WHAT is meant by the physical or chemical constitution of a celestial body, or of any luminary whatever, whether star or sun, planet or moon; or, aswe are treating of comets only, what is meant by the physical or chemical constitution of a comet?

We have presented for our consideration a question the nature of which is easily explained and not less easily understood; but it is one that the best-informed of astronomers would find it difficult to answer in its full integrity.

By comparison with the bodies that we see on the surface of the earth and with the terrestrial globe itself, considered as a whole, we shall proceed to explainwhat is meant by the physico-chemical constitution of a comet.

The earth is a globe, more accurately, a spheroid, whose form and dimensions are perfectly defined and well known, at all events as far as concerns its solidcrust, the atmosphere that surrounds it, and the rocks and strata near its surface.

Comets are, from all these points of view, their movements alone excepted, conspicuously different from the earth and the rest of the planets.

The memoir of Lalande and the panic of the year 1773–Letter of Voltaire upon the Comet–Announcement in the Gazette de France and the Memoirs of Bachaumont– Catalogue given by Lalande of comets which up to that time had approached nearest to our globe.

In the spring of the year 1773 a singular rumour, soon followed by a strange panic, obtained in Paris and rapidly spread throughout France. A comet was shortly to appear upon the earth's track, to come into collision with our planet, and thus infallibly bring about the end of the world. The origin of this rumour was a memoir which Lalande was to have read before the Academy of Sciences on April 21; it was, however, not read, but the title alone was sufficient to create a popular ferment. The work of the learned astronomer was entitled Reflexions sur les Comètes qui peuvent approcher de la Terre. It was speedily imagined, and without the smallest foundation–for nothing of the kind was to be found in the memoir–that a comet predicted by the author was about to dissolve the earth on May 20 or 21, 1773.

Rapid undulations occasionally observed in the light of cometary tails ; observations of Kepler, Hevelius, Oysatus, and Pingre; comets of 1607, 1618, 1652, 1661, and 1769–Undulations in the tails of the comets of 1843 and 1860; do these undulations arise from a cause peculiar to the comet itself, or do they depend upon the state of the atmosphere?–Objection made by Olbers to the first of these hypotheses ; refutation by M. Liais.

The tails of certain comets have exhibited variations of brilliancy, sudden changes of intensity, analogous to the phenomena of the same kind which are observed in the aurora borealis, and which, it is believed, have been remarked in the zodiacal light. This fact was unknown to the ancients ; and when Seneca speaks of the augmented or diminished brilliancy of comets, it is evident that he alludes to the changes produced, in the course of their apparition, by the variations of their distance from the earth. He compares them ‘to other stars which throw out more light and appear larger and more luminous in proportion as they descend and come nearer to us, and are smaller and less luminous as they are returning and increasing their distance from us.’ (Quæstiones Naturales, vii. 17.)

Kepler is the first observer who has made mention of these singular changes.

Introduction of poisonous vapours into the terrestrial atmosphere–The end of the world and the imaginary comet of Edgar Poe; Conversation of Eiros and Charmion –Poetry and Science; impossibilities and contradictions.

We now come to that other cometary influence which we have already alluded to, an influence capable of changing the air we breathe by the introduction of foreign effluvia.

Nothing within the range of fact and observation, up to the present time, affords ground for belief in such an influence. But this hypothesis has had the fortune to be presented in a striking and practical form by a modern writer of powerful imagination. The American poet Edgar Poe, whose Extraordinary Histories are known to everyone, has placed in the mouth of a being who has suffered death, an account of the destruction of the world by the near approach of a comet. We subjoin the principal portion of this wonderful dream, in which Eiros relates to Charmion the circumstances which put an end to the world.

‘ The individual calamity was, as you say, entirely unanticipated, but analogous misfortunes had been long a subject of discussion with astronomers. I need scarce tell you, my friend, that, even when you left us, men had agreed to understand those passages in the most holy writings which speak of the final destruction of all things by fire as having reference to the orb of the earth alone.

If we arrange in the order of date the various apparitions of comets (hat have been recorded, and note how these bodies appear in different regions of the heavens, and how some pursue a direct and others a retrograde course; or, better, if we study their elements in a catalogue, our attention is at once arrested by the diversity of these elements, which seem connected by no relation.

It may, however, be instructive to examine, by comparing these materials, whether any law presides over the distribution of comets in time and space. We shall, therefore, give a rapid résumé of the analysis we have made with this object. We have taken the catalogue published by Mr. Watson at the end of his work on Theoretical Astronomy as the basis of our investigation.

In this catalogue, which we reproduce at the end of this work, we find 279 comets arranged in the order of their successive apparitions, from the most ancient times to the commencement of the year 1867; we have ourselves completed it for the seven following years, including also the first half of the year 1874; so that the total number of comets in the catalogue is by this means increased to 311, a number very inferior, not only to the actual number of comets, but to the number of those which have received mention in history.

Do the nuclei of comets exhibit phases?–Polarisation of cometary light–Experiments of Arago and of several contemporary astronomers–The light of nebulosities and atmospheres is partly light reflected from the sun.

In the last century astronomers were almost entirely preoccupied with the study of cometary movements, the nature of cometary orbits, the periodicity of comets, and with every question, in fact, that tended to prove that, like the planets, these bodies are subjected to the universal law of gravitation. Astronomical physics was then hardly recognised, and conjecture filled the place of modern analytical research. It was doubtless owing to this preoccupation that comets were at that time looked upon as bodies of kindred nature to the planets. There was a kind of reaction against the ancient hypothesis of terrestrial meteors and transient fires. ‘Planets are opaque bodies,’ says Pingré; ‘they only send back the light which they receive from the sun. We ought not, perhaps, to conclude definitively that comets are also opaque bodies ; it is not absolutely proved that a luminous body may not circulate around some other body. But the light of comets is feeble and dull; its intensity varies; we can perceive in it sensible inequalities and even gaps. It does not appear that these phenomena can be explained otherwise than by supposing comets to be opaque bodies, possessed of no other light than that which they receive from the sun, and surrounded by an atmosphere similar to that of the earth. Clouds are formed within this atmosphere, just as in our ownatmosphere; these clouds weaken or totally intercept the rays ofthe sun, and successively deprive us of the sight of a portionof the comet. This hypothesis would explain everything. . .’

Transformation of a nebula which has entered into the sphere of the sun's attraction; continuous parabolic rings of nebulous matter–Similarity between the elements of the orbits of meteor streams and cometary orbits–The August stream; identity of the Leonides and the comet of 1862–Identity of the Perseids and the comet of 1866 (Tempel)–The shooting stars of April 20 and the comet of 1861–Biela's comet and the December stream–Did the earth encounter Biela's comet on November 27,1872?

It still remains to explain the origin of meteor swarms or streams, and the reason of their annual periodicity and the maxima which appear at dates separated by intervals of several years. For this purpose it will be necessary for a moment to quit the domain of fact and consider some theoretical speculations.

The swarms of shooting stars appear to be constituted, as it were, of aggregations of particles separated from one another by some distance. But if, instead of seeing them on their arrival in the proximity of the earth, in contact with its atmosphere, it were possible to contemplate them from a distance in the heavens, the whole of these myriads of particles, whether illuminated by the sun's rays or shining by their own light, would appear to the observer like a cloud or nebulosity.

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′