comet, which continued visible 70 days. The tail is said to have covered a fourth part of the heavens, and occupied four hours in rising.

43 B.C.

In this year, during the celebration of the games held in honour of Venus, a great comet appeared at Rome, which was visible before sunset. This celestial prodigy was supposed to have some connection with the departed soul of Julius Cæsar, whose assassination had shortly before occurred. 389 A.D. There appeared a comet, which is said to have almost rivalled Venus in brightness. The tail, which was of immense length, was curved like a scimitar. The apparition of so extraordinary a phenomenon excited universal terror.

891 A.D. Contemporary European writers allude to a great comet which appeared in this year. In the Chinese annals, wherein allusion to it is also to be found, it is stated that the tail was 100° in length. 1106. A great comet appeared, which was visible over all Europe. The tail is said to have resembled a fiery beam. According to Matthew Paris, it was visible in the day-time.

1264. This year was distinguished by the apparition of a magnificent comet, which is alluded to by several European writers, and is also mentioned in the Chinese annals. The tail is said to have attained a length of 100°.

1402. Two comets of extraordinary splendour appeared in the course of this year. The first became visible in the beginning of spring, and towards the close of March was so bright as to be visible in the day-time. The second comet of the year was not less conspicuous than the first, if we are to believe the statements of contemporary writers. The tail is said to have extended from the horizon to the zenith. It is stated, also, as in the case of the first comet, to have been visible in full daylight.

1456. A magnificent comet was visible throughout all Europe. The tail is said to have been 60° in length. The apparition of the phenomenon excited universal terror, in consequence of its being simultaneous with the capture of Constantinople by the Turks. With the view of averting the evil influence of its presence, Pope Calixtus II. ordered prayers to be offered up in all the Western churches; he also, in a famous bull, anathematised at once the Turks and the comet. It has been satisfactorily established in modern times that this was one of the early apparitions of Halley's comet.

1472. The comet of this year was undoubtedly the most splendid of the century. Towards the end of January it was visible in full daylight. In Europe Regiomantanus observed it. In China its successive positions with respect to the stars were also carefully recorded.

1531. An early apparition of Halley's comet. Observed in Europe by Peter Apian, at Ingoldstadt. An account of this apparition is also to be found in the Chinese annals.

1532. A comet appeared this year, which is stated by Cardan to have been visible in full sunshine.

1556. Apparition of a great comet, which has been supposed by some astronomers to be identical with the comet of 1264.

1577. The comet of this year is memorable in history from having furnished the data which enabled Tycho Brahé to demonstrate that the regions traversed by cometary bodies in general lie beyond the moon's orbit.

1607. An apparition of Halley's comet. The phenomenon was observed on the Continent by Kepler and Longomontanus, and in En land by the celebrated mathematician, Hariot. The head is said to have equalled in size the planet Jupiter, but to have shone by a pale and watery light. The tail, which was of a very conspicuous brightness, was about 7° long.

1618. The third comet of this year was one of the most splendid of which history makes mention. Longomontanus states that the tail was 100° long.

1652. Apparition of a conspicuous comet, which is minutely described by Hevelius. 1664-5-8. Each of these years was distinguished by the apparition of a comet of considerable brightness.

1680. The comet of this year is, for several reasons one of the most remarkable of ancient or modern times. It was first seen by Godfrey Kirch, at Coburg in Saxony, on the 14th of November. After its passare of the perihelion on the 20th of December, it shone with great splendour, the tail appearing in some places to extend over an arc of 90. This comet approached nearer the sun than any other comet recorded in history, with the exception of the great comet of 1843. It has been already stated that the observations of this comet furnished the data by means of which Newton was enabled to demonstrate that the orbits of comets are conic sections, having the sun situate in their common focus.

1744. This was the most brilliant comet of the 18th century. It was discovered at Haarlem by Klinkenberg, on the 9th of December, 1743. On the 7th of February the tail was 20° in apparent length. On the 1st of March, when the comet passed the perihelion, it was seen in full daylight. Remarkable physical changes were observed to occur in the head of this comet, on the occasion of its approach to the perihelion. 1759. An apparition of Halley's comet.

1769. This comet is memorable for the immense tail by which it was accompanied. Its passage of the perihelion took place on the 8th of October. On the 10th of September its tail appeared at Paris to be

ARTS AND SCI. DIV. VOL. III.

60°. According to Pingré, the apparent length of the tail in tropical countries measured 97°. 1807. The comet of this year was very conspicuous to the naked eye. It was first discovered at Castro Giovanni in Italy, on the 9th of September, by Parisi, an Augustine monk. The passage of the perihelion occurred on the 19th of the same month. This comet was carefully observed by Sir William Herschel, who, in a paper published in the 'Philosophical Transactions of the Royal Society' for 1808, has recorded many interesting facts respecting it.

1811. The first comet of this year is in many respects one of the most remarkable of modern times. It was discove ed. by M. Flaguerges, at Viviers, on the 26th of March. The passage of the perihelion took place on the 12th of September. From that time till the end of the year it formed a very conspicuous object in the heavens, the effect being enhanced by the circumstance of its apparent path lying so near the North Pole that it always remained above the horizon. It was last seen in Siberia, by Wisniewski, a Russian astronomer, on the 17th of August, 1812.

1835. An apparition of Halley's comet.

1813. One of the most splendid comets recorded in history. It was seen with the naked eye, close to the sun, in Italy, the Cape of Good Hope, and America, on the 28th of February, the day of its passage of the perihelion. In some places the tail was observed to extend over an arc of 65°. It generally disappeared from observation about the beginning of April. This comet is remarkable for having approached nearer the sun than any other comet of modern times.

1853. A very fine comet appeared in the autumn of this year. Throughout Europe it was distinctly visible to the naked eye shortly after sunset.

1858. The comet discovered by Donati on the 4th of June in this year, is one of the most splendid of which history makes mention. It first became generally visible to the naked eye on the 5th of September. The passage of the peribelion took place on the 30th of that month. The comet attained its greatest slendour about the 10th of October. The tail then appeared to extend over an arc of about 40°. The comet ceased to be visible in Europe about the 20th of October, but it continued to be observed by Mr. Maclear at the Cape of Good Hope till the beginning of March in the following year.

Theory of the Movements of Comets.

The theory of the movements of comets resolves itself into two great subjects of research. One of these relates to the determination of the orbit of a comet from a definite number of observed positions, supposing it to revolve in a conic section around the sun; the other takes cognizance of the effects of planetary perturbation upon its motion. Newton's method for determining the elements of a comet's orbit was founded on the hypothesis of its revolving in a parabola. His opinion indeed was, that all comets revolve in very elongated ellipses; but he remarked, that in any of such cases the orbit near the perihelion does not deviate sensibly from a parabola. By supposing the path of the comet to be parabolic, the investigation of its elements is considerably simplified; but even with this assumption the problem is one of the most difficult in astronomy. It is plain, also, that the parabolic hypothesis cannot assign the major axis of the orbit, nor consequently the time of revolution. Newton, as already stated, remarked that the time of revolution might be found by comparing the intervals which elapsed between the apparitions of comets having the same parabolic elements. For this purpose, it is necessary to form a catalogue of the elements of all those comets the orbits of which have been computed; then, when a new comet has been observed, and its parabolic elements calculated, a reference to the catalogue will serve to indicate whether it has been observed on any former occasion; and if the newly calculated elements should thus turn out to be identical with those of any comet in the catalogue, the interval between the passages of the perihelion will give the time of revolution, supposing the two apparitions to be consecutive. In this way Halley determined the time of revolution of the comet which bears his name; and the same mode of ascertaining the periodicity of a comet is, in consequence of its easy application, constantly practised in the present day. The method most commonly used for computing the parabolic elements of a comet is one invented by the German astronomer Olbers, towards the close of the last century.

It is plain that the determination of the elements of a comet's orbit upon the parabolic hypothesis, is subject to the defect of not giving the major axis by direct investigation. In order to ascertain this element, it is necessary that the comet should have been observed at two consecutive passages of the perihelion. Nay, it may happen that, although the comet has been observed on more than one occasion, the apparitions may not be consecutive; and yet there exists no criterion by which a definitive conclusion on this point may be arrived at. Geometers have accordingly investigated methods for computing the elements of a comet's orbit, independently of any hypothesis with respect to the species of conic section in which it may be revolving. In this branch of research, Laplace and Gauss have laboured with eminent success. According to the method devised by the latter geometer, the six elliptic elements of a comet's orbit may be derived from three (in some cases four) observed positions of the body.

When a comet has once been discovered, three observed positions generally suffice, by the aid of Olbers's method, for ascertaining the

F

parabolic elements of its orbit. If the places calculated from these elements satisfy all the subsequent observed places of the comet within the limits of the errors of observation, or if the observations can be satisfied by any possible correction of the original elements, it may be concluded that the comet revolves in an orbit which is sensibly parabolic. But if the observed places exhibit a systematic deviation from the corresponding results assigned by the parabolic elements, an indication is thereby afforded that the comet really moves in an ellipse or an hyperbola, and the orbit may be investigated de novo by means of Gauss's method. In this way the six elliptic elements of a comet may be at once obtained, by a process founded on merely three observed positions of the body.

The earliest general method for computing the effects of planetary perturbation on the movements of comets is due to Lagrange. The process devised by that geometer is founded on the application of mechanical quadratures to the theory of the variation of elements. The orbit of the comet is divided into a number of distinct sections, and the influence of planetary perturbation upon the elements is separately computed for the individual ares. In applying this process to each fresh elements are obtained which are employed in the computations of the arc immediately following. In regard to many comets, the influence of planetary perturbation is sensible only in the vicinity of the perihelion, and in such cases it is only there that the application of the method becomes necessary. It may be remarked that Lagrange's theory of cometary perturbation has served as the basis of all subsequent researches on the subject.

arc,

Periodic Comets.

§ (1) Comets which have returned to their perihelia since the establishment of their periodicity. Halley's Comet.-An account has already been given of the circumstances connected with the perihelion passage of this famous comet in 1759. The next return to the perihelion occured in the year 1835. In 1812, the Academy of Sciences of Turin proposed the investigation of its perturbations as the subject of a prize. Damoiseau, an eminent French geometer, was on this occasion the successful competitor. He found that the comet would pass through the perihelion on the 4th of November, 1835.

In 1829 M. De Pontécoulant obtained the prize of the Academy of Sciences of Paris, for his researches on the same subject. The result of his first investigation indicated that the comet would pass the perihelion on the 7th of November, 1835; but on subsequently taking into account the disturbing action of the earth, and employing more accurate values of the masses of the other disturbing bodies, he found that the passage of the perihelion would take place on the 16th of November.

The perturbations of the comet on the occasion of the same perihelion passage, also formed the subject of elaborate investigations by Rosenberger and Lehmann, two German mathematicians. Rosenberger found that the comet would pass through the perihelion on the 11th of November; according to Lehmann the passage would take place on the 26th of the same month.

It was expected by astronomers that the comet would become visible about the beginning of August. This conjecture received a satisfactory confirmation. The comet was first discovered on the 5th of August, at the Observatory of Rome, by MM. Dumouchel and De Vico. Towards the end of September it became visible to the naked eye. It attained its greatest brilliancy about the middle of October. The head then resembled a star of the second magnitude. The tail exhibited an apparent length of about 20° in the countries of Northern Europe, but in southern climates it was observed to extend over an are of 30°. The comet was not much seen in the northern hemisphere after the middle of November, having been shortly afterwards lost in the sun's rays. Early in the following year it was seen at the Cape of Good Hope, by Sir John Herschel and Mr. Maclear, and continued to be observed till the 12th of May.

The most complete investigation of the elements of the comet, founded on all the most trustworthy observations made in 1835-6, is due to Westphalen, a German astronomer of great promise, who shortly afterwards died at an early age. In order to exhibit the accordance which existed between theory and observation in this instance, we subjoin the elements of the comet as assigned respectively by Pontécoulant and Westphalen.

The mean of these periods is 761 years. The deviations from this result represent the effects of planetary perturbation.

Encke's Comet.-This comet, as in the case of the famous comet of Halley, had been observed on several occasions of its return to the perihelion, before its periodicity was established. On the 26th of November, 1818, a comet was discovered by Pons at Marseille, the parabolic elements of which were soon afterwards found to resemble those of comets observed in 1805, 1795, and 1786. M. Encke was induced by this circumstance to investigate an elliptic orbit for the comets, and he found the time of revolution to be somewhat more than 1200 days. In 1822, on the occasion of its next return to the perihelion, it was rediscovered by M. Rümker, at Paramatta, in New South Wales. Upon tracing back its motion it was found to be in reality identical with the comets of 1805, 1795, and 1786, but a comparison of the earlier with the more recent perihelion passages seemed to indicate that the time of revolution was gradually becoming shorter. Professor Encke was induced in consequence to suspect the existence of a resisting medium, and adopting such an hypothesis, he calculated beforehand the perihelion passage of 1825. The results were found to present a satisfactory agreement with those derived from observation, and on every subsequent occasion of the comet's return to the perihelion, its motion has been successfully computed beforehand by Professor Encke, on the supposition of a resisting medium. The following table, extracted from a paper by Professor Encke, exhibits the gradual shortening of the time of revolution, as indicated by observation. This remark does not apply to the perihelion passages corresponding to the periods 1786-95, 1795-1805, and 1805-19, which were not observed, and are therefore necessarily the results of calculation.

These results indicate that the times of the successive revolutions are gradually shortening at the rate of about ths of a day, or somewhat more than two hours and a half. No other comet has hitherto offered any evidence of a similar shortening of the time of revolution. The existence of a resisting medium cannot therefore be considered as established beyond doubt.

It might be supposed that the effect of a resisting medium would be to lengthen the time of the comet's revolution, rather than to shorten it as observation indicates. It is true, indeed, that the direct effect of such a resistance is to retard the orbital motion of the comet, and so far to prolong the time of revolution; but on the other hand, this diminution of the tangential motion allows the central body to act with greater efficacy in drawing the comet towards the centre. Now, according to the theory of central forces, the nearer a body approaches the centre of force the less must be the time of a complete revolution. The following are the elements of this comet corresponding to the perihelion passage of 1858:

Available for 1858, Oct. 185, Berlin mean time and mean equinox.

Biela's Comet.-This comet was discovered on the 27th of February, 1826, by Biela, an Austrian officer residing at Josephstadt, in Bohemia. Parabolic elements of its orbit were shortly afterwards calculated, and the results are found to bear a strong resemblance to the elements of comets observed in 1805 and 1772. This circumstance induced Gambart, the director of the observatory of Marseille, and Clausen, assisting astronomer at the observatory of Altona, simultaneously to compute elliptic elements of the comet's orbit. The results obtained by those two astronomers presented a satisfactory agreement with each other, and represented the observed places of the comet much better than the elements originally computed. The time of revolution was found to be nearly 66 years. Professor Santini of Padua computed the effects of planetary perturbation for the next return to the perihelion. By a discussion of the observations made subsequent to the discovery of the body, he found that it passed through the perihelion on the 18th of March, 1826, and that the corresponding time of revolution was 2455-176 days. Computing, then, the disturbing forces of the Earth, Jupiter, and Saturn, he found that the effect of their combined influence would be to shorten the time of revolution by 10.023 days, and that the comet would consequently pass through the perihelion on the 27th of November, 1832. It is a curious fact, that this comet, a little before its arrival in the perihelion, passes through the descending node of its orbit at only a very short distance from the earth's orbit. Great fears of a collision of the two bodies were consequently entertained, when it was announced as the result of astronomical calculation, that at the instant of the passage of the comet through its descending node on the 29th of October, the earth would be travelling in the same region. However, an exact computation of the earth's motion relatively to the comet had the effect of dispelling these apprehensions; for it was found that, although the comet would pass from the north to the south of the ecliptic on the 29th of October, the earth would not arrive in the same heliocentric longitude before the 30th of November.

The comet, on the occasion of its re-appearance, was first perceived on the 23rd of August by the observers of the Collegio Romano at Rome. Its passage through the perihelion took place within a few hours of the time fixed by the calculations of Professor Santini. The next passage of the perihelion took place in 1839, but the circumstances of its motion being unfavourable for observation, the comet passed unperceived. The results of Professor Santini's calculations showed that the next perihelion passage would take place on the 11th of February, 1846. On this occasion the comet did not pass unobserved. It was re-discovered, independently, on the 28th of November by Professor Encke, at Berlin, and Signor De Vico at Rome. During the interval of its visibility it underwent a singular change, having separated into two distinct fragments which continued to travel together at a distance of 3′ or 4′ from each other. This singular phenomenon appears to have been first unequivocally observed on the 12th of January, 1846, by Lieut. Maury, of the Observatory of Washington, (U.S.). One of the comets was considerably fainter than the other. Both bodies were seen for the last time on the 16th of April, 1846. Professor Plantamour computed the elements of the orbit described by each comet, taking into account the perturbations produced by the Earth, Jupiter, and Mars. The places of the two bodies when computed from these elements, were found to agree very nearly with the observed places. Professor Plantamour determined the mutual distances of the two bodies, and obtained the following results :—

The comet was again observed on its return to the perihelion in 1852. The appearance which it presented on that occasion continued to afford evidence of the disruption which it suffered in 1846, both fragments being still visible. Circumstances do not seem to have been favourable for its re-discovery at the time of the perihelion passage of 1859.

Faye's Comet.-On the 22nd of November, 1843, a comet was discovered by M. Faye, at the Royal Observatory, Paris, the observations of which it was found impossible to satisfy by a parabolic orbit. Dr. Goldschmidt was induced by this circumstance to compute an elliptic orbit for the comet, and obtained results which agreed very well with the observed motion. The time of revolution is 2718 days, or 7:44 years. The comet has been re-observed on the occasions of its return to the perihelion in 1851 and 1858.

Brorsen's Comet.-On the 26th of February, 1846, M. Brorsen discovered at Kiel, in Denmark, a small telescopic comet, which was speedily found to revolve in an elliptic orbit, the time of revolution being about 56 years. It was not observed on the occasion of the passage of the perihelion in 1851, but it was found at the next return to the perihelion in 1857.

D'Arrest's Comet.-This comet was discovered by Professor D'Arrest, at Leipsic, on the 27th of June, 1851. It was speedily found to

revolve in an elliptic orbit, the period being about 64 years. The circumstances of the next perihelion passage were calculated by M. Villarceau, of the Imperial Observatory, Paris; and by the aid of an ephemeris, due to that astronomer, the comet was re-discovered at the Cape of Good Hope by Mr. Maclear, on the 5th of Decembe, 1857. The comet continued to be observed till the 18th of January, 1858. During the whole period of its visibility it presented a very faint aspect. § (2) Comets which have been found to revolve in elliptic orbits, but which have not been re-observed since the discovery of their periodicity. A considerable number of comets in addition to those contained in the foregoing list, have exhibited traces of a deviation from parabolic motion, and elliptic orbits have in consequence been calculated for them, the results derived from which have been found to satisfy the observations with a greater or less degree of precision. We proceed to notice briefly the results relative to the periodic time which have been obtained for a few of the more interesting of those bodies.

Comet of 1680.-Halley was of opinion that this comet was identical with comets which appeared in the years B.C. 43, A.D. 531, and 1106; and he hence inferred a period of 575 years. Professor Encke, however, has found by an investigation, based upon all the recorded observations, that the most probable value of the periodic time is 8800 years; but he remarks that in consequence of the large probable errors of the data, the observations of the comet may be tolerably satisfied by an ellipse with a period of 805 years, or even by an hyperbolic orbit. The observations of Flamsteed and Newton alone indicate a period of 3164 years.

Lexell's Comet.-This comet was discovered by Messier in 1770. Astronomers having been unable to satisfy the observations by parabolic elements, an elliptic orbit was computed by Lexell, who found the periodic time to be somewhat more than five years. The comet was carefully searched for on the occasion of the next two expected returns to the perihelion, but it was not discovered in either instance, nor has any trace of it been ever since obtained. Lexell endeavoured to account for this curious circumstance by remarking that previous to 1770 the comet had always been invisible, but that having passed very close to the planet Jupiter in the year 1767, it was thrown into a new orbit, and rendered visible; and that, in 1779, having again approached very near to the same planet, it was thrown again into a new orbit, and thereby rendered invisible. The researches of Lexell were subsequently confirmed by Laplace, but Le Verrier has in recent times called in question some of the data on which they rest. Comet of 1811.-The orbit of this famous comet has formed the subject of an elaborate investigation by Argelander, who found it to revolve in an ellipse, with a period of 3065 years. This result must be understood, however, as corresponding only to the time of the perihelion passage. Argelander has shown that the disturbing forces of the planets must exercise a very considerable influence on the time of the next perihelion passage. By computing the perturbations up to May, 1827, he found that the time of revolution would be shortened from that cause to the extent of no less than 177 years.

Comet of 1815.-This comet was discovered on the 6th of March, 1815, by the celebrated astronomer Olbers. Bessel subjected the observations to a thorough discussion, when he found the comet to revolve in an ellipse, with a period of 74 years. Taking into account the effects of planetary perturbation, which he found would accelerate its movement to the extent of two years, he finally ascertained that the comet would again arrive in the perihelion in the month of February, 1887.

De Vico's Comet.-On the 22nd of August, 1844, De Vico discovered at Rome a comet which was found to revolve in an elliptic orbit, with a period of about five and a half years. It was carefully searched for at the perihelion passages of 1850 and 1855, but in neither instance has it been discovered.

Peters' Comet.-This comet was discovered by Dr. Peters at Naples, in 1846, and was found by him to revolve in an elliptic orbit with a period of 12.85 years. According to this result the comet ought to have returned to the perihelion in the year 1859, but hitherto it has not been found.

Winnecke's Comet.-On the 8th of March, 1858, Dr. Winnecke discovered a comet at Bonn, which he found to revolve in an elliptic orbit with a period of 5.549 years. He also established its identity with a comet observed in the year 1819. Supposing it to have made seven revolutions since that year, the time of a complete revolution would be 5541 years, a result agreeing almost exactly with that obtained by a direct investigation of the elliptic elements.

Donati's Comet.-The observations of this famous comet appear to be best satisfied by an elliptic orbit. According to Professor Stampfer of Vienna, the period of revolution is 2141 years. However, until the totality of the observations shall have been subjected to a complete discussion, it will be impossible to arrive at any definitive result on this point.

Number of Comets.

It is impossible to form any opinion with respect to the number of comets which are liable to visit our system. Multitudes of those bodies, whether from their faintness or the circumstances of their movements, will doubtless for ever elude observation. Arago gives

7 apparitions of Halley's comet.

approach the sun :

Date of Apparition

of the Comet.

14 apparitions of Encke's comet.

6 apparitions of Biela's comet.

2 apparitions of Faye's comet.

46 apparitions of comets revolving in elliptic orbits, or of which two passages of the perihelion may perhaps have occurred.

151 apparitions of comets revolving in parabolic orbits.

The following statement of the progress of cometary discovery during the five years which have elapsed subsequently to the year 1853, will enable the reader to form an opinion with respect to the activity which pervades this department of astronomical science :

1843

1680

1689

1826

1847

1816

Perihelion Distance. 475,000 miles 570,000

1,900,000

2,565,000

3,990,000

4,560,000

Newton found by calculation, that the comet of 1680, on its passage

of the perihelion was subjected to a heat 2000 times greater than that of red-hot iron. The great comet of 1843, which approached nearer the sun than any other comet recorded in history, must have been exposed to a heat of still greater intensity. Laplace, availing himself of Black's beautiful discovery of the principle of latent caloric, considered that the heat abstracted by the cometic particles in the course of passing into the vaporous state would serve to moderate the effect of the solar heat at the perihelion, and upon this ground he concluded, that the nucleus of a comet is not necessarily a solid body.

When one of the more conspicuous comets is advancing towards the perihelion, it is seen to undergo a succession of changes in the head action of the sun, appear to have been first remarked by Hooke in the and tail. These singular phenomena, which are evidently due to the

course of his observations of the comets of 168 and 1682. The following extract from his observations of the comet of 1682 (an apparition of Halley's comet), will enable the reader to form some idea of the through the perihelion on the 15th of September:It is right to state that the comet passed changes to which we refer.

August 26. At seven in the evening I delineated the figure and shape of the comet, exactly like that I saw through my fourteen-feet telescope, which will appear more plain by the fifth figure than I can otherwise well express it.* It had a pretty bright round nucleus, and about that was an atmosphere of thinner light which was terminated towards the sun with a round figure. That part of this halo, or lighter atmosphere towards the sun, was not so bright or radiant as another kind of light which seemed to issue from the nucleus or star both ways at right angles with the axis through the sun, which lighter issuings bent into a kind of parabolic figure, within the former halo or atmosphere, and was terminated within it, and seemed to form, as it were, a second parabolical termination towards the sun, in the apex of which parabola was the bright nucleus, and this brighter parabolic line of light seemed as gross or thick as the nucleus itself. This issued on both sides, but that on the right hand, or the northernmost, was much more conspicuous; insomuch that that on the left hand, or towards the south, was to be seen but sometimes, but that on the other side was very plain and conspicuous, and seemed like a stream of flame

Comet of 1680

Comet of 1769

Comet of 1811 (October 15)

Comet of 1843

Great comet of 1858 (Donati's).

Length of Tail.

Physical Constitution of Comets.

The substance of which a comet is composed appears to be of remarkable tenuity. This has been abundantly proved by the circumstance of the smallest stars being seen through their structure without undergoing any sensible diminution of light. Sir John Herschel, in a paper on Biela's comet, published in vol. vi. of the Memoirs of the Astronomical Society,' has mentioned a fact which affords a striking illustration of the translucency of cometic matter. The comet having passed over a small cluster of stars of the sixteenth or seventeenth magnitude, the appearance presented was that of a nebula, partly resolvable into stars. The most trifling fog would have effaced the stars; but in the present instance they still continued to be visible, although the cometic matter interposed between them and the observer must have been at least fifty thousand miles in thickness.

The question whether the nucleus of a comet is in any case a solid body has been often discussed, but no definitive conclusion has been arrived at. The passage of the nucleus of a comet over a star might be supposed to supply a useful criterion for deciding this point, but no instance of the actual occurrence of such a phenomenon has ever been satisfactorily established. Newton was of opinion that the nuclei of comets must necessarily be solid bodies, since otherwise they would in many cases be dissipated in space by the intense heat to which they are subjected on the passage of their perihelia. But whether the nuclei of comets be solid or not, it is certain that their masses must be very inconsiderable. This is evident from the circumstance of their producing no sensible derangement in the motions of the planets, however near they approach them. In the year 1779 Lexell's comet passed through the middle of the system of Jupiter's satellites, but none of those bodies appeared to be in the slightest degree affected by its attractive force.

Allusion has been made to the great heat which many comets must undergo on their passage of the perihelion. The following table of

just as such a blown flame of a candle will do, if it be made by a gentle blast. This I remarked very carefully, to see whether I could find, by any succeeding observations, any alteration of the magnitude, figure, brightness, or position, in respect of the comet's axis. These two bright spoutings of flame or light turned or bent upwards from the sun, and after a short space seemed to unite into the axis or middle of the blaze, and form the shape of the outside of a flame of a candle tapering to a point: the fainter part also without it seemed to taper much in the same manner. I saw also several coruscations or flashings of the flame, shooting out to a great distance into the blaze."

Phenomena of a similar nature were remarked by Heinsius in the course of his observations of the great comet of 1744. In more recent times, Halley's comet (1835), and the great comet of 1858, exhibited analogous changes previous to their passage of the perihelion. It has been already stated that Halley's comet passed through the perihelion on the 15th of November. Previous to the 2nd of October, the appearance which it presented was that of a round nebulous disk, with a faint nucleus in the centre. On that evening, however, the nucleus became exceedingly bright, and there was seen to issue from it a cone of light, which first extended a short distance in the direction of the sun, and then bent back as if impelled by some intense force in the opposite direction. This outstreaming cone of light continued to be seen until the 22nd of October, subject however to violent changes when observed from night to night. It is worthy of remark that simultaneously with these changes the tail was observed to increase gradually in length. The phenomena observed during the apparition of the great comet of 1858, were of a still more complex nature than those which characterised any previous comet, but it would be out of place here to enter into any minute details. On the 16th of September there commenced a series of luminous emissions from the nucleus, which continued till the passage of the perihelion at the end of the month. These were followed by a succession of envelopes of a paraboloidal form surrounding the head, and which were seen in greater or less number at the various observatories throughout Europe and America till the disappearance of the comet about the 20th of October. The development of the tail when a comet is advancing towards the perihelion affords also a striking indication of the action of the sun,

These drawings are given in Hooke's Posthumous Works.'

ness of the comet.

sun.

39,000,000 14,000,000

The shortening of the tail between the 12th and 17th of September is due entirely to the effect of moonlight. To the same cause may be partly attributed the change from the 10th to the 15th of October, but it is no doubt mainly due to the rapid diminution of the brightNumerous theories of the origin of the tails of comets have been proposed from time to time, but as they are all open to serious objections we do not think that any useful purpose would be served by giving an account of them on the present occasion. It is plain from the observations of the more conspicuous comets, that the tail is fed by the matter raised from the nucleus by the action of the There would appear to be in this case two forces acting upon the cometic particles, independently of the force of gravitation. First, we have indications of a force violently ejecting the particles to a short distance from the nucleus; and secondly, we have equally clear evidence of a repulsive force of great intensity directed upon the comet from the sun, and driving the particles so ejected to an immense distance in space. The most probable view of the nature of these powerful forces is that which attributes them to electrical agency, but no satisfactory theory of the subject has yet been advanced by any inquirer. COMITIA. Comitium originally signified a place of meeting, as the name imports. Varro, De Ling. Lat.'l. v., s. 155. Plutarch (Romulus,' xix.) says that the plain where the Romans met the Sabines, in order to agree on the terms of a treaty, was called "comitium," and Niebuhr, History of Rome' (i. 291, ed 1851), in narrating the history of the union of the two towns of Roma and Quirium, and accounting for the steps by which the union was effected, mentions the old legend of a place of meeting for the Roman and Sabine kings and senates, called comitium from the fact of its lying between the Palatine and Capitoline hills. Whether it was intended to preserve a record of the old place of meeting, or was simply used as a topographical description must be left to conjecture. The word was retained and applied to a particular part of the Forum, where for many a day the remembrance of the old rivalry was preserved by the two statues of Romulus and Tatius that were erected in it. The plural "comitia" denotes general assemblies of the Roman people, convened by the constitutional authority of some magistrate, in order to enact or repeal anything by their suffrages. One set of comitia was named "calata," from the old word calare, to call or convene (A. Gellius, 15, 27), where the people were summoned to be witnesses to certain solemn acts, or certain things then announced to them.

There were three kinds of Roman comitia :

1. Curiata, so called because the people met and voted in curia. Romulus, it is said, divided the whole Roman people into three tribes, and each tribe into ten curiæ, which were subdivided into decads, being, as Niebuhr contends, the same as houses, so that each curia containing 10 houses, the 3 tribes numbered 300 in all. Now, as no houses but those which composed the three ancient tribes were essential parts of the state, in consequence of which the patricians could boast that they alone (gentem habere) had a house (Livy x. 8), this division, so essential to the patrician order, was in close connection with it, and therefore, when the political importance of the plebeians rose, the curiæ sank and, except in the continued observance of their sacra, for some years after their political degradation, fell into oblivion. The word curiæ is derived from curare, to take care of or superintend civil and religious affairs (Varro, ' De L. L.' v. 165, and vi. 46); though another and somewhat plausible etymology is that which connects it with the Sabine word Quiris or Curis. (See Smith's Latin Dictionary,' sub verbo.) Each curia formed a separate community for the celebration of sacred rites, for which purpose a particular priest, called curio, was attached to each curia, and a decurio, or captain and burgess to each decad or house. But all the curia were under the superintendence of a curio maximus. A separate place

which was also called curia, was assigned to each curia for performing its sacred rites. The members of a curia were called curiales. There is some obscurity and doubt about the ancient constitution of the curia and comitia curiata. However, it seems certain, that the curiæ had the superintendence of sacred matters, that all the public power was united and centralised in the comitia curiata, and that the patrician order must have possessed a great preponderance in them. (See Niebuhr's Rome,' vol. i., on the Curies.') In these comitia laws were made or repealed, peace or war declared; (as to treaties of peace, however, see 'Dict. of Gr. & Rom. Antiq.' Comitia, p. 332 b), the affairs of the curiæ and gentes or houses decided, capital crimes judged, and the king as well as the other chief magistrates of the state elected. The place of meeting (comitium) was in the forum, and in its northern corner were the rostra. There was no fixed time for the meeting of the curiæ, but they met as business required, and were held in the presence and under the protection of the priests, their president being the king, or an interrex in the ante-republican times, and some high patrician magistrate, a consul, prætor, or dictator, under the Republic, while none but the populus or the patrician members of the curiæ had a right to take part in these assemblies.

Servius Tullius having instituted the comitia centuriata, and the plebeians becoming powerful through the comitia tributa, the comitia curiata gradually lost almost all political power. However they still institution of the comitia centuriata, denoted every law made by the passed enactments under the title of leges curiata, which, before the comitia curiata; but afterwards that term was limited to express a few political rights, still reserved to the latter comitia, particularly that of granting military power (imperium) to those magistrates who were elected in the comitia centuriata, which could only confer civil power (potestas). Finally, the political influence of the comitia curiata was reduced to a mere formality, and represented, in Cicero's time, by thirty lictors. Still, a shadow of the old institution was preserved in the continuance of the patrician comitia calata used principally for the adrogationes. Though their political power was lost, the curiæ retained their religious functions till the last times of the republic, and always elected the curio maximus and the flamens. Their number was never augmented, as was the case with the tribes. Before proceeding to the second class of comitia, a few words on the subject of a peculiar authority possessed by the comitia curiata will not be inappropriate, for a more full account of which the reader is referred to Niebuhr's Hist, vol. i. ch. 21. The election of the kings was, it is known, in the hands of the curies, but in addition thereto, to this body belonged the conferring the imperium (Cic. De Rep.' ii. 13): hence they not only could elect, but they could annul that election: the first it is said was done by the populus, the second by the senate or patres. and to effectuate that second decision, a law was passed by the patres, called by Cicero lex curiata de imperio, and by Livy, auctoritas patrum (Livy i. 17). The conclusion that Niebuhr draws from this identity of the auctoritas patrum, and the lex curiata de imperio is, that the comitia of the curies and the assembly of the patricians were identical.

2. Centuriata. Servius Tullius, according to tradition, in order to diminish the power of the patricians, and to elevate the plebeians without giving them any power, made a new division of the Roman people into six classes, which were subdivided into centuries or votes. There has been much dispute about this division and the number of the centuries; and the controversy scarcely admits of decision, as the ancient writers (Livius, i. 43, Dionys. Halicarn., Antiq. Rom.,' i. 19-22, and Cicero, ' De Republica,' ii. 22) are of different opinions. But the nature of the institution is not so doubtful. According to the more probable opinion (that of Dionys. Hal.), the 6 classes contained 193 centuries. The first class consisted of 18 centuries of knights and 80 centuries of those (ditissimi) whose fortune amounted to at least 100,000 ases; the second class (ditiores) contained 22 centuries, and consisted of those who possessed at least 75,000 ases; the third (divites) 20 centuries, and consisted of those who had a property of 50,000 ases at least; the fourth class (mediocres) 22 centuries, of those who possessed 25,000 ases at least; the fifth class (modici) 30 centuries, of those who possessed 12,500 ases; the sixth class contained but one century of capite censi, that is, persons counted by head and not by estates: they were also called proletarii, or ærarii.

According to this division the Roman people met in the comitia centuriata, in order to vote in centuries on public matters; that is, a decree of the assembly was made by counting the votes of the centuries. As the first class alone contained more centuries than all the other classes together, it may be said that, as Romulus had created an aristocracy of birth by his division of curixe, so Servius Tullius created an aristocracy of fortune by his new division. In order to prevent that disadvantage, when the plebeians had obtained more power, the century which was to give its suffrages first was appointed by lot. The century upon which the lot fell was called prærogativa. The other centuries voted according to the order of their classes, and were called jure vocatæ. The decision by lot being regarded as a divine omen, the centuria jure vocata commonly followed the vote of the centuria prærogativa; and thus the power of the first class was balanced in some measure. A contest however sometimes arose whether a matter was to be decided in the comitia centuriata or tributa.

Every Roman citizen in the best sense of the words (civis optimo jure)