Slow improvement in the beliefs relative to comets–Bayle's remarks upon the comet of 1680–Passage from Madame de Sevigné's letter referring to this comet and the last hours of Mazarin–In the eighteenth century belief in the supernatural exchanged for belief in the physical influence of comets–Remains of cometary superstitions in the nineteenth century–The comet of 1812 and the Russian campaign ; Napoleon I. and the comet of 1769; the great comet of 1831 in Italy.

We have just seen that the superstitious ideas of the Middle Ages were yet dominant in the height of the Renaissance, since a man of learning like Ambrose Paré–no astronomer, it is true–could attribute to comets the same malign influences as those ascribed to them in- the year 1000, when the end of the world was confidently expected. Nor could it be otherwise, science not having then assigned to comets, in common with other extraordinary meteors, their true place in the order of nature.

Little by little, however, healthier ideas make their way, and to the supernatural influence of comets we shall now see gradually succeed in the minds of men of science and the more enlightened of the people the idea of an influence purely physical, at first under the form of simple hypotheses, and afterwards as a probability deduced from observations and facts.

Experiments and hypotheses of Tyndall–Originality of his theory; objections and omissions–Is this theory incompatible with that of a repulsive force?

A new theory of cometary phenomena which has been proposed by Professor Tyndall, one of the most distinguished of contemporary physicists, in our opinion merits special attention. In the first place, because we believe it to be altogether new and original ; and, in the second place, because it is derived, not from à priori conceptions, like so many other theories in astronomy and physics, but from accurate experiments and their interpretation.

The study of the action of radiations upon very rarefied media of gaseous matter first led Professor Tyndall to consider the mode of production of the phenomena presented by the heads and tails of comets. Of the undulations proceeding from any luminous source, such as the sun, some have a purely calorific action ; these are those which have the greatest amplitude or are least refrangible; the undulations which constitute or produce light come next in the order of length of wave or refrangibility; the shortest waves are those which manifest themselves exclusively by chemical action.

Estimations of the apparent dimensions or brilliancy of comets–Ancient comets said to be brighter than the sun–Comets visible to the naked eye and comets seen at noonday; great comets of 1744 and 1843.

We will now enter into some particulars respecting the dimensions of comets, their atmospheres, nuclei, and tails. In order to form correct notions concerning this portion of our subject, it is important to distinguish between real and apparent dimensions. This is elementary, but it is here even more necessary than elsewhere, because, from the very nature of cometary orbits, the comet itself, whether periodical or nonperiodical, may be situated at the moment of its appearance either very near to or very far from the earth; so that on two successive apparitions the same comet may appear of very different aspect and dimensions, and at one time may present itself as a very conspicuous body in the sky, at another may be hardly visible, or perhaps not visible at all without the aid of a telescope. We have already alluded to this point when speaking of the difficulty of recognising the identity of a new comet with one before observed by its external aspect; and we must here call attention to it again, when we are comparing different comets in respect to their dimensions, either real or apparent.

Researches of Huggins, Secchi, Wolf, and Rayet–Spectra of different comets : bright bands upon a continuous luminous ground–Analysis of the light of Ooggia's comet in 1874–Chemical composition of different nuclei and nebulosities.

Physicists, it is well known, recognise three orders of spectra as produced by sources of light when a luminous beam emanating from these sources has been decomposed in its passage through a prism or a system of prisms.

A spectrum of the first order consists of a continuous coloured strip, exhibiting neither dark lines, nor bright bands separated by dark intervals ; it is, in fact, the solar spectrum, more or less brilliant in colour, and of more or less extent, but destitute of the fine black lines which belong to the spectrum of the sun. Incandescent solids or liquids produce these continuous spectra. Spectra of the second order are those which arise from sources of light composed of vapours or incandescent gas; they consist of a greater or less number of lines or brilliantly coloured bands, separated by dark intervals; the number, the position, and consequently the colours of these lines or luminous bands are characteristic of the gaseous substance under ignition. Every chemically simple body, every compound body which has become luminous without decomposition, has a spectrum peculiar to itself.

M. Chacornac's observations upon the comet of 1862–Formation of luminous sectors emanating from the nucleus–Oscillation of aigrettes, and flowing back of the nucleal matter.

We are now about to give our attention to the evolutions of the luminous sectors of the great comet of 1862, which, on the contrary, presented oscillations analogous to those exhibited by the aigrettes of Halley's comet. We shall follow the development of these phenomena by means of the observations of the late M. Chacornac.

On August 10, 1862, M. Chacornac detected in the head of the comet the presence of a luminous aigrette, a brilliant sector directed towards the sun. This sector, which at three o'clock in the morning included an angle of 46°, had, by two o'clock on the following day, opened ‘ like the corolla of a convolvulus, and included 65°. On the 10th the nucleus presented the appearance of a rocket, having a diameter much more extended in the direction of the radius vector than at right angles to it.’ It is worthy of remark that the contrary was the case with the nuclei of the comets of 1858 and 1861, which were flattened in the direction of the radius vector. On the 11th the two diameters were nearly equal. New sectors disengaged themselves successively from the nucleus, and on August 26 M. Chacornac determined that between the 10th and the 26th they had succeeded each other to the number of thirteen.

Possibility of our globe passing through the tail of a comet–Has such an event ever taken place?–The great comet of 1861–Relative positions of the earth and one of the two tails of that comet–Memoir of M. Liais and the observations of Mr. Hind.

Thus far, in treating of the possibility of a rencontre between a comet and the earth, we have more especially had in view the nucleus, or rather that portion of the comet's nebulosity which constitutes the coma. The effects of the rencontre have been studied on certain hypotheses respecting the mass and physical constitution of the comet whose nucleus we have supposed to be solid; this is far from certain, and, in any case, seems to be exceptional, as it is only in certain comets that the head is sufficiently condensed to exhibit a luminous nucleus.

A rencontre, of much greater probability, is that which would arise from the passage of the earth through the voluminous nebulosity of which the tail is formed. In all probability the masses of these appendages are all but inappreciable. Whatever opinion we may form of their nature, whether we regard them with Cardan and certain savants of our day as purely optical effects without material reality, or see in them the most tenuous portions of the atmosphere of the comet projected by a repulsive force, it appears certain that they consist of quantities of matter of extremely slight mass, and of even less density.

Is there any example in history of the division of a comet into several parts?–The comet of B.C. 371–Ephorus, Seneca and Pingre–Similar observations in Europe and China–The Olinda double comet, observed in Brazil, in 1860, by M. Liais.

The doubling of Biela's comet did not fail to direct attention to the several instances on record of analogous phenomena which had hitherto been looked upon as little worthy of belief. It was then remembered that Democritus had, according to Aristotle, related the fact of a comet having suddenly divided into a great number of little stars. It was this, perhaps, that gave rise to the opinion of certain philosophers of antiquity that comets were composed of two or more wandering stars. Seneca, in endeavouring to refute this opinion, mentions the account, given by Ephorus, the Greek historian, of the division of the comet of the year B.C. 371 into two stars. He thus expresses himself:–

‘ Ephorus, who is far from being an historian of unimpeachable veracity, is often deceived–often a deceiver. This comet, for example, upon which all eyes were so intently fixed on account of the immense catastrophe produced by its apparition– the submersion of the towns of Helice and Bura – Ephorus pretends divided into two stars. No one but himself has related this fact. Who could possibly have observed at what moment the comet dissolved and divided into two? Andbesides, if this division was actually seen to take place, howis it that no one saw the comet form itself into two stars ? Why has not Ephorus given the names of these two stars ?’

At a distance from the sun the nebulous agglomerations which constitute a comet preserve a spherical or globular form, a certain indication that their molecules obey the preponderating action of the nucleus. This form would be preserved if no foreign influence interfered to derange their mutual positions or to disturb the general equilibrium.

But the comet, when approaching its perihelion, is subjected more and more to the attractive power of the sun, whose enormous mass suffices to change the spherical form of the cometary nebula, to render it more and more ellipsoidal, and finally to carry away beyond the sphere of the attraction of the nucleus whole strata of the nebulosity. This is proved beyond a doubt, as we have seen, by the analysis of M. Roche. In addition to the action of the solar mass there is likewise the action of radiated heat from the sun, which determines changes of great importance : the emission of vaporous matter from the nucleus, luminous jets, aigrettes, and successive concentric envelopes. If the tails of comets, as everything leads us to believe, are material realities, and not simple visual effects; if they are molecules detached from the nebulosity and projected far beyond it by a repulsive force, we may say that, having passed beyond the preponderating action of the nucleus, they have for the moment become foreign to the comet itself, which has thus suffered a portion, however small, of its matter or its mass to escape.

Prevalence of popular superstitions–Comets announce wars, plagues, the deaths of sovereigns–Terrors of the year 1000; comets and the end of the world–Gian Galeazzo Visconti and the comet of 1402–-Ambrose Paré celestial monsters– Halley's comet and the Turks; origin of the Angelus de Midi–The comet of 1066 and the conquest of England by the Normans ; apostrophe to the comet by a monk of Malmesbury.

If a complete history were desired of all the superstitions which, during the Middle Ages and in modern times, have obtained with respect to comets, it would be necessary to pass in review every apparition of these stars, together with such incidental phenomena as the Aurora Borealis, new and temporary stars, bolides, &c, all of which have been converted by popular credulity into as many prodigies. Interesting in a scientific point of view, this long enumeration derived from the naïve chronicles of the time, the only documents available in the absence of a more complete and intelligent record, would be but a tedious study of human errors; a constant and monotonous repetition of the same absurd beliefs. To this state of things savants have themselves contributed, as at the epoch when these voluminous records were compiled cometary influences were still believed in, and the erudite of the day shared the universal prejudice.

Comets appear in all regions of the heavens–Effects of parallax–Apparent motion of a comet, in opposition and in perihelion, moving in a direction opposite to the earth–Hypothetical comet of Lacaille; calculations of Lacaille and Olbers concerning the maximum relative movement of this hypothetical comet and the earth.

The orbits which the planets describe about the sun are not circles, but oval curves, termed ellipses; these ellipses differ but little from circles; that is to say, their eccentricities are small. Moreover, the planes of the orbits in which they move are inclined at small angles to the plane of the ecliptic. Hence it follows that their apparent paths are confined to a comparatively narrow zone of the heavens, which zone is called the zodiac. If we imagine these curves pressed down, as it were, upon the ecliptic they will appear as nearly concentric circles described about the sun, and so disposed as not to intersect each other. The distances of the earth and of each of the planets vary according to the position occupied by these bodies in their respective orbits; but these variations are confined within very narrow limits, and hence it follows that the velocities of the planets change so slightly that the difference is all but imperceptible. The mean diurnal motion of Mercury, which of all the planets moves the most rapidly, amounts to only 4°5′

Predominance of atmosphere in comets–Luminous sectors; emission of vaporous envelopes from the nucleus in the comets of 1835, 1858, 1860, and 1861–Formation of envelopes in Donati's comet; progressive diminution of the velocity of expansion in emissions from the nucleus.

The planets, as seen through a telescope, are bodies of regular form and definite dimensions, probably invariable, so far as we can judge from observations made in the short period of two centuries and a half that has elapsed since telescopes have been invented. A globular mass, solid or liquid, surrounded on all sides by a light and comparatively thin aëriform envelope, is perhaps, from a physical point of view, the simplest description of a planet. The comparative stability is due, on the one hand, to the preponderance of the central globe, where general phenomena are modified only at long intervals ; and on the other to the trifling depth of the atmosphere, the portion of the planet the most subject to variation and internal change.

In comets, we have seen, this relation is reversed, and the atmosphere or nebulous envelope constitutes the entire body, or at all events greatly preponderates. At the utmost we can only conjecture that in some comets the nucleus is solid or liquid. Certainly its volume is generally but a very insignificant portion of the entire nebulosity, even if we except the tail.

Kepler's remark upon the number of comets–Comets observed–Comets calculated and catalogued–Conjecture as to the number of comets which traverse the solar system or belong to it; calculations and estimates of Lambert and Arago–Calculation of the probable number of comets from the actual data; Kepler's remark verified.

‘ Comets are as numerous in the heavens,’ said Kepler, ‘ as fishes in the ocean, ut pisces in oceano’. In quoting this comparison of the great astronomer we only follow the invariable custom of all the authors who have hitherto treated the question of the number of comets; but we remark that the expression employed by Kepler is only the result of an opinion which is little more than a conjecture, and that the words ought to be taken in their poetical rather than in their literal sense; but, making allowance for some exaggeration in the expression, we shall see that Kepler was justified in considering the numberof comets as very great.

Our inquiry, it is evident, must be confined to comets which are liable temporarily to traverse our system, or to revolve for ever about the sun as an integral part of the solar system. Any attempted estimate of comets situated outside this sphere, beyond our range of vision, and exterior to the planets which belong to our group, could not rest upon any certain data.

Study of the question by Arago–The calorific action of comets upon the earth appears to be inappreciable–Comparison of the meteorological statistics of various years in which comets did and did not appear–The meteorological influence of a comet is not yet proved by any authentic fact.

We have already said how general a consternation was created in 1832 by the announcement that Biela's comet would pass within a very short distance of the orbit of the earth. Arago made it the occasion of one of those brilliant and interesting notices in which he endeavoured to destroy existing prejudices, and to render the simple truths of astronomy better and more generally understood. The heading of one section of this notice was–

‘ Will the future Comet modify in any appreciable degree the Course of the Seasons of the year 1832?’ To this question he replies in the following terms :– ‘ The above title will doubtless call to mind the beautiful comet of 1811, the high temperature of that year, the abundant harvest following, and, above all, the excellent quality of the comet wine. I am therefore well aware that I shall have to contend with many prejudices in order to establish that neither the comet of 1811, nor any other known comet, has ever occasioned the smallest change in the seasons.

Opinions entertained by astronomers of the last century: Gregory, Maupertuis, Lambert–Calculations of Lalande ; comets move too rapidly in the vicinity of the earth for the effects of their attraction to come into play–Opinion of Laplace– The collision of a comet with the earth; its effect according to the mechanical theory of heat.

It is interesting to note the opinions formed by savants a century ago respecting the probable effect of a collision between a comet and the earth. Further on we shall speak of the theological romance invented by Whiston for the scientific explanation of the Deluge. According to Whiston the famous comet of 1680, after having, 4000 years ago, produced the universal deluge, is destined to accomplish the destruction of the world, and our globe will be ultimately set on fire by the same comet which had previously inundated it.

Whiston wrote at the end of the seventeenth century. In the middle of the eighteenth century theological speculations engaged but very slightly the attention of astronomers ; but that a very exaggerated idea continued to prevail respecting the amount of injury which the proximity of a comet or its collision with the earth would be capable of producing is undoubted.

In 1742 Maupertuis, in his Lettre sur la Comète, writes as follows: ‘ With their variety of movements it is clearly possible for a comet to encounter some planet or even our earth upon its way; and it cannot be doubted that terrible results would ensue.

Comets participate in the diurnal motion of the heavens. During the time of their apparition they rise and set like the sun, the moon, the stars, and the planets. In this respect, therefore, they do not differ from other celestial bodies.

Let the observer, when a comet is in sight, note the point in the heavens which it occupies when his attention is first directed to it. This is easily done by referring the nucleus, the brilliant point from which the tail proceeds, to two adjacent stars. Let a certain time elapse–an hour, for example; at the end of that time the three luminous points, the two stars and the comet, will be found to have changed their position with respect to the horizon, each having described an arc of a circle in the heavens. The common centre of these arcs is the celestial pole, a point situated within a very small distance of the pole-star; the lengths of these arcs depend upon the interval of time between the observations, and the angular distance of each body from the pole. The direction is that of the general movement of the heavens and the stars; that is to say, from east to west.

We have here, then, a fact which clearly teaches us that a comet moves in regions beyond the atmosphere of the earth; for the diurnal motion is an apparent motion, foreign to the comet, and belongs in reality to the observer, or, as we may say,to the observatory.

I have great pleasure in introducing to English readers M. Guillemin's valuable and interesting work on comets. When rapid progress has been made in any branch of science, it is generally very difficult for anyone, who has not been actually concerned in the investigations in question, to obtain accurate information of the state of our knowledge; and for this reason a book, such as the present, which gives an account of the new results that we owe to very recent researches, really confers a benefit upon many persons who, though taking a strong interest in the subject, have necessarily been quite unable to follow its development in the periodical publications of English and foreign scientific societies. There is no work that at all occupies the ground covered by that of M. Guillemin; and as the subject is one which, always of high interest, has in the last few years acquired, great importance in consequence of Schiaparelli's discovery of a connexion between comets and shooting-stars, I was anxious that it should appear in our language.

Complexity and extent of the question–The law of gravitation suffices to explain the movements of comets–Lacunse in the theory; acceleration of the motion of the comets of Encke and Faye–Origin of comets; their systems–Questions relative to their physical and chemical constitution–Form of atmospheres; birth and development of tails.

Let us glance back for a moment at the contents of the preceding chapters.

We there find many facts accumulated, observations both interesting and instructive, phenomena whose variations suggest reflections without limit concerning the nature of the bodies to which they relate. Nevertheless, do these collected facts permit a clear and certain reply to the simple question: What is a comet?

I say a simple question, for so, as a rule, it is thought to be by non-scientific people; but in reality there is no question more complex. In order to attempt to reply to it, or at least to relate what is known for certain about comets, and to pass in review the most probable conjectures on doubtful points, we must proceed methodically, and thus as it were divide the difficulty.

A first natural division of the subject is at once apparent, it seems to us, from the exposition of cometary phenomena which has been made in the preceding chapters.

Comets lost or strayed: the comet of 1743; the comet of Lexell, or 1770; perturbations caused by Jupiter; in 1767 the action of Jupiter shortens the period, and in 1779 produces an opposite effect–Comet of De Vico; short period comets of 1783, 1846, and 1873.

During the month of February 1743 a comet was observed at Paris, Bologna, Vienna, and Berlin whose parabolic elements were calculated by Struyct and Lacaille. A mathematician of our time, M. Clausen, identified it as a comet of short period, performing its revolution in five years and five months. Is this, as has been supposed, the same comet that was seen in November 1819? If so, its period must have greatly changed, since the calculations of Encke assign to the latter a period of about four years and ten months.

The comet of which we are now about to speak is celebrated in the history of astronomy. The following extract from a memoir by M. Le Verrier gives an account of the circumstances of its first apparition:–

‘ Messier perceived, during the night of the 14–15th of June, 1770, a nebulosity situated amongst the stars of Sagittarius, but not discernible by the naked eye ; it was a comet first coming into view. On the 17th of June it appeared surrounded by an atmosphere the diameter of which was about 5′ 23″. In the centre appeared a nucleus; its light was bright, like that of the stars. Messier estimated its diameter at 22 seconds.