1856.] Eccentric Movements of Comets. great eccentricity of their orbits, comets are generally visible only during a few days or weeks, and in a small part of their orbits. Hence arises the difficulty of computing the time of revolution of those bodies from a single apparition, so as to predict their next return to the perihelion. It will be readily understood from these remarks that the orbit of a comet must be computed for one apparition at least, before we can expect that the astronomer shall be enabled to predict its future return to the perihelion. Some persons, in the height of their ignorance, have supposed that astronomers ought to be enabled to predict all cometary apparitions whatever; not reflecting that in the vast majority of such instances there are no existing records of the comet having been visible on any former occasion. When the great comet of 1843 burst suddenly upon the observations of European astronomers, M. Arago was violently assailed by some of the Parisian journalists because he had not predicted the visibility of the comet. This circumstance has induced him to insert in the work we are now considering some excellent remarks on the subject of cometary predictions. After pointing out the various conditions which must be satisfied before the astronomer can predict the return of a comet, he tinues: con Now, I ask, can it be reasonably expected that the astronomer will be able to predict the arrival within the sphere of our visibility of comets which for ages remain lost in the depths of the most distant regions of space, which no one has ever perceived, the action of which on the bodies of the solar system is utterly inappreciable, as well by reason of the excessive rarity of the nebulous matter of which they are composed, as in consequence of their prodigious distance? A celestial body reveals itself to mankind either by becoming visible, or by its producing sensible effects. The body which has never been seen, which has never produced any sensible displacement, is to us as if it had never existed. The announcement of the apparition of a comet which is totally unknown would partake of sorcery, and not of true science. Astrology itself did not push VOL. LIII. NO. CCCXVIII. 739 its pretensions so far, even when it was in the height of its fervour. But it will be said that the comet of March, 1843, was not found in the conditions alluded to; it was observed in 1668. I shall grant, if the reader choose, that the comet of 1843 was seen in 1668; my concessions will go no further. To see a comet and to observe it are two things totally different. The observations, properly so called, alone determine the form and position of the orbit described. Now there is only one decisive means of recognising a comet in its different apparitions, and that is, the complete resemblance of the orbits. He who regards the heavens as a simple contemplator renders as little service to astronomy as if he were blind. The eccentric movements of comets have frequently suggested the idea whether one of such bodies might not on some occasion come into violent collision with the earth. The comet which has been named after its discoverer Biela, is remarkable for intersecting the plane of the ecliptic at a point very near the path of the earth. Its motion on the occasion of its passage of the perihelion in 1832 having been calculated beforehand, it was found to approach so near the earth's orbit that great alarm was generally apprehended lest the two bodies should actually come into collision. It was ascertained, however, upon closely scrutinizing the relative movements of the two bodies, that no such eventuality could possibly occur. It appeared that the comet would pass through the plane of the ecliptic (when it would approach nearest the earth's path) on the 29th of October, 1832, but that the earth would not arrive in the same position in the course of its annual motion until the 30th of November. It was found, in fact, that the distance between the two bodies was not at any time less than forty-five millions of miles. Arago has applied the calculus of probabilities to the question what chance there is of a comet coming into collision with the earth. Let us consider a comet of which we know nothing else than that when it is passing through the perihelion it would be nearer the sun than we are ourselves, and that its diameter would be equal to one-fourth of that of the earth: the calculus of probabilities shows that out 3 B of 281 millions of chances there is only one which is unfavourable-that there exists only one which could lead to a collision of the two bodies. . . . . Here we wish to determine, without knowing anything of the form and position of the orbit of the comet, to how many chances of collision the earth is exposed. It is thus that we have found, with respect to the nucleus, properly so called, one chance of collision-one adverse chance to 280,999,999 favourable chances; in regard to the nebulosity considered according to its usual dimensions, the unfavourable chances would be ten or twenty to the same number of 281 millions. Let us admit for a moment that the comets which might happen to come into collision with the earth by their nucleus were to annihilate the whole human race, then the danger of death which would result to each individual from the apparition of an unknown comet would be exactly equal to the risk which he would run if there was in an urn only one white ball to a total number of 281 millions of balls, and that his condemnation to death would be the inevitable consequence of the white draw. Every man who consents to make use of his reason, however attached he may be to life, will laugh at the idea of so remote a danger. Well, the day on which a comet has been announced before it has been observed, the day before its path can have been determined, it is to each inhabitant of our globe the white ball of the urn of which I have just spoken. An opinion has long prevailed very extensively that comets exercise an influence on the weatherthat they occasion epidemics and produce various other terrestrial effects. According to Forster,* it is certain that since the Christian era the most unhealthy periods are precisely those during which some great comet has appeared; that the apparitions of these bodies have been accompanied by earthquakes, eruptions of volcanoes, and atmospheric commotions, while no comet has been observed during salubrious periods. M. Arago has remarked that in order to establish the existence of cometary influences, it would be necessary to prove that the alleged effect manifested itself generally at every place on the earth's surface. In 1665 the city of London was ravaged by a fright ful plague. If we desire, with Mr. Forster, to see in this the effect of the sufficiently remarkable comet which appeared in April of the same year, let it then be explained how this same body did not generate any malady at Paris or even in a great many towns in England very near the metropolis. If an attempt were seriously made to establish the generality of the effect, science would acknowledge the legitimate character of such researches, although no reliable conclusions could be expected to be deducible from them, considering the extreme rarity of the matter of which the substance of cometary bodies consists. But when an author attaches to the date of the observation of a comet the remark, that in Westphalia all the cats were sick; at the date of a second the circumstance, it must be admitted by no means analogous to the preceding, that an earthquake destroyed at Peru the cities of Lima and Callao; when he adds that during the observations of a third comet, an aerolite penetrated into an elevated tower in Scotland, and broke there the mechanism of a clock; or that in winter the wild pigeons appeared in America in numerous flocks; or again, that Etna and Vesuvius vomited torrents of lava,-this author makes to no purpose a great display of erudition. If upon thus registering contemporary events he pretended to have established new relations, he would not commit a less grievous mistake than the woman of whom Bayle speaks, who, having never put her head out of the window without seeing carriages in the Rue St. Honoré, imagined that it was the sole cause of their passage. M. Arago remarks that he should have desired very much for the honour of science and modern philosophy, that he could have dispensed with taking into consideration the ridiculous ideas which have been propounded on the subject of cometary influences; but he had acquired the conviction that even in our own times there are not wanting persons of intelligence who entertain a firm belief in the reality of such absurd notions: The celebrated traveller, Rüppell, wrote from Cairo on the 8th of November, 1825: The Egyptians think that * Illustrations of the Atmospheric Origin of Epidemic Diseases. Chelmsford. 1829. 1856.] Speculations about the Influence of Comets. the comet actually visible is the cause of the violent earthquakes which we have felt here on the 21st of August, and that It also exercises its malignant influence over the horses and asses which die from bursting. The truth is, that they die of hunger, the forage failing in consequence of the imperfect inundation of the Nile.' If indiscretions were not forbidden here, I should easily convince the reader that in the matter of comets all the Egyptians are not upon the banks of the Nile. I shall say then only: Listen when you are present at one of those brilliant réunions, where are gathered together those whom it is usual to call the social notabilities listen for a single instant to the long discourse of which the future comet furnishes the text, and then decide if we can congratulate ourselves upon that pretended diffusion of intelligence which so many optimists love to point out as the characteristic feature of our age. As regards myself, I have long since abandoned these illusions. Under the brilliant and superficial varnish with which the purely literary studies of our colleges almost invariably invest all classes of society, we generally findlet us be brief-a complete ignorance of those beautiful phenomena, of those grand laws of nature, which are our best safeguard against prejudice. It would be vain to enumerate the multitude of speculations which comets have given rise to. Among the possible effects attributed to them, it has been suggested whether a comet, by approaching too near the earth, might not exercise so strong an attraction upon that body as to derange the form of its orbit, and cause it to revolve permanently as a satellite around the comet. If such an eventuality should occur, it would necessarily follow, since the orbits of comets are very eccentric, that the earth would approach very near the sun at the perihelion, and would recede to a great distance from it at the aphelion. The materials of which the earth is composed would thus be liable to evaporate and congeal by turns, and every kind of animal and vegetable life would perish. M. Arago has considered the influence of the great comet of 1680 under this point of view. This comet is remarkable for having approached nearer the sun than any other recorded in history, with the exception of the great comet of 1843. Newton calculated that at the time of its 741 passage of the perihelion, on the 17th of December, it was subjected to a heat two thousand times greater than that of red-hot iron. M. Arago, while remarking that this result is founded upon imperfect data, is of opinion that the question is beset with difficulties which render it impossible to ascertain what would be the real condition of the earth if it were transported into a position so close to the sun. At first the earth would no doubt experience in its solid envelope a heat twenty-eight thousand times more intense than the heat of summer; but soon all the seas would be transformed into vapours, and the thick stratum of clouds which would hence result would perhaps protect it from the conflagration which might be dreaded at first sight. Thus it is certain that the vicinity of the sun would induce a great increase of temperature without our being able, by the nature of things, to assign its numerical value. Equally certain would be the condition of the earth if it were transported to the aphelion of the comet. In 1820 Captain Franklin and his companions endured at Fort Enterprise cold of 49° 7′ centigrade below zero. The mean temperature of the month of December at that locality was 35° below zero. On the other hand, those who may be desirous of consulting the Notice which I have devoted to the temperatures of different kinds of animals, will see that it has been demonstrated by experiment that under certain hygrometric circumstances man is capable of supporting a heat of 130° centigrade, a heat 30° greater than that of boiling water. Thus, nothing serves to establish that if the earth became a satellite of the comet of 1680, the human race would disappear, from thermometric influences. The author concludes his interesting exposition of the theory of comets with a short chapter on the question of the habitability of those bodies. He remarks that an opinion generally prevails that animated beings could not exist in an absolute vacuum, or in a medium of a very high temperature; but without supporting this opinion by better arguments than if a person who, never having seen any fishes, were to maintain upon that ground alone that life in the water is impossible. Religious scruples have also tended to complicate the problem. The following is the mode in which Fontenelle, as early as the year 1680, replied to this kind of difficulty: There are some persons who imagine that there exists some danger to religion in placing inhabitants elsewhere than on the earth. But we must here expose a slight error of the imagination. When you are told that the moon is inhabited, you forthwith represent to yourself men of the same nature as yourselves; and then if you are somewhat of a theologian you find yourself involved in difficulties. The posterity of Adam has not been enabled to reach the moon, or to send colonies thither. The men who are in the moon are not then sons of Adam. Now it would be embarrassing in theology that there should exist men who were not descended from Adam. The objection turns then entirely upon those men in the moon. For my own part I do not put men there; I assign to the moon inhabitants who are not at all men. What are they, then? I have not seen them; it is not from having seen them that I speak of them. However (says the ingenious secretary of the Academy), although I believe the moon to be an inhabited land, I do not refuse to live on civil terms with those who think differently, and I am always prepared to adopt their opinion with honour if it should have the advantage over mine. I engage in these questions only in the way that persons engage in civil wars, wherein the uncertainty of the final result induces the individual who has compromised himself always to maintain a good understanding with the opposite party. The volume of which we have thus briefly noticed the contents, concludes with an exposition of the knowledge which we possess relative to the physical constitution of the two inferior planets, Mercury and Venus. The planet Mercury is usually so close to the sun that it is only under favourable circumstances that it can be seen with the naked eye. It is a remarkable fact that Copernicus never saw this planet, in consequence of the vapours arising from the mouth of the Vistula, which generally concealed from his observations the region of the heavens near the horizon. The planet Venus, which revolves in a much larger orbit, is on the contrary the most conspicuous object in the stellar vault, and is frequently visible for several weeks in succession. When it is near its greatest elongation, this planet may even be seen distinctly with the naked eye. The following anecdote in connexion with this fact is related by Arago:— Bouvard has related to me that General Bonaparte, upon repairing to the Luxembourg, when the Directory was about to give him a fête, was very much surprised at seeing the multitude collected in the Rue de Tournon pay more attention to the region of the heavens situate above the palace than to his person, or to the brilliant staff which accompanied him. He enquired the cause, and learnt that these curious persons were observing with astonishment, although it was noon, a star which they supposed to be that of the conqueror of Italy: an allusion to which the illustrious general did not seem indifferent when he remarked the radiant body with his own piercing eyes. The star in question was no other than Venus. Cassini, and several astronomers of the last century, suspected from their observations that Venus is accompanied by a satellite; but modern observers have been unable to verify the existence of such a body. M. Arago has examined this question with his usual care, but he leaves the reader to form an opinion for himself on the subject. It is a curious fact that of the four planets of moderate dimensions whose orbits are comprised within the zone of the minor planets, the earth should seem to be the only one which is dignified with a satellite. We may remark in conclusion, that the work, of the first volume of which we have thus attempted to give a brief description, is extremely well adapted for diffusing a knowledge of astronomy among the large class of persons who have imbibed a taste for science, but are strangers to the technical language in which its grand truths are usually shrouded. The editors have appended a series of foot-notes, which cannot fail to prove serviceable to the reader, either for illustrating the text or for rectifying some point in the history of the science. 1856.] 743 POLITICAL RUMINATIONS. DURING the late war we ven tured to express three confident and decided opinions:- -1. That it would not be of long duration; 2. That Sebastopol must fall; 3. That the Conferences at Vienna would prove a failure, but that negotiations for peace would shortly be resumed with a successful result. We do not recal these facts for the purpose of laying claim to any rare political sagacity, but rather to show that it is possible sometimes to speculate on human affairs with something like certainty. Our first opinion, then, was based upon the conviction that the combatants were not fairly matched. Russia, in a state of military preparation as complete as she could hope to attain, had to encounter France equally prepared; England was not so ready, but, from her inexhaustible energy and resources, likely to be in the highest vigour at the moment when both her enemy and her ally were beginning to feel severely the pressure of war. Russia, single-handed, had to encounter these redoubted allies under circumstances the most disadvantageous. One arm, her navy, was at once disabled; she had to convey her troops and muniments of war over vast inhospitable steppes, at a prodigious loss both of matériel and of men, while her antagonists had a safe, regular, and comparatively easy transport. It was impossible that a struggle carried on upon such conditions as these could be a prolonged one. The only question, indeed, was whether the balance was likely to be redressed by the accession of any other belligerent. Prussia, a time-server as usual, was sure not to commit herself against the winning side. Austria was in fact the only Power whose policy might influence the fortune of the war. But Austria was in this dilemma :-on the one hand, to declare against the Western Allies was to lose her Italian provinces, and to risk the loss of Hungary; on the other, Vienna itself lay open to the invasion of her powerful neighbour. Austria, therefore, had the weightiest reasons not only for maintaining a strict neutrality, but for exerting herself to put an early period to a war the flames of which might ultimately reach her own territory, in spite of all her efforts to keep them aloof. While it was the fashion in this country to rail at the Cabinet of Vienna for not joining in a contest in the issue of which they had as great an interest as the Western Powers, we always maintained that they had a difficult card to play. The policy of that Court, unquestionably, was to put an end to the war; and though the negotiation at Vienna was premature, we hardly doubted that peace would be brought about through the intervention of Austria. The course of events pointed to an early opportunity of renewing these friendly offices. All relief to Sebastopol by sea being cut off, the fall of that place became only an affair of time; and looking to the enormous losses the Russians must sustain as the siege advanced, and the increasing difficulty of repairing them, it required little prophetic skill to hazard an opinion that the campaign of 1855 would close the war. That event has now come to pass, and the only question is, have France and England obtained a result worthy of their alliance, and of the great object for which they fought? fought? England, conscious of power, and accustomed to the first place in arms, was emulous, if not jealous of her illustrious ally and rival. Though she had in the first campaign amply vindicated her old renown, she felt vexed and even humiliated by a series of reverses, the results of palpable blunders and incapacity. But at the commencement of this year, all traces of such miscarriages had been removed. A splendid army, in high spirits and perfect condition, was encamped in the Crimea. A fleet, such as had never before been seen upon her waters, or perhaps upon any sea, was ready for any operation which could be undertaken in maritime warfare. And as England thus showed, all furnished, all in arms,' awful in her strength, in her pride, and in her will, the spirits of her sons mounted high, and every man felt that we were on the eve of some achievement worthy of her name. |