that the glare of the planet in Herschel's great specula must have rendered almost impossible the perception of objects so minute and so close to its disc, appears tolerably decisive to the contrary. Ura.n.u.s is thus attended by four moons, and, so far as present knowledge extends, by no more. Among the most important of the ”negative results”[240]
secured by La.s.sell's observations at Malta during the years 1852-53 and 1861-65, were the convincing evidence afforded by them that, without great increase of optical power, no further Neptunian or Uranian satellites can be perceived, and the consequent relegation of Herschel's baffling quartette, notwithstanding the unquestioned place long a.s.signed to them in astronomical text-books, to the Nirvana of non-existence.
FOOTNOTES:
[Footnote 195: _Op._, t. i., p. 107. He interposed, but tentatively only, another similar body between Mercury and Venus.]
[Footnote 196: _Allgemeine Naturgeschichte_ (ed. 1798), pp. 118, 119.]
[Footnote 197: _Cosmologische Briefe_, No. 1 (quoted by Von Zach, _Monat. Corr._, vol. iii., p. 592).]
[Footnote 198: Second ed., p. 7. See Bode, _Von dem neuen Hauptplaneten_, p. 43, _note_.]
[Footnote 199: The representative numbers are obtained by adding 1 to the following series (irregular, it will be observed, in its first member, which should be 1/2 instead of 0); 0, 3, 6, 12, 24, 48, etc. The formula is a purely empirical one, and is, moreover, completely at fault as regards the distance of Neptune.]
[Footnote 200: _Monat. Corr._, vol. iii., p. 596.]
[Footnote 201: Wolf, _Geschichte der Astronomie_, p. 648.]
[Footnote 202: Such reversals of direction in the apparent movements of the planets are a consequence of the earth's revolution in its...o...b..t.]
[Footnote 203: _Dissertatio Philosophica de Orbitis Planetarum_, 1801.
See Wolf, _Gesch. d. Astr._, p. 685.]
[Footnote 204: Observations on Ura.n.u.s, as a supposed fixed star, went back to 1690.]
[Footnote 205: He had caught a glimpse of it on December 7, but was prevented by bad weather from verifying his suspicion. _Monat. Corr._, vol. v., p. 171.]
[Footnote 206: Planetary fragments, hurled _in any direction_, and _with any velocity_ short of that which would for ever release them from the solar sway, would continue to describe elliptic orbits round the sun, all pa.s.sing through the scene of the explosion, and thus possessing a common line of intersection.]
[Footnote 207: _Phil. Trans._, vol. xcii., part ii., p. 228.]
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