(278.)

(279.) Division

of the comet in

1846.

though the perturbations of Jupiter were, as usual, large.

Biela's Comet has been since recognised in 1846 and 1852, but it was not seen in 1839. It does not appear to be admitted that it shows any acceleration due to a resisting medium. Its perihelion distance is, however, considerable.

At the apparition of 1846 an extraordinary circumstance occurred. When discovered in the end of the body of November 1845 it appeared round and single. On the 19th December it was observed by Mr Hind to be elongated, and ten days later was seen in America (and soon after at Cambridge and elsewhere) to have divided into two seemingly distinct nebulous parts. These continued to subsist and move independently throughout the remainder of the apparition the real distance of the centres being about 150,000 English miles. In 1852 the comet was rediscovered at Rome; the division into two still subsisting, but the interval of separation being increased about eight-fold.

(280.) Other

comets.

Besides the comets of Encke and Biela, there are several others which are suspected on good grounds periodical to have periods of from 5 to 74 years, their aphelia all lying in tolerable proximity to the orbit of Jupiter. But among these the return of only one has yet been verified by observation; namely, the comet of Faye, which, after passing its perihelion 17th October 1843, returned to it 3d April 1851, within an hour of the time predicted by M. Leverrier. The motion of comets of short period seems to be invariably direct or conformable to that of the planets. The inclination of their orbits to the Ecliptic is usually moderate.

(281.) Great

comets of

1811 and

1843.

Great Comets of 1811 and 1843.-The finest comets of the last hundred years were those of 1811 and 1843. The former was observed for a length of time altogether unusual, having been visible from March 1811 to August 1812. There is pretty good reason to think that its period is not much less than 3000 years. The comet of 1843 was even more splendid, but its flight was more rapid, and it was not favourably seen in northern latitudes. It was visible at many places in broad daylight when less than 4° from the Sun, and at one time a tail 65° in length could be traced. The circumstance which distinguishes this comet from all others which have been computed is the smallness of its perihelion distance, which was only of the radius of the Earth's orbit, or the comet approached the Sun's body within one-seventh of his radius. The solar disk then subtended an angle at the comet of 1214°, or the glare was equal to that of 47,000 suns as seen by us! The heat to which the comet was exposed is supposed

to have exceeded 24 times that concentrated by our most powerful burning-glasses by which even rock crystal has been fused.2

Planets or

MR HIND.-Discovery of New Planets.—We have (282.) spoken in a former section (161), of the discovery of Discovery four small planets or asteroids between the orbits of of new Mars and Jupiter. They were found between the years Asteroids. 1801 and 1807. An interval of nearly forty years elapsed without any addition to the members of our system. In 1845 a new asteroid, Astræa, was found by M. Hencke; the following year was distinguished by the discovery of Neptune under unparalleled circumstances; and since 1847 every year, down to the present time (1855), has added to our knowledge of the group of asteroids.

Among the discoverers of these planetary bodies Mr Hind has been distinguished by frequent success, under circumstances which appeared by no means peculiarly advantageous. This indefatigable observer and computer commenced (I believe) his astronomical career as one of the assistants at Greenwich, and afterwards had the sole charge of the private observatory of Mr Bishop, a wealthy citizen of London, together with the use of a fine refractor equatoreally mounted. It is within the Regent's Park, close to the smoke of the metropolis, that Mr Hind has discovered a larger number of planetary bodies than any other person living. Next to him M. de Gasparis of Naples has been most successful. Unquestionably the impulse towards these new discoveries has been given by the indefatigable industry of astronomers (principally those of Germany), in constructing minutely accurate star-maps. Mr Hind is also advantageously known by the discovery of several comets, and by his ingenious observations in sidereal astronomy, especially on variable stars. I shall here give a table of the asteroids in the order of discovery as at present known (July 1855).

April 17 June 24

Aug. 22

(283.) Mr Hind.

M. de Gas

paris.

List of the Asteroids.

(284.) New Secondary Planets.

MR LASSELL.-New Secondary Planets.-Mr Lassell of Liverpool deserves a more lengthened notice than our limits will permit, not only as a disMr Lassell. tinguished discoverer, but as one whose success cannot be too widely made known as an encouragement to others. This gentleman, engaged in mercantile pursuits in an eminently commercial town, possessing little leisure and no enormous fortune, has contrived, in the intervals of business, to construct with his own hands telescopes which in accuracy of definition appear to rival any which art stimulated by national liberality has yet constructed elsewhere, and to use them with a degree of skill and success which has not been exceeded (nor in some respects equalled) by any astronomer whether professional or otherwise. I speak, let it be observed, of accuracy of definition, such as is necessary to display minute points of light, like the satellites of Uranus. In respect of the amount of illumination requisite for the display of many diffuse faint objects among the nebulæ, the gigantic telescopes of Herschel and Lord Rosse are of course superior.

(285.)

His instru- in 1840.

ments.

Mr Lassel's observatory near Liverpool was erected The principal instrument is a reflecting telescope of 24 inches aperture (completed, however, only some years later), mounted equatoreally, an arrangement requiring great mechanical skill, but, as the results show, most effectually accomplished. The speculum was worked and polished by machinery constructed by Mr Nasmyth, but principally devised by Mr Lassell, after he had examined and tried Lord Rosse's method. I should think it must be admitted to be the most perfect optical work of its kind ever made: for I believe there is no test object in existence which Mr Lassell has not seen with it; in fact he has discovered the most delicate tests himself,—the 6th star of the group ◊ Orionis (though not first seen

1 From Mr Lassell's observations at Malta, 1852-3.

(287.)

Saturn.

Till 1848 only seven satellites of Saturn were admitted. The two closest to the planet were detected and one of by Sir William Herschel in 1789, and have been seen by very few astronomers since. During five years' residence at the Cape, Sir John Herschel never but once obtained even a doubtful glimpse of the closest with an 18-inch mirror. The third, fourth, and fifth were discovered by Cassini in 1684; the sixth and most conspicuous by Huygens in 1654: the outermost by Cassini in 1671. To these an eighth satellite, intermediate in position between the two last, was added by Mr Lassell on the 19th September 1848. By a singular coincidence, it was recognised as a satellite the very same evening by Mr Bond of Cambridge (in America) with the great Munich refractor. The new body was called Hyperion, in conformity with Sir John Herschel's suggestion of distinguishing the satellites as well as the planets by mythological names. On the 22d November 1850 Mr Lassell saw at once Saturn with his whole train of eight satellites-a glorious spectacle probably enjoyed by no other astronomer. In the same month of November Faint ring Mr Bond discovered a faint or dusky ring of Saturn of Saturn interior to the two long known. It is probably ne- by Mr bulous, for by Mr Lassell's observations and Mr Bond. Jacob's it appears to be transparent.

discovered

of Uranus.

Sir William Herschel thought that he recognised (288.) six satellites of Uranus. The second and fourth of Satellites his table have been observed by several astronomers, particularly Sir John Herschel and M. Lamont. Their periods are 8d 17h and 13d 11h. To these Mr Lassell, aided by the fine climate of Malta (to which for two seasons he removed his telescope), has conclusively added two more: one, appearing to coincide with the closest of Herschel's, of which the period is 4d 3h 28m, and one still nearer the planet revolving in 2d 12m 29s, the shortest orbital revolution in the solar system. Mr Lassell doubts seriously the existence of any other satellite.2

2 See Astron. Society's Notices, xiii. 151; and xiv. 133.

Astrono

my.

(289.) As it is absolutely necessary to bring this chapter Continua- to a speedy close, and as I have already anticipated, tion of the in the account of Sir William Herschel's discoveries, history of Sidereal part of what relates to the recent history of the Astronomy of the Fixed Stars, particularly the "Constitution of the Heavens," and the motion of our system in space, I shall condense within brief compass a few leading facts connected, in the first place, with the Orbits of Double Stars, the Brightness of Stars, and the constitution of Nebula, and these subjects I shall connect with the names of the elder M. Struve, Sir John Herschel, and Lord Rosse; the second topic shall be the Parallax and distance of the fixed stars, as ascertained more particularly by the late professors Henderson and Bessel.

(290.) FRIEDRICH GEORG WILHELM STRUVE has been the M. Struve. most assiduous observer of double stars since the time of Sir William Herschel. No discovery in this department can for a moment compete with the great one of the orbital revolution of one star round another. But M. Struve, by devoting his chief energies during the most active years of his life, since 1813, to the assiduous continuation of Herschel's observations, has added immensely to our knowledge of these systems, and has earned the reputation of one of the most skilful of modern practical astronomers. His most elaborate observations were made at the Russian Observatory at Dorpat, with a noble refractor by Fraunhofer, nearly 10 inches aperture, and 13 feet in focal length. He has published three works on the subject of double stars, one in 1824, one in 1837, and one in 1852, besides minor papers. The second of these works contains the particulars of about 3000 double stars, deduced from a survey of the heavens, in which at least 120,000 stars were examined.

(291.) His re

double stars.

M. Struve's papers are distinguished by the elaboration of the reductions, and of the statistical researches on sults deduced from them. In his last publication he has made an interesting estimate of the number of true double stars in the heavens, which, it appears, is much greater in proportion to the whole number than is usually believed. But it is first necessary to distinguish those which are physically double from those which are merely apparently or optically so. The criteria on which he principally depends are— (1.) the fact of observed orbital revolution; but as this is established in comparatively few instances, he very reasonably admits (2.) a common proper motion of the two components as a proof of their connection. He thus finds the evidence for physical duplicity to be much stronger for the closer double stars, and also for brighter or nearer stars, as compared with those of less magnitude. On the whole he concludes, that of

Observa

Besides these important works on Sidereal Astro- (293.) nomy, M. Struve is well known as the head and di- Directs the rector of perhaps the best organized observatory in tory of the world, that of Pulkowa near St Petersburg, of Pulkowa. which he has published a very interesting description. Besides other noble instruments, it contains the finest refractor in Europe, that by Merz, 15 inches in diameter, and 22 feet focal length. The observations with this noble telescope are chiefly made by his son M. Otto Struve, the author of many good papers. We have seen in a former section (252), that we owe in great part to M. Struve the conduct of the most extensive trigonometrical operation ever undertaken.

his astrono

SIR JOHN FREDERICK WILLIAM HERSCHEL, Son of (294.) Sir William Herschel, whilst conversant with almost Sir John every branch of science, has devoted himself with re- Herschelmarkable success to the cultivation of Sidereal As-mical catronomy. Bearing a name honoured and revered reer. by all, his career at Cambridge reflected upon it fresh lustre; the variety and extent of his acquirements gave him a reputation amongst his college contemporaries, afterwards fully confirmed by the not more impartial voice of mankind at large." He was senior wrangler in 1813. "Since that time he has been indefatigable as an author:-first, in systematizing the higher mathematics, and in forwarding their study in his own university;-afterwards by treatises contributed to the Encyclopædia Metropolitana on Sound, Light, and Physical Astronomy, which still rank among the clearest, completest, and most philosophical in our language. About the same time he wrote experimental essays on different branches of Chemistry, Magnetism, and Optics, and commenced his purely astronomical investigations, chiefly on nebulæ and double stars, partly in conjunction with Sir James South, of which the details are given in different volumes of the Astronomical, and of the Royal Society's Transactions. These memoirs collectively include a complete revision of the objects of the same description catalogued and classified by Sir

servations

at the Cape of Good

Норе.

William Herschel." 1 But the most considerable monument to Sir John Herschel's love of science is the record of his four years' labours for the advancement of Sidereal Astronomy at the Cape of Good Hope, where he applied his father's methods of obSir J. Her-servation to the southern hemisphere. His Results schel's ob- of Astronomical Observations, which fill a large quarto volume, and which include "the completion of a Telescopic Survey of the visible Heavens, commenced in 1825," form one of the most considerable and most interesting works of our time. The instruments employed were a 20-feet reflector, of 18 inches aperture, and a 7-feet achromatic, with 5 inches of aperture. With these the nebulæ and double stars of southern skies were examined and measured, and that wonderful "Gauging of the Heavens" completed, of which I have spoken in the account of Sir William Herschel (201). There is an admirable chapter on the apparent magnitude of the stars, to which I shall refer presently, and one on Halley's comet, besides other matters of interest.

(295.)

His high

character amongst

Since his return to England in 1838, Sir John Herschel has withdrawn from the labours of practical astronomy, but he continues to advance different bis contem- branches of science, and to expound them by his able poraries. and lucid writings in a way which has made his authority equally respected by philosophers and by men

of the world. The career of Sir John Herschel has been marked by an almost total absence of the element of ambition, so often a powerful excitement in the pursuit of discovery. Had he sought notoriety and posthumous fame, he would have confined his efforts within a more circumscribed range. But his versatile talents sought their appropriate exercise in all departments of exact science, and even (it is believed) in pursuits widely distinct from these, in natural history, belles lettres, and the fine arts. In all this he no doubt considered simply the useful and pleasurable employment of his mental activities. Truth seemed to him as desirable whether attained by the labours of others or by his own; and in his numerous writings he has expounded these with a zest which a less generous spirit might have reserved for his peculiar achievements. What he may have lost in future fame by this enlargement of his sympathies and interests, he has gained in the respect and good-will of all his contemporaries. Sir John Herschel recently filled the post of Master of the Mint, to which, like his illustrious predecessor Newton, he devoted a considerable share of his time. His general eminence as a man of science has been acknowledged by his nomination in 1855 to the distinguished honorary position of one of the eight foreign Associates of the French Academy of Sciences. Orbits of Double Stars.-Though not absolutely On the or- the first to apply calculation to the orbits of double stars, this step in their theory may not unfitly be con

(296.)

bits of double

stars.

1 Quarterly Review, vol. lxxxv., p. 2.

nected with the name of Sir John Herschel, from the ardour of his researches and the neatness of his methods. To Savary of Paris is due the merit of ascertaining the form and position of the orbit of Ursa Majoris in 1827, which was followed by a more purely analytical method by M. Encke, and one chiefly graphical by Sir J. Herschel,2 in which angles of position of the component stars are used nearly to the exclusion of the more doubtful measures of distance. On the whole, these investigations not only confirm Sir William Herschel's anticipations, but render it highly probable that the relative orbits are really ellipses, and consequently that the law of force is that of the inverse square of the distance. The reader will find in Sir J. Herschel's Cape Observations a very curious discussion of the orbit of y Virginis, a remarkable double star, whose interval was in 1783 five seconds and two-thirds, which diminished till 1836, when the two stars appeared united in one, as seen even in the best telescopes. This was the perihelion passage of these two suns, and the angle of position must then have varied (could it have been measured) at the rate of 70° per annum, or 1° in 5 days. The following are some of the best ascertained periods of sidereal revolutions in years :

Herculis 36-4; Ursa Majoris 61-5; a Centauri 77; p Ophiuchi 80 or 90'; Coronæ Borealis 600 or 700. M. Mädler, Admiral Smyth, and Mr Hind, have added much to our knowledge of this interesting subject.

Brightness of Stars, and Variable Stars.-Sir John (297.) Herschel has attempted by an elaborate system of on the brightness inter-comparison to assign the correct relative bright-of stars. ness of the stars, and to give precision to the ordinary terminology of Magnitudes. His "Method of Sequences" described in his Cape Observations, appears to be one of the happiest specimens of generalization which experimental science affords. Whilst regretting the impossibility of here giving even the slightest sketch of it, I cannot but recommend it to the student of natural philosophy as a model of research. Having ascertained, in a way independent of every sort of hypothesis, the relative brightness of the stars upon the scale of Magnitudes usually adopted, but which is wholly arbitrary, Sir J. Herschel proceeds, by properly photometric methods, to give a scientific precision to this notation; and he arrives at this singular and fortunate conclusion, that by adding a small and constant correction to the received scale of Magnitudes, the numbers will represent the distances of the respective stars from our system on the supposition of an intrinsic equality in the brightness of the stars themselves.

This subject naturally includes that of Variable (298.) Stars, which may be divided into those which under-Variable go periodic or irregular fluctuations, and the latter' class may embrace new stars, and stars which have

2 Astronomical Society's Memoirs, vol. v.

stars.

-his re

disappeared. The last two hundred years have not presented any such astonishing phenomena as the new stars recorded in the sixteenth and seventeenth centuries; but singular variations in the brightness of some of the most conspicuous stars, as a Orionis and Argûs, have been discovered by Sir John Herschel. Several stars of short and irregular periods of varying brightness were recorded towards the close of last century. But upon this very interesting subject I must content myself with referring to the details given by Professor Argelander in the third volume of Humboldt's Cosmos.

(299.) EARL OF ROSSE. Latest observations on Nebula.— Lord Rosse It is a remarkable circumstance, that as the reflectflecting ing telescope was a British invention, so the more telescopes. important improvements and applications of it have been almost confined to the United Kingdom. It is also worthy of notice that the manufacture has prospered more in the hands of amateurs than of regular opticians. Sir William Herschel appeared at one time to have brought the invention to its highest perfection, but the Earl of Rosse has made an important step farther; not only by constructing a larger telescope than had been made before, but by adapting machinery driven by steam power, to the grinding and polishing of the mirror; so that the largest speculum may be finished with nearly the same accuracy and expedition as the smallest. The chef d'œuvre of Lord Rosse is a telescope of 6 feet aperture, and 53 or 54 feet of focal length. It was completed in the latter part of 1844, and erected at Parsonstown in Ireland.

encountered.

(300.) Let me here record the important fact, that neither Difficulties rank nor wealth could absolve Lord Rosse from those toils and disappointments which attend all new and original efforts. There is no royal road to such triumphs. The Irish nobleman owes his success entirely to his unwearying perseverance and mechanical skill. Even his assistants were countrymen instructed by himself in his own workshops, where the very steam-engine which drives the polisher was fabricated. His labours to improve the telescope date from 1828 (when he was Lord Oxmantown), or even earlier, and they appear to have been unremitting until 1844; indeed I might say until the present time. Commencing with a variety of ingenious attempts to correct spherical aberration, and to overcome the extreme difficulty of procuring large castings of so excessively brittle a material as speculum-metal, they terminated in the rejection of all minor helps and expedients, and in the fortunate completion of immense mirrors at a single casting, and of correctly parabolic figure when ground and polished. The speculum of his largest telescope weighs four tons. It was polished in six hours, and its surface is con

siderably more than twice as large as that of Sir William Herschel's forty-feet instrument.

construc

tion.

We cannot enter into the details of the methods, (301.) which evince no small mechanical skill and scientific Methods of ingenuity, together with a perseverance admirable in itself. I will only mention how the upper and lower surfaces of the casting were made to cool nearly equally fast. To effect this the lower surface of the mould (which naturally retains the heat more than the upper) was made of iron, a good conductor, whilst the upper surface was made of sand. This effected the purpose; but it being found that air bubbles entangled in the fluid metal could not escape beneath, and injured the casting, the iron bed was constructed of hoops set on edge and closely packed, the crevices allowing the escape of air, whilst the cooling proceeded as before; and this ingenious contrivance was perfectly successful.

tion to Ne

Many difficulties in detail have been found in the (302.) mounting and use of so gigantic a mass, particularly Applica on account of the distortion of the mirror by flexure. bulæ, and But these have gradually been surmounted by Lord its results. Rosse. His published observations (Philosophical Transactions, 1850) relate almost entirely to objects of the class of nebulæ ; and as I cannot enter into details, I may state the general results in two or three sentences. (1.) As might have been expected, many nebula which resisted the power of former telescopes (for, except in rare instances, nothing greater than eighteen-inch apertures have been directed to them) have been "resolved" into stars by the six-feet speculum. (2.) The aspect of a great number of nebulæ described by the two Herschels is materially modified by the power of the telescope to embrace the fainter prolongations of these singular objects. In general, the symmetrical forms are very much cut up and confused, and in many cases vanish altogether. (3.) Instead of these, a certain species of symmetry, of a vague yet very remarkable description, has been detected by Lord Rosse, probably for the first time. It is a spiral arrangement of the nebulous coils round a centre, resembling somewhat the spiral emanations of revolving fireworks. The well-known nebula No. 51 of Messier's Catalogue shows this in a remarkable manner.

Some observations have been made upon the moon. It is much to be desired that these were continued, and that the planets could also be observed; but I believe that the climate of Parsonstown affords but few nights favourable to observation.

(303.)

HENDERSON AND BESSEL. Parallax and distance (304.) Henderson

of the Fixed Stars.-THOMAS HENDERSON, at one -his birth time government astronomer at the Cape of Good and characHope, and subsequently professor of practical as-ter. tronomy in the university of Edinburgh, and his