LIBRARY OF 
 
 . B. GRISWOLD. 
 
 ^~ 
 
 THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 GIFT OF 
 
 Mrs. Franziska Roos 
 
A POPULAR 
 
 TREATISE ON COMETS, 
 
 BY 
 
 JAMES C. WATSON, A.M., 
 
 FORMERLY PROFESSOR OF ASTRONOMY, AND NOW PROFESSOR OF PHYSICS, UT 
 TEX UNIVERSITY OF MICHIGAN. 
 
 ER1T QUI DEMONSTRET ALIQUAXDO, IN QUIBU8 COOTS PARTIBUS CURRANT, 
 CCM TAX SEDUCTI A CETERIS ERRENT, ET QCANTI QUALESQUE SUIT. 
 
 Seneca, Quasi. Nat^ lib. rii., c. 27. 
 
 PHILADELPHIA: 
 JAMES CHALLEN & SON. 
 
 DETROIT : RAYMOND & LAPHAM. CHICAGO : S. C. GRIGGS & CO. 
 
 ANN ARBOR: SCHOFF t MILLER. 
 1861. 
 
Entered, according to Act of Congress, in the year 1861, by 
 
 JAMES C. WATSON, 
 
 in the Clerk's Office of the District Court of the United States for the Eastern 
 District of Pennsylvania. 
 
 STEREOTYPED BT J. FAGAN FEINTED BY I. ASUMEAD. 
 
 Astronomy 
 GIFT 
 
NN33 
 
 TO 
 
 E. B. WARD, ESQ., 
 
 OF DETROIT, 
 
 AS A TESTIMONIAL OF THE LIVELY INTEREST WHICH HE ! 
 TAKEN IN THE PROMOTION OF THE STUDY OF THE 
 PHYSICAL SCIENCES, THIS WORK IS MOST 
 RESPECTFULLY INSCRIBED, BY THE 
 
 AUTHOR. 
 
 2C6 
 
PREFACE. 
 
 THE announcement of the discovery of a comet has 
 always been received with great interest by both the 
 learned and unlearned. By the former, it is hailed 
 with delight, as affording new proofs of the perfection 
 of the laws which regulate the motions of the celestial 
 bodies ; while to the latter, the appearance of a comet 
 of any considerable magnitude furnishes, too often, the 
 basis of the most foolish and superstitious fears, being 
 regarded as the forerunner of pestilence, famine, or 
 some other dreadful calamity. 
 
 The time, however, has now come when all such 
 fears should be at an end, and it has been for the pur- 
 pose of disseminating correct notions in regard to the 
 nature and physical constitution of these wonderful 
 bodies, among those not versed in astronomical science, 
 that this volume has been prepared. The subject is 
 here treated in the most complete and candid manner, 
 and all technicalities are studiously avoided. In a 
 word, the object has been to convey to every intelligent 
 reader all the knowledge in regard to the motions and 
 character of these erratic bodies possessed by the pro- 
 fessional astronomer of the present day, without making 
 it necessary to introduce the symbolic operations of 
 mathematical analysis. 
 
 (vii) 
 
viii PREFACE. 
 
 To accomplish this, it has been necessary in some 
 instances to state facts without giving the processes 
 which astronomers have employed in their determina- 
 tion ; but the statements which have been made may 
 be relied on as being accurate. Whenever the strictly 
 popular character of the work has permitted a satis- 
 factory explanation of the practical, as well as the 
 theoretical, details of the calculations, it has been 
 attempted. 
 
 Although it may be said that the age in which we 
 live is indeed distinguished for the clearest and most 
 enlarged views of social and political science, yet it 
 is not less marked by the disposition, so unequivocally 
 and universally manifested, to reject the inordinate 
 estimate heretofore placed upon merely ornamental 
 literature; and while it does not refuse their jusi rank 
 and influence to such studies, it admits of that high 
 consideration to which they are entitled, the sciences 
 which explain the beautiful phenomena of the physical 
 world. It is therefore encouraging to know that the 
 demand for some portion of scientific knowledge, and 
 the desire to be informed of what is passing in that 
 universe of which our earth is so minute and appa- 
 rently insignificant a member, no longer confined within 
 the sacred precincts of astronomical observatories and 
 academies of science, has spread throughout the whole 
 extent of civilized society. To supply wants thus 
 created, so far as they relate to those beautiful and 
 mysterious worlds known as comets, this book is now 
 offered with the hope that it may merit the approbation 
 of an enlightened public. 
 
 J. C. W. 
 ANN ARBOR, MICHIGAN, September, 1860. 
 
CONTENTS. 
 
 CHAPTER I. 
 
 Introductory remarks Characteristic difference between 
 comets and planets Superstitious fears excited by the 
 appearance of comets Opinions held by the ancients 
 Fears excited in the middle ages and at the present day 
 Influence of comets on the weather The great dry fogs 
 of 1783 and 1831 Possibility of a collision between a 
 comet and the earth Effect of such a collision True 
 theory of cometary influences PAGE 13 
 
 CHAPTER II. 
 
 Apparent forms of comets Description of some of the most 
 remarkable comets which have appeared during the period 
 of authentic history The great comets of 1556, 1680, 
 1769, 1807, 1811, 1843, 1853, and 1858 Number of 
 comets 77 
 
 CHAPTER III. 
 
 Orbits of comets Computation of an orbit Ephemeris of 
 a comet Periodic comets Halley's comet Encke's 
 comet Biela's comet Faye's comet De Vico's comet 
 Brorsen's comet D' Arrest's comet Tuttle's comet 
 Winnecke's comet LexelFs comet of 1770 Other 
 periodic comets Relation supposed to exist between the 
 comets of short period and the asteroid planets Possi- 
 bility of a collision between a comet and planet Probable 
 appulse of two comets 149 
 
x CONTENTS. 
 
 CHAPTER IV. 
 
 Physical constitution of comets Perturbations of the comets 
 by the planets The masses of comets Effect of intense 
 heat and cold on their form Light of comets Tails of 
 comets Theory of the formation of the tail Influence 
 of the tail on the motion of the nucleus in its orbit 
 Origin of comets Formation of new worlds Nebular 
 hypothesis 273 
 
 CHAPTER V. 
 
 On the existence of a resisting medium in space Character 
 of the resisting medium Its effect on the motions of the 
 heavenly bodies The stability of the solar system 
 Influence of the comets General remarks 333 
 
A 
 
 POPULAR TREATISE 
 
 ON 
 
 COMETS. 
 
 CHAPTER I. 
 
 INTRODUCTORY REMARKS CHARACTERISTIC DIFFERENCE BETWEEN 
 
 COMETS AND PLANETS SUPERSTITIOUS FEARS EXCITED BY THE AP- 
 PEARANCE OF COMETS OPINIONS HELD BY THE ANCIENTS FEARS 
 
 EXCITED IN THE MIDDLE AGES AND AT THE PRESENT DAY INFLU- 
 ENCE OF COMETS ON THE WEATHER THE GREAT DRY FOGS OF 1783 
 
 AND 1831 POSSIBILITY OF A COLLISION BETWEEN A COMET AND 
 
 THE EARTH EFFECT OF SUCH A COLLISION TRUE THEORY OF 
 
 COMETARY INFLUENCE. 
 
 THERE is no department of human learning and 
 research in which the mind has been exercised with 
 more transcendant success than in the theory of 
 astronomy. The development of the legitimate 
 results of the theory of gravitation, first discovered 
 by Newton, and its extension to the far distant 
 regions of the Universe, has enabled the astronomer 
 of the present day to comprehend, at one glance, as 
 it were, the past, the present, and the future changes 
 of entire systems. It has enabled him to show that 
 '2 (13) 
 
14 TREATISE ON COMETS. 
 
 even in our own solar system, with its planets and 
 their satellites, where everything is in a state of 
 motion apparently the most complex that could be 
 devised, although the mutual attraction of the 
 bodies which compose it, causes the orbits alter- 
 nately to contract and expand, and their planes to 
 rock slowly up and down, yet, that stability is there. 
 It permits him also to declare that this unseen agent 
 of the Supreme Intelligence, mysterious in its nature 
 as spirit itself, connects the parts of the universe so 
 intimately, that action is answered instantaneously 
 by reaction through distances which elude even the 
 grasp of the imagination ; and yet the law of this 
 force, though the most general and exalted that man 
 has discovered, is so simple that the effects of gravi- 
 tation, however numerous and complicated, have 
 been, or may be, predicted with unerring certainty. 
 Thus do we find that as far as telescopic vision can 
 extend, in the most remote realms of space, sun 
 revolves around sun, and system around system, in 
 obedience to the same power that causes the rains 
 to descend and the tides to flow. 
 
 If we go back to the earliest ages of the world, 
 and trace forward the progress of astronomical dis- 
 covery until the time of Kewton, we shall find that, 
 notwithstanding the perseverance and acuteness of 
 the many illustrious astronomers who preceded him, 
 there were many phenomena presented in the hea- 
 
COMETS AND PLANETS. 15 
 
 vens which, with their knowledge of laws and forces, 
 baffled every attempt at explanation, but which now 
 present us only with striking proofs of the operations 
 of the attraction of gravitation. The planets, which 
 were never so distant as to escape vision, having 
 perpetually afforded opportunities for observation, 
 were known to move in obedience to some definite 
 law of nature, no matter what that law might be. 
 But there was another class of celestial bodies whose 
 motions seemed to be at variance with any force 
 which could harmonize with the actual motions of 
 the planets; and these, from the circumstance of 
 their having presented, in most instances, the phe- 
 nomenon of a bright head or nucleus, with a train 
 of fainter light bearing some resemblance to a tuft 
 or lock of hair, were called comets (from the Latin 
 word coma, signifying a bunch of hair), in order to 
 distinguish them from the other luminaries, which, 
 whether near or remote, apparently fixed or mova- 
 ble, never exhibited any similar appearance. The 
 planets, too, were known to move regularly, in their 
 annual course, from west to east, in paths which 
 were nearly circular, and which were confined to a 
 very narrow belt or zone of the heavens ; and con- 
 sequently they were always visible, except when in 
 the immediate vicinity of the sun. But the case 
 presented by the comets was far different. They 
 were seen only for a brief period, and were observed 
 
16 TREATISE ON COMETS. 
 
 to move over a wide extent of the heavens, and in 
 every direction with reference to the path pursued 
 by the earth. Some were found to move in orbits 
 whose planes must be nearly at right angles with 
 the orbits of the planets, and others were found to 
 move nearly in their plane. Some were seen to 
 move from west to east, in the direction of the 
 planetary motions, while an equal number retro- 
 graded, or from east to west ; and from the appear- 
 ances which they presented from day to day, it was 
 evident that they approached very near the sun, and 
 then retreated to the most remote regions of space. 
 It was without doubt this mystery which attended 
 their motions, as well as that of their physical cha- 
 racter, which has in all ages produced that sensation 
 which we shall presently notice. The planets being 
 always present, before the invention of the tele- 
 scope, attracted no very considerable attention; 
 but the comets, by their unusual aspect, their sud- 
 den arrival, and the prodigious velocity with which 
 they winged their flight through our system into the 
 deep recesses of the heavens, excited the most vivid 
 impressions on the imagination. "We shall find, 
 therefore, that, since of all the celestial phenomena 
 they are indeed the most wonderful, and especially 
 calculated to excite in the minds of the ignorant 
 the idea of supernatural agency, they have been 
 universally regarded with the greatest apprehension. 
 
FEARS EXCITED BY COMETS. 17 
 
 Moreover, when we consider the wonderful charac- 
 teristics which mark their flight through space, the 
 suddenness and frequency of their appearance, and 
 the incongruity of their motion towards the sun 
 from all regions and in all directions, it will not 
 perhaps seem strange that, in ages of ignorance and 
 superstition, they have been regarded as the demon- 
 strations of the wrath and as the harbingers of the 
 vengeance of offended deities ; and that, among the 
 dreadful evils which they were supposed to forerun, 
 were pestilence, famine, war, and political and 
 physical convulsions. 
 
 It has been well remarked, that it is an inherent 
 attribute of the human mind to experience fear, and 
 not hope or joy, at the aspect of that which is unex- 
 pected and extraordinary. Thus it is that the 
 strange form of a large comet, its faint nebulous 
 light, and its unpredicted appearance in the vault 
 of heaven, have in all countries been almost inva- 
 riably regarded by the people at large as some new 
 and formidable agent inimical to the existing state 
 of things. The sudden occurrence and short dura- 
 tion of the phenomena lead to a belief in some 
 equally rapid and portentous reflection of its agency 
 in terrestrial affairs ; while, on account of the almost 
 infinite variety of the phenomena here presented, 
 it is easy to find events that may be regarded as the 
 fulfilment of the evil foretold by the appearance of 
 
18 TREATISE ON COMETS. 
 
 these mysterious bodies. Accordingly, we find, by 
 reference to the early history of nations, when their 
 philosophy was essentially ideal and superstitious, 
 every celestial phenomenon which seemed to them 
 to be extraordinary and at variance with the general 
 harmony of nature, was regarded as indicative of 
 some peculiar state of the weather, or of some event 
 connected with the characters and fortunes of indi- 
 viduals. In climates where the sky was in a state 
 of almost perpetual serenity, the magnificence of the 
 starry heavens could not fail to excite the wonder 
 and admiration of the inhabitants. Attracted thus 
 unconsciously to mark the aspects of the heavenly 
 bodies, they would, by continuous observation, soon 
 acquire sufficient familiarity with the general ap- 
 pearance of the celestial vault, to detect any unu- 
 sual appearance which might there be exhibited. 
 While thus occupied in observing phenomena, they 
 would necessarily endeavor to seek out causes and 
 effects ; and, since it is inherent in the human mind 
 to attempt to pry into the future, it may readily be 
 perceived that their methods of reasoning would be 
 vitiated by labored attempts to connect the aspects 
 of the heavens with terrestrial events. Speculating 
 further, and reasoning by analogy, they would 
 necessarily ascribe motion to the impulse of a living 
 being, and as the simplest solution of every such 
 appearance, they would suppose life to be inherent 
 
OPINIONS OF THE ANCIENTS. 19 
 
 in the body which moves. There is no language 
 which does not bear the traces of this belief. Every 
 fountain and every stream is at first conceived to be 
 "living water," every motion to be the breathing 
 of a spirit, and the clouds floating on high to be 
 borne along on "the wings of the wind." In this 
 way, therefore, they would finally arrive at the con- 
 clusion, whether legitimate or not, that the heavenly 
 bodies which were found to be in motion, must be 
 in a state of animation ; and hence it would be easy 
 to imagine that their power must be incomparably 
 superior to any that could be produced by human 
 agency. When such an idea gains admission, the 
 imagination knows not where to stop ; and it has 
 been asserted, as a natural sequence, that this ideal 
 and visionary philosophy would go still further, and 
 maintain that sympathies are established between 
 the most remote parts of the universe. All the divi- 
 sions of nature would be found to harmonize and 
 reciprocally actuate each other. The stars may 
 affect the earth, but the earth also affects the stars ; 
 and hence it would be contended, that it is possible 
 to acquire such a knowledge of their mutual actions, 
 as not only to predict the changes which the former 
 may produce in the latter, but, to a certain extent, 
 to regulate the operations of the cause and to modify 
 the degree of the effect. 
 
 Such we conceive to be the primitive tendency 
 
20 TREATISE ON COMETS. 
 
 of the mind of man in the early and barbarous state 
 of society, in the study of astronomical science ; but 
 in more advanced stages of civilization, the opinion 
 that the heavenly bodies have an influence over 
 terrestrial affairs, might appear to be confirmed by 
 observations and inductions which are not essentially 
 of so visionary a character. Certain appearances in 
 the heavens being associated with corresponding 
 changes in the seasons, might be supposed to be 
 either the efficient causes or the invariable signs of 
 these changes. The variations in the temperature 
 and density of the atmosphere, the ebbing and 
 flowing of the tides of the sea, and the fertility of 
 the earth, were known to bear some intimate rela- 
 tion to the sun and moon ; and, consequently, it was 
 very naturally concluded that the planets and the 
 comets must have as great an influence over the 
 bodies and minds, the actions and fortunes of men, 
 as those more conspicuous luminaries have over the 
 vast realms of the ocean, the air, and the earth. 
 The myriads of stars which were seen scattered 
 here and there in bright profusion over the entire, 
 surface of the heavens, would, on account of their 
 great number, be supposed to have been appointed 
 to regulate the destinies of the numberless indi- 
 viduals who inhabit the earth, to each of whom a 
 particular star was appointed, as the guide of his 
 conduct and as the arbiter of his fate ; and, since 
 
OPINIONS OF THE ANCIENTS. 21 
 
 many of the stars were not observed to have any 
 apparent connection with the great changes which 
 have from time to time taken place in human affairs, 
 it would be supposed that it was their exclusive 
 province to preside over the incidents which occur 
 in the minuter portions of the world. But the 
 comets, from their sudden and yet casual appear- 
 ance, and their hideous aspect, would be regarded 
 as announcing those greater convulsions which have 
 disturbed the nations of the earth. This opinion 
 would be strengthened by the fact that it was gene- 
 rally admitted that there existed some intimate 
 relation between mind and matter, and that the 
 state of the human body and that of the mind are 
 very closely connected. Now, since the bodily con- 
 stitution is sensibly affected by the modifications of 
 the atmosphere, it was very naturally concluded that 
 the heavenly bodies might, through the medium of 
 the atmosphere, affect the human body, and also, 
 through the intervention of the body, affect the dis- 
 position and passions of the mind. It is well known 
 that climate has a most powerful agency in the for- 
 mation of human character, and consequently, if a 
 variation of a few degrees in the mean temperature 
 of any locality is sufficient to account for the different 
 varieties of intellectual capacity, for the strength or 
 weakness of passion, for the liveliness or defect of 
 imagination, and for the activity or torpor of all the 
 
22 TREATISE ox COMETS. 
 
 faculties, it would not be unreasonable to suppose 
 that these varieties could be ascribed to influences 
 from the celestial regions. Such a conception seems 
 to harmonize completely with the doctrines advanced 
 by some of the ancient philosophers, of the exist- 
 ence of two separate and distinct kinds of influence, 
 the one immediate and the other remote, the one 
 discoverable by the senses, the other eluding the 
 most careful observation. 
 
 We have thus discussed, somewhat in detail, what 
 seems to be the only reasonable manner of account- 
 ing for the superstitious notions connected with 
 unusual celestial appearances, which have prevailed 
 in every age of the world and in every nation. We 
 have been specific in the statement of these tenden- 
 cies of the mind, in order that, although we may 
 be amused at the recital of the follies and absurd- 
 ities of those who fostered such delusive supersti- 
 tions, we may still look beyond the immediate pic- 
 ture presented, and in the dim vista, it may be, 
 witness the development of each peculiar system 
 from what might be considered perfectly legitimate 
 assumptions. With these preliminary remarks, we 
 are prepared to consider the various opinions in 
 relation to the appearances of comets, which have 
 been entertained in each successive age of the 
 world, commencing with that epoch, where the 
 obscurity of tradition is dissipated by the light of 
 
OPINIONS OF THE ANCIENTS. 23 
 
 authentic history, and tracing them forward, step 
 by step, until the present day. 
 
 The ancients for the most part regarded comets 
 as simple meteors, generated by inflammable vapors 
 in the earth's atmosphere ; and supposed that when 
 they were once extinguished, they were lost for ever. 
 Some, however, believed them to be distinct celes- 
 tial bodies, situated beyond the moon. Diodorus 
 informs us that the shepherd astronomers of Chaldea 
 considered the comets as subject to the same dynam- 
 ical laws as the planetary bodies, but revolving in 
 orbits which receded to a greater distance from the 
 earth. The ancient Chinese, unlike the Chaldeans, 
 were satisfied with simply extended observations of 
 comets, and did not attempt, even in the slightest 
 degree, the process of generalization and of deriving 
 any knowledge in regard to their nature and phys- 
 ical peculiarities. 
 
 Epigenes, who had studied among the Chaldeans, 
 very singularly maintained that there were two 
 kinds of comets, the one stationary and pouring 
 forth their heat in all directions, the other diffused 
 like hair and traversing the stars like the planets. 
 There were many also who believed that comets 
 were formed by a conjunction of two planets, the 
 light of both being confused into one, exhibiting 
 the phenomenon of an elongated star ; or, that being- 
 very near each other, the atmosphere (which they 
 
24 TREATISE ON COMETS. 
 
 supposed to pervade all space) was enlightened by 
 both, producing the appearance of a comet. Apol- 
 lonius Myndius maintained that a comet, instead of 
 being composed of one or more planets or wandering 
 stars in conjunction, was itself the same as a planet, 
 although its form, unlike that of the other planets, 
 was not globular, but more extended. He did not, 
 however, suppose them to have any regular course, 
 but to wander here and there, at random, through- 
 out the entire heavens. The same opinion* was 
 entertained by Zeno the Stoic. He supposed that 
 the comets have their regular courses, and that they 
 reappear after the lapse of a very long interval of 
 time. The majority of the Stoics, however, ima- 
 gined the comets to be created in the atmosphere, 
 and accounted for their motion by supposing that 
 the rapid combustion which they believed to take 
 place, impelled them forward in order to obtain a 
 sufficient supply of fresh air. 
 
 Aristotle, in an attempt to explain the theory of 
 the tails of comets, was led to believe that there 
 existed some intimate relation between the comets 
 and the Milky Way. He supposed that the myriads 
 of stars which compose this starry zone, give out a 
 self-luminous, incandescent matter. He therefore 
 regarded the nebulous belt which separates the 
 different portions of the vault of heaven as a large 
 comet, the matter of which it is composed being 
 
OPINIONS OF THE ANCIENTS. 25 
 
 inqessantly changing, by assimilating new particles 
 and giving off others. He also regarded comets as 
 presaging violent storms of wind and rain ; and, on 
 account of their slow motion, declared that they 
 must be of immense weight. The remark has 
 indeed been very appropriately made, that, since 
 Aristotle exercised so great an influence throughout 
 the Middle Ages, it is very much to be regretted 
 that he was so averse to those grander views of the 
 elder Pythagoreans, which inculcated ideas so nearly 
 approximating to truth respecting the structure of 
 the Universe. He asserts that comets are transitory 
 meteors, belonging to our atmosphere, in the very 
 book in which he cites the opinion of the Pytha- 
 gorean school, according to which these cosmical 
 bodies are supposed to be planets, having long 
 periods of revolution. Panetius supposed the comets 
 to be delusive appearances, produced by certain un- 
 explained and indeterminate conditions of the atmo- 
 sphere ; and denied that they exhibited, either in 
 their motions or appearances, any of the attributes 
 of material bodies. 
 
 Such are some of the many curious theories which 
 were advanced by the ancient philosophers, in order 
 to account for the phenomena presented by the 
 comets. We must not, however, omit to give the 
 opinions held by Seneca, who, among the Greeks 
 and Romans, stands preeminent for his very accu- 
 3 
 
26 T E E A T I S E N C M E T S . 
 
 rate notions in regard to the real character of these 
 mysterious bodies. In speaking of the various 
 theories which prevailed at his day, he says : " I do 
 not agree with our philosophers, for I do not think 
 that a comet is a sudden fire, but that it is to be 
 regarded among the eternal works of nature ; " and 
 afterwards, in recapitulating his own theories, he 
 adds : " The time will come when some one shall 
 show us in what regions the comets wander, why 
 they are so different from the other celestial bodies, 
 and what and how great they are. Let us, there- 
 fore, be content with what we know, and leave it to 
 those who shall succeed us to discover facts which 
 our day and generation will not be able to unfold." 
 The arguments by which Seneca enforced his own 
 individual opinions, and the variety of the pheno- 
 mena which he cited in support of each hypothesis, 
 were wholly insufficient, as we shall see, to dissem- 
 inate and perpetuate correct ideas of the character 
 of the cometary worlds. The philosophy of those 
 who succeeded him was so impregnated with a kind 
 of religious superstition, that no argument, however 
 plausible or cogent, seemed sufficient to disabuse 
 the minds of the people of those ideas of fear which 
 the appearance of a comet invariably excited. The 
 tendency of the human mind to connect terrestrial 
 events with celestial phenomena, which we have 
 already discussed somewhat in detail, together with 
 
PEARS EXCITED BY COMETS. 27 
 
 the ignorance and stupidity with which a bigoted 
 and infatuated priesthood announced the most 
 ahsurd and ridiculous doctrines, with the mistaken 
 prestige of Divine authority, had prepared the 
 people for the adoption of the most ludicrous and 
 sup'erstitious fears. Thus, while we are prepared to 
 excuse the folly of less enlightened nations, we 
 cannot avoid contemplating, with mingled feelings 
 of ridicule and amusement, the devices employed 
 even at a late day, by nations professing to have 
 arrived at an advanced stage of civilization, in order 
 to frighten away comets and counteract their sup- 
 posed pernicious and baneful influences. Although 
 the recital of the fears produced by comets in vari- 
 ous ages of the world, and of the concurrent events 
 which they were supposed to presage, may not be 
 considered essentially necessary to an understanding 
 of the true theory of the influence of these bodies, 
 yet we deem it proper to give a few instances at 
 least, in order that, separated as we are from the 
 influences which operated in those times, we may 
 regard them in all their unmasked absurdity. We 
 shall therefore proceed at once to give some ex- 
 amples. 
 
 We are informed by Seneca, on the authority of 
 Calisthenes, that immediately prior to the great 
 earthquake in Achaia, in the year 373 B. C., there 
 appeared a comet of enormous size, which was the 
 
28 TREATISE ON COMETS. 
 
 specific cause of the destruction of the cities of 
 Bura and Helice. Those cities were submerged by 
 the sea, and the appearance of the comet at the 
 same epoch, and its disappearance very soon after 
 that event, led to a very general belief that its office 
 was directly connected with the destruction of both 
 cities. The earthquake here referred to was, per- 
 haps, the greatest and most disastrous which had 
 ever been felt in that portion of the globe. It is 
 not strange, therefore, that Aristotle should have 
 shared the general belief; and, accordingly, we find 
 that he makes mention of it as presaging the dread- 
 ful events which took place in the same year in 
 which it appeared. He informs us, also, that in the 
 year 468 B. C., there appeared a large comet, which 
 was supposed to have some agency in producing the 
 famous fall of aerolites near ^Egos Potamos. One 
 of these meteoric stones, celebrated in antiquity, is 
 said to have weighed nearly two tons. It was seen 
 to fall from an immense cloud of fire and smoke ; 
 and the occurrence of such an unusual event, to- 
 gether with the phenomena presented by the cornet, 
 produced a most profound sensation among all 
 classes. Aristotle mentions another comet which 
 appeared in the year 341 B. C., which was supposed 
 to have caused a terrific storm near Corinth. 
 
 In the year 43 B. C. a comet made its appearance 
 which was so brilliant as to have been visible to the 
 
FEARS EXCITED BY COMETS. 29 
 
 naked eye in the daytime, when in the immediate 
 vicinity of the sun. It was regarded by the Romans 
 as the soul of Julius Caesar who had lately been 
 murdered transferred to the heavens. The star 
 of the Magi, which signalized the birth of Christ, 
 is numbered among the comets by many modern 
 writers on the subject. The probability of the iden- 
 tity of the star of the Magi and a comet which 
 appeared the same year, has been doubted. The 
 comet is described as producing a light rivalling the 
 splendor of the noonday sun. It is said to have 
 appeared during an interval of twenty-four days, 
 and its magnitude was so great that it extended 
 over nearly fifty degrees of the heavens, so as to 
 occupy more than three hours in rising and setting. 
 A comet appeared in the year 54 of our era, which 
 was supposed to have presaged the death of the 
 Emperor Claudius, which took place on the 13th of 
 October of the same year ; and a comet which ap- 
 peared in the year 63 was supposed to have been 
 the cause of earthquakes in Achaia and Macedonia. 
 Josephus informs us. that just before the destruction 
 of Jerusalem by Titus Vespasian, in the year 69, 
 there appeared, hanging over the city, a flaming 
 torch; and ten years later, while Vespasian was 
 suffering from the disease of which he died, a comet 
 was seen, which continued in sight during a long 
 time. In the latter part of the year 190, or in the 
 3* 
 
30 TREATISE ON COMETS. 
 
 beginning of 191, a "hairy star" was seen, which 
 Herodian informs us was the cause of many prodi- 
 gies which appeared at the same time. Stars were 
 continually seen by daylight, and some of them ap- 
 peared to be stretched out lengthways, and seemed 
 to be suspended in the air. 
 
 The death of the Emperor Constantine is said to 
 have been preceded for several days by the appear- 
 ance of a comet of extraordinary magnitude, and 
 another comet which appeared in the year 400 was 
 believed to have foreboded the most frightful dis- 
 asters. We are informed by the historians of the 
 Western Empire that the misfortunes with which 
 Constantinople was threatened by Gain as, in the 
 same year, were so great that they were undoubt- 
 edly announced by the appearance of the comet, 
 which is described as being one of the most terrible 
 ones on record. It appeared above the city, with a 
 tail in the form of a sabre, extending from the high- 
 est region of the sky almost to the horizon. Two 
 years later another comet made its appearance, 
 which seemed fairly to rival this one. It was be- 
 lieved to have caused tempests and frequent thunder, 
 numerous eclipses of the moon, prodigious hail 
 storms, and spontaneous conflagrations. It was 
 said to have been attended by birds of evil augury, 
 and to have announced the arrival of Alaric, King 
 
FEARS EXCITED BY COMETS. 31 
 
 of the Visigoths, in Italy, an event which caused 
 the greatest possible consternation at Rome. 
 
 A comet which appeared immediately before the 
 birth of Mohammed was subsequently supposed, by 
 the followers of the Prophet, to have announced his 
 birth, in a manner precisely similar except in de- 
 tail to the star which appeared at the birth of 
 Christ, 
 
 A comet appeared in March, 1402, which was so 
 brilliant as to be distinctly visible at mid-day ; and 
 a second comet, which appeared in June of the same 
 year, was visible several hours before sunset. This 
 comet was believed to presage the death of John 
 Galeas Visconti. That prince, being a believer in 
 astrology, had consulted the charlatans of his court 
 in reference to such an event, and the fright pro- 
 duced by the appearance of the comet may possibly 
 have contributed to the fulfillment of the prediction 
 of his death. Another conspicuous comet appeared 
 in 1532, which was also stated to have been visible 
 before sunset. It created a most profound sensa- 
 tion throughout France, Germany, and Northern 
 Italy, where it was considered as the announcement 
 of the death of Sforza II. 
 
 The celebrated comet of Halley, which we shall 
 subsequently refer to, performs its revolution around 
 the sun in a period of 75 years, and, consequently, 
 has been visible at many different epochs. It may 
 
32 TREATISE ON COMETS. 
 
 not be uninteresting, therefore, in order to show the 
 discrepancies of all superstitious theories, to give, 
 in a connected manner, all the various events which 
 the same body has been supposed to announce at 
 different visits to our immediate vicinity, although 
 the fact of the periodicity of the comet was unknown 
 to those who attributed to it such remarkable quali- 
 ties. In 1005 the appearance of this comet was 
 attended by a great famine ; in 1080 by an earth- 
 quake ; in 1155 by a cold winter and the failure of 
 crops ; in 1230 by rain and inundations (part of 
 Friesland was overwhelmed, with 100,000 inhabit- 
 ants) ; in 1304 by great drought, and intense cold 
 in the following winter, succeeded by a pestilence ; 
 in 1380 by a still more destructive contagion ; and 
 in 1456 by wet weather, inundations, and earth- 
 quakes. At its appearance in 1456 the comet is 
 represented as being of an " unheard of magnitude," 
 and as having a tail which extended over sixty de- 
 grees of the heavens, or nearly from the zenith to 
 the horizon. It was visible during the month of 
 June, and spread terror throughout Europe. It was 
 supposed to announce the future success of the 
 Turks under Mohammed II., who was then engaged 
 in the subjugation of the Christian nations. He 
 had already taken Constantinople in part, had 
 advanced his forces even as far as the walls of Vi- 
 enna, and had thus struck terror into the whole 
 
F E A it ,S EXCITED BY C M E T S . 33 
 
 Christian world. Pope Calixtus II., terrified at the 
 appalling spectacle presented by the comet, and 
 aroused by what seemed to be the inevitable fate 
 of Christianity, directed the thunders of the Church 
 against the enemies of the faith, both terrestrial 
 and celestial. It had become imperatively necessary 
 that something should be done immediately to 
 counteract the baneful influences which were ope- 
 rating both above and below ; and, consequently, the 
 Pope ordered public prayers, and issued a bull, in 
 which he anathematized not only the Turks, but the 
 comet ; and in order to perpetuate this manifestation 
 of the power of the Church, he ordained that the 
 bells should be rung at noon, a custom which is 
 still observed in various Catholic countries. But, 
 as Pontecoulant has aptly remarked, neither the 
 progress of the comet, nor the victorious arms of 
 the Mohammedans, were arrested. The comet tran- 
 quilly proceeded in its orbit, passing through its 
 appointed changes, regardless of the thunders of 
 the Vatican, and the Turks under Mohammed esta- 
 blished their principal mosque in the church of St. 
 Sophia. 
 
 In the year 1531 the appearance of this comet is 
 said to have been attended by great floods ; in 1607 
 by extreme drought, followed by a most severe 
 winter ; in 1682 by floods and earthquakes ; in 1759 
 by rains and storms, and slight earthquakes ; while 
 
34 TREATISE ON COMETS. 
 
 in 1835, the date of its last appearance, nothing, so 
 far as we are aware, has been attributed to it, ex- 
 cept, it may be, some local epidemic or unfavorable 
 state of the weather. 
 
 It is related that a comet appeared in the year 
 590, to the presence and influence of which was 
 ascribed a fearful epidemic, which prevailed in that 
 year, in the crisis of which the patients were seized 
 with violent paroxysms of sneezing, often followed 
 by death. It has been asserted, also, that it became 
 the custom, when these paroxysms manifested 
 themselves, for the bystanders to address their 
 benediction to the sufferer, exclaiming, " God bless 
 you;" and to this has been attributed the custom 
 which is prevalent at this day, to address one who 
 happens to sneeze with the same words. 
 
 In a work on the atmospherical origin of epi- 
 demic diseases, written by an English physician, 
 about thirty years since, it is asserted that, since the 
 Christian era, the most unhealthy periods have been 
 precisely those in which some great comet appeared; 
 that such appearances were accompanied by earth- 
 quakes, volcanic eruptions, and atmospheric com- 
 motions, while no comet has been observed during 
 healthy periods. Among the many cometary influ- 
 ences enumerated are the following: Hot seasons 
 and cold, tempests, earthquakes, volcanic eruptions, 
 hail, rain, and snow, floods and droughts, famines, 
 
FEARS EXCITED BY COMETS. 35 
 
 clouds of midges and locusts, the plague, dysentery, 
 and influenza. Each affliction is assigned to its 
 comet, whatever kingdom, city, or village, the 
 famine, pestilence, or other visitation may have 
 ravaged. In making thus, from year to year, a 
 complete inventory of the misfortunes of this lower 
 world, the question has very properly been asked, 
 who would not have foreseen the impossibility of 
 any comet approaching the earth, without finding 
 some portion of its inhabitants suffering under some 
 affliction ; and who would not have granted at once, 
 what Lubienietski has written a large work to prove, 
 that there never was a disaster without a comet, nor 
 a comet without a disaster ? These are only a few 
 of the influences recorded in the work above re- 
 ferred to, but there is one in particular which must 
 be regarded as the masterpiece of absurdity, namely, 
 that the appearance of the great comet of 1668 pro- 
 duced a remarkable epidemic among the cats in 
 Westphalia. 
 
 The great comet which appeared in 1811 is said 
 to have produced some of the most remarkable 
 results. Among others, it is asserted, in a standard 
 English periodical, published a few years subsequent 
 to the appearance of the comet, that its influence 
 produced a mild winter, a moist spring, and a cold 
 summer; that there was not sufficient sunshine to 
 ripen the fruits of the earth ; that, nevertheless 
 
36 TREATISE ON COMETS. 
 
 (such was the cometic influence), the harvest was 
 abundant, and some species of fruits, such as melons 
 and figs, were not only plentiful, but of a delicious 
 flavor; that wasps rarely appeared, and flies became 
 blind and died early in the season ; that in the 
 neighborhood of London, numerous instances oc- 
 curred of women bearing twins, and it even hap- 
 pened, in one instance, that the wife of a shoemaker 
 in Whitechapel had four children at a birth ! All 
 these singular influences were assigned to this 
 comet. In Germany the somewhat remarkable 
 crop of grapes produced in that year was duly 
 acknowledged to be the effect of the comet ; and 
 the wine manufactured therefrom, which received 
 the appellation of the comet wine, obtained a popu- 
 larity and demand which has rarely, if ever, been 
 equalled. 
 
 The celebrated traveller Riippel, in writing from 
 Cairo on the 8th of October, 1835, observes that the 
 Egyptians thought, that the comet then visible was 
 the cause of the great earthquake which was felt in 
 that country on the 21st of August of that year, and 
 that the comet also exercised so malignant an influ- 
 ence over some of the lower animals, that horses 
 and asses perished in great numbers. The truth 
 was, he adds, that the poor animals died of star- 
 vation, their usual forage having failed in conse- 
 quence of the insufficient inundation of the Nile. 
 
FEARS EXCITED BY COMETS. 37 
 
 Such are a few examples of the many curious and 
 absurd notions of cometary influences which have 
 prevailed at various periods in the history of the 
 world. We might suppose that in these enlightened 
 times, no such attributes would be assigned to these 
 chaotic worlds, especially in countries where edu- 
 cation is generally diffused among the people, and 
 where the arts and sciences have been carried to a 
 high state of perfection. We ought to expect to 
 find, in these cases, that the appearance of the comet 
 would be everywhere hailed with delight, as fur- 
 nishing new proofs of the beautiful harmony which 
 characterizes the motions of the heavenly bodies. 
 But such is not generally the case. The same ten- 
 dency of the human mind to connect terrestrial 
 events with celestial phenomena, exhibits itself even 
 when civilization, culture, and refinement appear to 
 have attained the foremost rank. We even find 
 that in the days of Shakspeare, the comets were re- 
 garded as being directly connected with the great, 
 more especially the calamitous events of nations, 
 and are thus introduced by the illustrious poet him- 
 self, in the lamentation which the Duke of Bedford 
 makes over the bier of Henry V. : 
 
 " Comets, importing change of times and states, 
 Brandish your crystal tresses in the sky ; 
 And with them scourge the bad revolting stars, 
 That have consented unto Henry's death." 
 
 4 
 
38 TREATISE ON COMETS. 
 
 Milton, too, though he lived in the days of Kepler 
 and Galileo, though he was imbued with all the 
 learning and philosophy of his time, and showed 
 that he was intimately acquainted with all the labors 
 of the philosophers who had preceded or were 
 cotemporaneous with him, does not scruple to call 
 in the aid of the malign power of comets, in order 
 to heighten his picture of Satan when preparing for 
 the combat: 
 
 . "On the other side, 
 Incensed with indignation, Satan stood, 
 Unterrified, and like a comet burned, 
 That fires the length of Ophiuchus huge 
 In the Arctic sky, and from its horrid hair 
 Shakes pestilence and war." 
 
 When such men as Milton and Shakspeare allude 
 to the comets in such a manner, even for the pur- 
 pose of illustration, we will not be surprised to 
 know that sentiments similar to these were enter- 
 tained by the people at large. But to come down 
 even to a later day, to the date of the last appear- 
 ance of Halley's comet (1835), we shall find that 
 notions fully as absurd as those which we have 
 already noticed, were received in France and else- 
 where. But a short time previous to the approach 
 of the comet to the earth and sun, Arago wrote as 
 follows : "I would have wished, for the honor of 
 modern philosophy, to be freed from the necessity 
 of taking serious notice of such absurdities ; but I 
 
I N F L U E X C E ON THE WEATHER. 39 
 
 have acquired personal knowledge that some refu- 
 tation of them is not useless, and that the advocates 
 of these influences have no inconsiderable number 
 of followers. Listen, when you are present at one 
 of those brilliant assemblies, where you meet what 
 is called good society ; listen to the talk, of which 
 the approaching comet furnishes the subject, and 
 then decide if we ought to boast of that diffusion 
 of knowledge, which so many declare to be the 
 characteristic feature of our times." 
 
 Those, however, who assign to the comets, not an 
 influence over the fortunes of nations and indi- 
 viduals, directly, but rather an influence over atmo- 
 spheric changes, may perhaps do so with a greater 
 show of probability. To maintain that the comets, 
 owing to their peculiar nature, may, to a certain lim- 
 ited extent, operate to produce some unusual state of 
 the weather, will not at first be regarded by a large 
 portion of even intelligent people, as really absurd. 
 There are numerous reasons, and even forcible ones, 
 in theory, which may be adduced in support of such 
 a hypothesis; but there are other considerations 
 which are of far greater importance, and have a 
 bearing in the opposite direction, which must not 
 be overlooked. It would, indeed, be easy to show 
 at once, upon general physical principles, that there 
 is no reason whatever why a comet should exert 
 any influence, even in the very slightest degree, 
 
40 TREATISE ON COMETS. 
 
 over the temperature of our seasons, or in producing 
 either droughts or rains ; but it will perhaps be 
 more satisfactory to refute it by showing that such 
 doctrines are not in conformity with observed facts. 
 This method of showing the utter fallacy of attri- 
 buting to comets any direct influence other than 
 what is due to their infinitesimal attractive influence 
 in accordance with the law of universal gravitation, 
 is most simple and easy. The appearances and 
 motions of the comets have been recorded. The 
 average daily, monthly, and yearly temperatures of 
 the weather, the direction and velocity of the winds, 
 the number and character of the storms, and the 
 extent of the droughts, have also been accurately 
 observed and recorded. To ascertain, therefore, 
 whether the comets really exercised any influence 
 on the temperature of the seasons, it is only neces- 
 sary to place in juxtaposition these two records, and 
 to examine whether there exists any general law or 
 analogy which is capable of exhibiting any corre- 
 spondence between them. 
 
 Arago has given a table, in which he has exhi- 
 bited in one column the temperatures of the weather 
 at Paris for every year, from 1735 to 1831, inclusive ; 
 and in juxtaposition with these he has stated the 
 number of comets which appeared, with their mag- 
 nitude and general appearance. The result is that 
 no coincidence whatever is observable between the 
 
INFLUENCE ox THE WEATHER. 41 
 
 temperatures and the number and appearance of 
 comets. Sometimes we find that the years of great- 
 est mean temperature were those in which several 
 comets appeared ; and, again, that they were those 
 in which no comet appeared. For example, in 
 1737, although two comets appeared, the mean tem- 
 perature was inferior to that of the preceding years, 
 in which none appeared. The year 1765, in which 
 no comet appeared, was hotter than the year 1766, 
 when two comets appeared, one of which was re- 
 markable for its splendor ; the year 1775, when no 
 comet appeared, was hotter than the year 1780, 
 which was marked by the appearance of two comets. 
 The temperature was still lower in 1785, in which 
 two comets appeared ; while, on the other hand, the 
 temperature of the year 1781 was greater, which 
 was likewise marked by the appearance of two 
 comets. 
 
 This question of the supposed connection between 
 the temperature and the appearance of comets, was 
 still farther discussed by Arago. He has given not 
 only the general temperatures, but also a table of 
 the years of greatest cold, of the years in which the 
 Seine has been frozen over, and also of the years 
 of greatest heat ; and he has shown that the corre- 
 sponding appearances of comets have been varied 
 without any connection whatever with these vicis- 
 situdes of temperature. Thus, 1806 and 1811 were 
 4* 
 
42 TREATISE ON COMETS. 
 
 both hot years the first, however, hotter than the 
 second ; and yet the first had only one comet, while 
 the second had two, one of which was the most 
 hrilliant, with perhaps a single exception, which has 
 appeared during the present century. Again, the 
 year 1826, in which five comets appeared, was not 
 so hot as 1831, which was characterized by the 
 appearance of only one comet. But it is, perhaps, 
 unnecessary to enter into any further detail in regard 
 to the thermal character of particular seasons as 
 compared with cometary appearances, and we may 
 therefore notice some of the more prominent results 
 arrived at by Arago. 
 
 Of seventy-four years, forty-nine were signalized 
 by the appearance of one or more comets, and 
 twenty-five by their non-appearance. The mean 
 temperature of the former years was found to be 
 51.6, and that of the latter 50.7, the difference, as 
 will be perceived, being actually less than a degree. 
 Again, of the forty-nine years in which comets ap- 
 peared, a single one was observed in twenty-five of 
 these years, and two or more in the remaining 
 twenty-four years. Now, if these bodies produced 
 any influence on the temperature, it might naturally 
 be expected, or rather it would be a necessary con- 
 sequence, that a difference would be found to exist 
 between the mean temperature of the latter and of 
 the former years. It was found, however, that the 
 
INFLUENCE ON THE WEATHER. 43 
 
 mean temperature of the twenty-five years in each 
 of which a single comet appeared, was 51. 6, while 
 that of the twenty-four years in which there were 
 several comets, was 51. 4; the difference being only 
 the fifth part of a degree, and even that being in a 
 direction contrary to the hypothesis that the comets 
 tend to augment rather than to diminish the tern- 
 perature. It is noticeable, however, in these com- 
 parisons, that as a general result, more comets are 
 seen in hot years than in cold ones, which may be 
 explained by the fact that the hot years, generally 
 giving a finer sky, are more favorable than the 
 colder ones for the discovery of those objects which 
 are usually too faint to be detected by the naked 
 eye. In the case of the large comets the circum- 
 stance just noticed would not produce any effect, 
 and accordingly we find that in the year 1843, the 
 date of the appearance of one of the most magni- 
 ficent comets on record, the average temperature 
 was more than a degree less than it had been for 
 several years previous. Again, the mean tempera- 
 ture of the years 1853 and 1858, in each of which 
 there appeared large and brilliant comets, did not 
 sensibly differ from the average temperature of a 
 series of years. Another fact which might be no- 
 ticed in this connection, is that experiment showed, 
 in the case of the great comet of 1811, that the 
 light emitted by the comet, when brought to a focus 
 
44 TREATISE ON COMETS. 
 
 by a large mirror, was not equal to a tenth part of 
 the light of the full moon. Moreover, it was found 
 to have no sensible effect on the blackened ball of 
 a thermometer, which was so delicate a test of vari- 
 ation of temperature that it would have indicated 
 the hundredth part of a degree. This alone shows 
 the total inefficiency of comets to influence our cli- 
 mate even in the slightest degree. In fine, the com- 
 plete discussion of the cometary and thermal obser- 
 vations, continued through the entire period during 
 which authentic records of each have been kept, 
 fully establishes the conclusion that there exists no 
 foundation whatever for the popular opinion that 
 the comets influence the seasons. 
 
 There is another point of view in which the sup- 
 posed agency of comets in producing atmospherical 
 changes has been presented, which requires reason- 
 ing of quite a different character from that which 
 we have already given. It is imagined that the 
 comet has a certain magnetic influence, of which 
 the law of intensity and of operation is unknown. 
 For example, it is now universally admitted that 
 there exists some direct connection between terres- 
 trial magnetism and the solar spots, and in the 
 same way it is conceived that there may exist also 
 a direct connection between the comets and our 
 atmosphere. Those who have advanced such theo- 
 ries have generally adopted Oersted's hypothesis. 
 
INFLUENCE ON THE WEATHER. 45 
 
 that light, heat, and electricity are produced in the 
 same manner, yet under different circumstances; 
 and that it is the ethereal fluid which serves for the 
 transmission of light by successive undulations, 
 which furnishes the agent for heat and electricity. 
 They conceive, therefore, that the comet is in a state 
 of electrical excitement, and that it operates by in- 
 duction through the ether, and affects the atmo- 
 sphere. This is a general statement of the theory, 
 although its development by different individuals 
 has led to a great variety of absurd, and, for the 
 most part, conflicting results. The best, and per- 
 haps *the only' argument necessary to refute these 
 doctrines, is the fact that observation fails to exhibit 
 any agreement or harmony in the phenomena which 
 could be reconciled with any recognized or supposed 
 law in physics. It fails in every instance to exhibit 
 the least connection between atmospherical changes 
 and cometary appearances ; and in view of all this, 
 we cannot conclude otherwise than that the appear- 
 ance of a comet is no more a prodigy, and has no 
 more influence upon the fate of men or of nations, 
 than the appearance of the moon or of any planet 
 or star in the firmament. In reality it cannot have 
 nearly so much ; for the moon, by causing the tides, 
 affects both the atmosphere and the weather, and 
 thus indirectly it affects also the human body. The 
 comets, on the contrary, are so distant, and either 
 
46 TREATISE ON COMETS. 
 
 their motions are so rapid or their substance so rare, 
 that none of them have been found to have any 
 material action on such of the planets as they may 
 have approached very near, although the planets 
 have been found to have a very considerable influ- 
 ence on them. 
 
 "We have thus shown conclusively that the general 
 opinion which prevails, even at the present day, 
 among all classes of people, that the appearance of 
 a comet has reference to, or rather causes, some 
 unusual state of the weather, is without the slightest 
 foundation, either in philosophy or in fact. The 
 arguments are such as to admit of no equivocation, 
 and every intelligent reader will at once perceive 
 how untenable and unworthy of confidence the 
 whole theory of cometary influence in terrestrial 
 affairs must be. There are, however, two other 
 cases connected with this branch of our subject, 
 which must not be passed by unnoticed, and which 
 we shall now consider, namely, the possibility of the 
 near approach of a comet so as to produce atmo- 
 spheric disturbances and currents, and also, the 
 possibility of a collision between a comet and the 
 earth, and the effect of such a collision. 
 
 We shall hereafter have occasion to show that 
 some of the comets have trains which extend to 
 such an enormous distance, that were the earth in 
 the line of the axis of the tail it would certainly be 
 
NEAR APPROACH TO THE EARTH. 47 
 
 enveloped in it, since the length of the tail often 
 exceeds the distance of the comet from the earth. 
 Should it happen, therefore, that on the approach 
 of a very large cornet to its perihelion, or point 
 nearest the sun, the earth should have a position 
 such that the sun and comet would appear to be in 
 conjunction, or nearly so, the inevitable result must 
 be that the earth would be surrounded completely 
 by the nebulous matter which forms the tail of the 
 comet, but which we shall find to exist in a state of 
 great attenuation. Thus, although the possibility 
 of such an event is fairly established, yet, if we rea- 
 son from the theory of probabilities, and introduce 
 the comets which have hitherto been observed as 
 the basis of our computations, we shall find that the 
 probability of an approach sufficiently near to en- 
 velop the earth in the tail, will not exceed the ratio 
 of one to one hundred millions. In other words, 
 let us suppose the number of comets which shall 
 pass through our system in a given time, and whose 
 perihelia are within the orbit of the earth, to be one 
 hundred millions ; then it is barely possible, or 
 rather probable, that one of these may envelop the 
 earth in its tail. When such an event will take 
 place, whether it ever has taken place, or whether 
 it ever will occur, is a problem which defies the 
 power of human analysis ; but since the possibility 
 
48 TREATISE ON COMETS. 
 
 cannot be doubted, it will not be improper to con- 
 sider what would be the probable effect produced. 
 
 It is well known, that since the comets are found 
 to be composed of matter extremely attenuated, the 
 tails will, as a necessary consequence, be even more 
 attenuated ; and in case the earth should be enve- 
 loped in one of these, it would hardly be expected 
 that the nebulous matter would even penetrate our 
 atmosphere to any considerable distance. There 
 might, however, exist such conditions, as regards 
 the nebulosity of the tail and the atmosphere of the 
 earth, as would result in the complete permeation 
 of the atmosphere by the matter of the tail. As a 
 general thing, no such result need be feared, in case 
 the circumstances were such as to render it possible ; 
 and even if it should really happen, the effect pro- 
 duced might be most beneficial to mankind. The 
 nebulous matter might be of such a character as to 
 cause the most fatal epidemics ; or, on account of 
 atmospheric disturbances and currents which it 
 would generate, it might, on the contrary, be such 
 as to produce a sanitary effect. That atmospheric 
 currents exist in certain parts of the globe, which 
 do exercise a healing influence over various kinds 
 of diseases, has been certainly established. For 
 example, the influence of the Harmattan, a peri- 
 odical wind, which blows three or four times a year 
 from the interior of the African continent towards 
 
THEHARMATTAN. 49 
 
 the Atlantic coast, between the latitudes fifteen 
 degrees north and one degree south, is thus de- 
 scribed by an English traveller: 
 
 " The periods of its prevalence are stated to be 
 chiefly from the end of November to the beginning 
 of April, its direction varying from east-south-east 
 to north-north-east. Its duration at any one time 
 varies from one to six days, and its force is always 
 very moderate. A fog, thick enough to render the 
 disc of the sun red, always accompanies this wind. 
 The particles deposited by this fog upon the leaves 
 of vegetables, and on the black skin of the natives, 
 appears always white, but the nature of this whitish 
 matter was not ascertained. It was remarked that 
 this fog was speedily dissipated by the sea; for, 
 although the wind was sensible on the sea at many 
 leagues from the coast, the fog became rapidly less 
 dense, and, at the distance of a little more than a 
 league, it disappeared. 
 
 " One of the characteristics of this wind and fog 
 is extreme dryness. When it continued for any 
 time, the foliage of the orange and lemon trees ex- 
 posed to it became shriveled and withered. So ex- 
 treme is this dryness, that the covers of books, even 
 when closed, locked in chests, and enveloped in 
 linen cloth, were curved by it, just as if they had 
 been exposed to the heat of a strong fire. The 
 panels of doors, and frames of windows, and the 
 5 D 
 
50 T R E A T I S E N C M E T S . 
 
 furniture, were often cracked and broken by it. Its 
 effects upon the human body were not less marked. 
 The eyes, lips, and palate were parched and painful. 
 If the wind continued unabated so long as four or 
 five days, the face and hands grew pallid. The 
 natives endeavored to counteract these effects by 
 smearing their skin with grease." 
 
 This description may serve to give a very correct 
 idea of the character of the Harmattan, which we 
 conceive to be not unlike what might be expected 
 in case the earth should be enveloped in the tail of 
 a comet. From what is said of its effect on the 
 human body, it might be conjectured that its final 
 influence would be highly insalubrious ; yet it is 
 further related that it proved to have the extreme 
 opposite quality. It was found that its first breath 
 completely banished intermittent fevers. It is as- 
 serted, also, that those who had been enfeebled by 
 the practice of excessive bleeding then prevalent 
 there, soon recovered their strength ; that epidemic 
 and remittent fevers, which had a local prevalence, 
 disappeared as if by enchantment; and that it ren- 
 dered infection incommunicable, even when applied 
 by artificial means, such as inoculation. 
 
 The immediate effect of the passage of the earth 
 through the tail of a comet, would be to produce 
 unusual disturbances of the atmosphere, partly by 
 currents and partly by electrical action ; and also 
 
DRY FOG! OF 1788. 51 
 
 to produce the most intense fogs. It has, therefore, 
 been conjectured that the great dry fogs which 
 spread over a large portion of the surface of the 
 earth in the years 1783 and 1831, were produced by 
 the near approach of a comet to the earth, and that 
 the tail of the comet must have either passed over 
 the earth or over a part of it. 
 
 The great dry fog of 1783 possessed several char- 
 acteristics which have induced the belief in its 
 cometary origin ; and one of these is the fact, that 
 it commenced on nearly the same day at places very 
 distant from each other. It lasted nearly a month, 
 and it was found that its position was not in the 
 least affected by the winds, which proves that the 
 atmosphere did not convey it over the regions in 
 which it prevailed. It was found, also, that it pre- 
 vailed equally at all accessible heights above the 
 surface, and that it was as dense upon the summits 
 of the Alps as upon the plains of France. It caused 
 a general sensation throughout Europe, and pre- 
 sented the following phenomena: Its density was 
 such, that in some places objects at the distance of 
 a few rods could not be distinguished, and even 
 such as could be distinguished appeared blue, or 
 else surrounded with vapor. The sun appeared red, 
 and without brilliancy, and could be gazed at with 
 the eye unprotected at mid-day ; while at its rising 
 and setting, it disappeared in a sort of haze before 
 
52 TREATISE ON COMETS. 
 
 it had approached within ten or twelve degrees of 
 the horizon. 
 
 This fog was first seen at Copenhagen on May 
 29th, and was preceded by a succession of fine days. 
 In other places it was preceded by a gale. In Eng- 
 land it came after continuous rains ; at La Kochelle, 
 it was seen on June 6th and 7th; at Dijon, on the 
 14th. It was noticed almost everywhere in Ger- 
 many, France, and Italy, from the 16th to the 18th ; 
 on the 19th it was observed at Franecker and 
 throughout the Netherlands ; on the 22d, at Spyd- 
 berg, in Norway ; on the 23d, at St. Gothard and at 
 Buda ; the 24th, at Stockholm ; the 25th, at Moscow ; 
 toward the end of June, in Syria ; and, on the 1st 
 of July, in the Altai. It will thus be perceived that 
 it "covered a part of the earth's surface, extending 
 north and south, from Africa to Sweden ; and we 
 might remark, in addition, that it prevailed on the 
 North American as well as upon the European 
 continent. 
 
 An examination of this fog showed that it was 
 absolutely dry, since the most delicate hygrometric 
 instruments exposed in it indicated the complete 
 absence of humidity. It was found, also, to have a 
 faintly luminous quality, such as might be supposed 
 to proceed from a slight degree of phosphorescence ; 
 and some observers maintained that they found 
 traces of acid in it. 
 
DRY FOG OF 1783. 53 
 
 In order to explain all these phenomena, which 
 had never before been noticed in the case of the 
 dry fogs which sometimes prevail in different parts 
 of Germany, the cometary hypothesis, which re- 
 ferred its origin to the nebulous and slightly phos- 
 phorescent matter of the tail of a comet which may 
 have been at that time passing over the earth, has 
 been advanced. To decide this question definitely 
 is a problem which admits of a ready and easy solu- 
 tion, and one which any one can fully comprehend. 
 All the phenomena presented in the case of the fog, 
 both in respect to time and place, we have already 
 minutely described, and it remains only to see 
 whether a comet could have occupied for so long a 
 time a position such as the admission of the come- 
 tary hypothesis would require. The tails of comets, 
 as we shall see hereafter, are usually in a direction 
 opposite to the sun ; and it results, therefore, that 
 in order to satisfy the necessary conditions, the 
 comet must have been almost exactly in conjunction 
 with the sun. $"ow, for reasons which it would be 
 useless to attempt to explain, but which may be 
 readily conceived of, there is no possible combina- 
 tion of the motion of the comet in its orbit, and of 
 the diurnal motion of the earth, which would be 
 compatible with the position and continuance of the 
 great dry fog of 1783. Had the interval during 
 which the fog prevailed been much shorter, and the 
 
54 TREATISE ON COMETS. 
 
 date of its first appearance general, or nearly so, 
 the hypothesis of a cometary origin might be pre- 
 sented with a much greater show of probability; 
 but when we remember that the angular motion of 
 a comet, properly situated, cannot be reconciled 
 with the continuance of any such phenomenon for 
 a period longer than a very few days, we can con- 
 clude, in view of all the facts, with the utmost cer- 
 tainty, that the phenomenon under consideration 
 did not arise from the immersion of the earth in 
 the tail of an unseen comet. 
 
 The great fog of 1831 was, in every respect, ex- 
 cepting only the extent of country over which it 
 prevailed, similar to that of 1783. It appeared in 
 the month of August, commencing on the north 
 coast of Africa on the 3d, at Odessa on the 9th, 
 throughout France on the 10th, in the United States 
 on the 15th, and in China during the latter part of 
 the month. Observers in the north of Africa, in 
 the south of France, in the United States, and in 
 China, reported that the disc of the sun seen 
 through the fog had the tint of azure, and, in some 
 places, of emerald green. During the day the light 
 blue of the sky appeared to be tarnished by a mix- 
 ture of a dirty color. At an altitude of a few de- 
 grees above the horizon, the blue of the sky was 
 suddenly interrupted, and was terminated above by 
 a ring of dull brown-red color, more or less clearly 
 
DRY FOGS. 55 
 
 defined. Distant terrestrial objects of a deep color 
 appeared to be nearly effaced, or covered by a bluish 
 veil. It was found, also, that this fog had a proper 
 light, similar to that described in connection with 
 the great fog of 1783, and it was asserted that this 
 apparently phosphorescent light was such that in 
 the absence of the moon, even at midnight, it was 
 possible to read the smallest written or printed 
 characters. 
 
 Another fog of similar character appeared in the 
 months of May, June, and July, 1834, which was 
 confined almost exclusively to the western portion 
 of Germany ; and it is recorded, also, that similar 
 fogs appeared in the years 526, 1721, and 1822. In 
 each of these instances, however, as we have already 
 noticed in the case of the fog of 1783, the hypo- 
 thesis of the immersion of the earth in the tail of a 
 comet is completely overthrown by the fact that the 
 fog, although extensively spread, was not contin- 
 uous, and certainly not uniform. Some parts of the 
 European continent were altogether or nearly free 
 from it, while in other parts it was developed in very 
 different degrees. The times of its continuance in 
 different places also varied much and irregularly, 
 and in such a manner as to be entirely incompatible 
 with the cometary hypothesis. Thus do we find that 
 each new case presented furnishes additional evi- 
 dence to show, that however mysterious and unac- 
 
56 TREATISE ON COMETS. 
 
 countable these phenomena may have been, yet that 
 we are of necessity compelled to refer them entirely 
 to terrestrial agency, and to admit that they had no 
 connection whatever with any celestial phenomenon. 
 At the times when these great fogs appeared, the 
 supposition of cometary agency became very preva- 
 lent ; but in the inquiry which was excited, failing 
 to obtain any tangible evidence in support of such 
 a hypothesis, various other theories were devised. 
 Lalande attributed the fog of 1783 to the quantity 
 of electricity developed during a very hot summer 
 that succeeded a moist winter ; while Cotte regarded 
 it as formed of metallic emanations united with 
 electricity, in consequence of the great heat and 
 numerous earthquakes which marked the summer 
 of 1783. It is a historical fact, that in the year in 
 which this fog appeared, there was a very violent 
 earthquake in Calabria, and a volcanic eruption in 
 Iceland, and very many have attributed the exist- 
 ence of this fog to these. It has been determined 
 by accurate researches that the column which rises 
 above a volcano, although bearing a very great 
 analogy to a column of smoke, is composed, for the 
 most part, of the vapor of water and of volcanic 
 ashes, with which are mixed transparent gases in 
 various quantities. "When the lava runs over the 
 side of the mountain, it carbonizes everything 
 which it meets, and an immense cloud of smoke 
 
DRY FOGS. 57 
 
 rises in the air. Considering, therefore, the fact 
 that an immense quantity of vegetables were con- 
 sumed in Iceland, as well as seventeen villages, we 
 can readily comprehend that the lava, when running 
 over a soil covered with these vegetables, might 
 have been able to produce this smoke, which the 
 north winds, then prevalent, would immediately 
 spread over a great portion of Europe. Add to 
 this, that the combustions of turf and the confla- 
 grations of forests were both extensive and frequent 
 during the summer of 1783, which was a very dry 
 one, and we can conceive of the distribution", in a 
 very short space of time, of the vapor, smoke, and 
 gaseous matter combined, over nearly the entire 
 surface of the globe, and under conditions, as re- 
 spects its general characteristics and electrical influ- 
 ence, producing all the phenomena which were 
 observed. 
 
 Such are some of the most prominent examples 
 of the supposed agency of comets in atmospherical 
 disturbances, by permeating the atmosphere of the 
 earth with their gaseous matter ; and although, in 
 each of these cases, this hypothesis has been found 
 to be insufficient for the explanation of the phe- 
 nomena, yet we cannot, on this account, venture 
 the assertion, that absolutely no such event, as the 
 cometary hypothesis under consideration would seem 
 to indicate, can ever take place. We have already 
 
58 TREATISE ON COMETS. 
 
 seen that there does exist a possibility of the pas- 
 sage of the tail of a comet over the surface of the 
 earth, so as to envelop either the whole or a part of 
 its surface ; but we have also seen that the proba- 
 bility of such an occurrence is so small that it can 
 hardly be expected ; and in proof of this assertion 
 we have the fact, that during the period of authentic 
 history, there is no phenomenon on record which 
 may not be satisfactorily explained without a resort 
 to any other than terrestrial agency, which would 
 otherwise be referred to a cometary origin. 
 
 Having thus disposed of cometary influences, so 
 far as they have been supposed to relate to the birth 
 and death of heroes, to the prevalence of famine, 
 pestilence, war, and various other political and 
 physical convulsions which have from time to time 
 taken place in the affairs of the world ; and having 
 shown conclusively, as must be admitted, that the 
 comets have no influence whatever over atmospheric 
 changes, either directly or indirectly, we are pre- 
 pared to consider the other question which we have 
 proposed as relating further to this branch of our 
 subject, namely, the possibility of a collision be- 
 tween a comet and the earth, and the probable effect 
 of such a collision. The question thus presented 
 is one which admits of the closest reasoning, and 
 one in which the results arrived at, admitting the 
 validity of the argument, must be morally certain. 
 
POSSIBILITY OF A COLLISION. 59 
 
 We are here not left to the guidance of facts con- 
 tradictory and ill authenticated ; but, on the con- 
 trary, the data are both abundant and accurate. 
 Under such circumstances, the case presented can- 
 not fail to be of the greatest 'interest, inasmuch as 
 it involves consequences which may ultimately prove 
 disastrous to the inhabitants of the earth, if not to 
 the existence of the planet itself in its present con- 
 dition. 
 
 When we consider the great number of comets 
 that are known to be continually traversing our 
 system, and also the fact that the orbits of some of 
 them, as we shall subsequently see, almost intersect 
 the orbit of the earth, we may not hesitate to admit 
 at once the possibility of a collision between the 
 earth and one of these bodies. The circumstances 
 under which such a catastrophe may happen for 
 such it would certainly be are these : The orbits 
 of the earth and comet must either actually inter- 
 sect each other, or at least approach so near at some 
 point, that the distance between the orbits shall be 
 less than the sum of the radii of the nucleus or 
 head of the comet and the earth ; and, the earth 
 and comet must both arrive at this point at nearly 
 the same instant. These conditions are indispen- 
 sable to a collision ; but it would, indeed, be pos- 
 sible, without producing a collision, for an approach 
 of a comet to the earth to such an extent that its 
 
60 TREATISE ON COMETS. 
 
 orbit would be changed, or else it might be drawn 
 to the earth, in ease the attractive force of the latter 
 exceeded that of the sun. Another indispensable 
 condition to the possibility of a collision, would be 
 that the earth and comet should move in contrary 
 directions, or, in other words, that the motion of the 
 comet should be retrograde. The planets all revolve 
 around the sun from west to east, while the comets, 
 as we shall notice, are not all found to move in the 
 same direction. Some of them move from west to 
 east, in the direction of the planetary motions, while 
 perhaps an equal number move from east to west, 
 or in a direction contrary to that of the planets. 
 Should the earth come in contact with a direct 
 comet, they would continue to move on in new 
 orbits, which would necessarily result from their 
 impinging ; and the deviation in either case would 
 be least when the planes of the two orbits were 
 nearly coincident, and when they crossed each other 
 very obliquely. Should the orbit, however, of a 
 comet which moves with retrograde motion, as 
 compared with the planets, intersect the orbit of the 
 earth, and the comet and earth happened to meet at 
 this point, the result would be an absolute collision, 
 the effect of which would certainly be disastrous to 
 the comet, if not to both bodies. Since, therefore, 
 the conditions on which a collision depends are 
 such as may actually exist, it may not be improper 
 
EFFECT OF A COLLISION. 61 
 
 to consider at once what must be the inevitable 
 result of such an event, before discussing the abso- 
 lute probability of its occurrence. 
 
 We have just remarked, that in the case of a 
 simple appulse between a comet and the earth, or 
 in the case that the motions of the comet and the 
 earth should both be nearly in the same direction, 
 the effect produced would be merely local in its 
 character, and might perhaps cause a deflection in 
 the path of the earth, and change its velocity. If, 
 however, the comet had no solid nucleus of any 
 considerable size, the effect either of an appulse, or 
 even of a direct collision, would not probably be 
 different from that produced by the passage of the 
 tail of a comet over the earth, which we have already 
 noticed. -The effect produced in this case would be 
 nothing more than an intermixture of the cometic 
 atmosphere with that of the earth. The extremely 
 light mass of the comet would, notwithstanding its 
 proximity, render it impossible that it could produce 
 any sensible effect, either on the annual or diurnal 
 motion of the earth ; and, consequently, no change 
 would be produced in our days, seasons, and years. 
 But in the case where the comet had a solid nucleus, 
 and withal moved in a direction contrary to the 
 annual motion of the earth, the most fatal effects 
 would be produced by a collision. The momentum 
 might indeed be sufficient to destroy the progressive 
 6 
 
62 TREATISE ON OOAJKTS. 
 
 motion of both bodies; and it would result from 
 this that the attractive force of the sun would cause 
 both to fall to his surface. Such would be the fate 
 of the earth if it were struck by a cornet, with a 
 mass only about four times that of the moon, and 
 moving with a retrograde motion at the rate of 
 about three hundred and fifty miles in a second of 
 time, and both bodies would fall to the sun in sixty- 
 four and a half days. In case any obstruction had 
 destroyed the centrifugal force of the great comet 
 of 1680, when nearest the sun, it would have fallen 
 to his surface in three minutes. 
 
 Let us, however, suppose a case which aifords a 
 greater degree of probability, since the one just 
 supposed is such as is almost, if not entirely impos- 
 sible, namely, that the shock experienced by the 
 earth from a collision w^as not such as to destroy the 
 centrifugal force. It can readily be understood, 
 without further explanation, that the effect of the 
 concussion in this case would be to change both the 
 axis around which it performs its diurnal revolution, 
 and the time of rotation. The fluids composing 
 the atmosphere, the oceans, seas, lakes, and rivers, 
 not partaking of that change, would be thrown 
 from their position of relative equilibrium. Violent 
 atmospheric commotions would ensue, and there 
 would also be a sudden rush of all the waters of the 
 ocean, lakes, and rivers from their former beds, 
 
EFFECT OF A COLLISION. 63 
 
 overflowing the land, and sweeping before them 
 animate and inanimate beings in one vast and un- 
 distinguishable ruin. The entire animal, and per- 
 haps the vegetable creation, would be destroyed by 
 a universal deluge, or by the violence of the colli- 
 sion. Every vestige of animated existence would 
 be forever obliterated, and all the monuments of 
 human industry overturned. It has, therefore, been 
 asserted that in this way we may explain why the 
 ocean has deserted the most lofty mountains, leav- 
 ing, however, incontestible evidence of its presence ; 
 why the animals and plants of the tropics may have 
 existed in the higher latitudes, where their relics 
 and footsteps are still seen. It h^s further been 
 asserted that this will explain the recent date of the 
 present races of men, whose earliest monuments go 
 back only a few thousand years ; since the human 
 race, reduced to a small group of individuals, in a 
 deplorable condition, occupied exclusively in pro- 
 viding for their physical wants, must necessarily 
 have lost the remembrances and records of all the 
 sciences and arts ; and when, later, new wants were 
 created by the progress of civilization, everything 
 was to be recommenced, as if no previous progress 
 had been made, and as if man had been for the first 
 time placed upon the earth. 
 
 It is well known, that all parts of the globe bear 
 testimony to the vast and -destructive floods of 
 
64 TREATISE ON COMETS. 
 
 mighty waters. The remains of plants and animals 
 deeply buried in the ground, and the "footprints" 
 in the solid rocks, declare that long before man 
 became its inhabitant, this earth of ours was ten- 
 anted by innumerable races of beings altogether 
 different from those which share with us the present 
 state of things. These must have ceased to exist 
 as animated beings thousands and thousands of 
 years ago, since the strata under which they are 
 found indicate that sometimes the waters have pre- 
 vailed for many centuries over the beds containing 
 them, and sometimes' the dry land. Thus, too, do 
 we find that enormous masses of rock have been 
 torn from their native hills, and have been carried 
 to distant regions, and these, together with the de- 
 posits of the natives of the deep on the tops of the 
 loftiest mountains, declare the irresistible force and 
 magnitude of the vast waves which, in remote 
 times, have carried destruction over the face of 
 nature. These and similar effects have been attri- 
 buted to the shock which may have been experi- 
 enced from a comet in those remote ages of the 
 world. 
 
 The appearance of the comet of 1680 led "Whiston, 
 the friend of Newton, and the man whom he had 
 designed as his successor, to make an attempt to 
 account for the Mosaic deluge by supposing the 
 earth to have been struck by a comet at that time. 
 
EFFECT OF A COLLISION. 65 
 
 Not content with simply conjecturing the possibility 
 of explaining the deluge in this manner, he at- 
 tempted further to exhibit a coincidence between 
 the theory and the circumstances of that great 
 catastrophe, as related in Genesis. 
 
 The biblical deluge happened in the year 2349 
 before the commencement of our era, according to 
 the modern Hebrew text ; and in the year 2926, ac- 
 cording to the Septuagint and Josephus. Whiston, 
 therefore, endeavored to prove that a great comet 
 may be supposed to have appeared at that time. 
 
 It is related, that a large comet appeared in the 
 year 1106, which resembled the blaze of the sun, 
 and had an immense train. In ascending still 
 higher, we find a very large and terrific comet, 
 designated by the Byzantine historians by the name 
 of Lampadius, whose appearance must have been 
 in the year 531 of our era. A comet also appeared 
 in the month of September, of the year 43 before 
 our era, shortly after the death of Julius Csesar, 
 which is said to have been very brilliant, and to 
 have been visible for some time before sunset. 
 
 A comparison of these dates reveals the fact that, 
 supposing these to have been successive appearances 
 of the same comet, which, it should be remarked, 
 is altogether conjectural, the period of revolution 
 of the comet of 1680 differs but little from 575 
 years. Adopting this hypothesis, and computing 
 
66 TREATISE ON COMETS. 
 
 backward, we find that four revolutions prior to 
 B. C. 43, would give the epoch of the deluge, ac- 
 cording to the modern Hebrew text, within six 
 years ; and by supposing that there had been five 
 revolutions, we obtain the date of the deluge, ac- 
 cording to the Septuagint, within eight years. From 
 these and other considerations, it has been inferred 
 that, as Whiston has supposed, there may have 
 been a comet near the earth at the period of the 
 deluge, although the probability of its having been 
 in collision admits of no such assumption. It ought 
 also to be remarked that the comet of 1680 moved 
 in the same direction, relatively, in which the earth 
 moves ; yet the plane of its orbit was so inclined 
 to that of the earth's orbit, that a collision, although 
 barely possible, yet could not have been so disas- 
 trous as might have resulted in case the motion had 
 been retrograde. 
 
 But without entering into a detailed account of 
 the arguments adduced by "Whiston in support of 
 his hypothesis, it will perhaps be sufficient to notice 
 only the general conclusions which he arrived at. 
 
 He asserts, that at the period of the deluge, the 
 comet of 1680 was only nine or ten thousand miles 
 from the earth ; and, consequently, that it attracted 
 the waters from the great deep, just as the moon is 
 known to attract the waters of the ocean. Its action, 
 on account of that great proximity, must have 
 
EFFECT OF A COLLISION. 67 
 
 tended to produce an immense tide ; and hence, he 
 contends, that since it is probable that the terrestrial 
 shell could not resist the impetuosity of the inun- 
 dation, it would break in at a great number of 
 points, and the waters, then free, would spread 
 themselves over the continents. To render this 
 intelligible, it might be well to state that he sup- 
 posed the earth to have been originally a comet. 
 He therefore regarded it as having, at or near 
 its centre, a nucleus, which is a hard and compact 
 substance, and which was formerly the nucleus or 
 head of the comet. He supposed, also, that the 
 matters of various natures, confusedly mixed, which 
 composed the nebulosity, subsided more or less 
 quickly, according to their specific gravities ; that 
 the then solid nucleus was at first surrounded by a 
 dense and thick fluid; that the earthy matters pre- 
 cipitated themselves afterwards and formed a cover- 
 ing over the dense fluid a kind of crust, which 
 may be compared to the shell of an egg ; that the 
 water, in its turn, covered this solid crust ; that in 
 a considerable degree it became filtered through the 
 fissures, and spread itself over the thick fluid ; and 
 that, finally, the gaseous matters remaining sus- 
 pended, purified themselves gradually, and consti- 
 tuted our atmosphere. Thus he makes it possible, 
 as he imagines, for the supposed shell to be broken 
 up by the rush of the exterior waters; and when 
 
68 TREATISE ON COMETS. 
 
 once broken up, the waters which he supposed to 
 have existed interior to this crust, and on which he 
 supposed it to rest, would gush forth, and produce 
 the phenomena given in Genesis, namely: "In the 
 second month, the seventeenth day of the "month, 
 the same day were all the fountains of the great 
 deep broken up." 
 
 Continuing his theory still further, Whiston sup- 
 posed that the nebulosity of the comet reached the 
 earth near Mount Ararat, arid that the range of 
 mountains intercepted the entire tail. He con- 
 cludes, therefore, that it would result, as a necessary 
 consequence, that the terrestrial atmosphere, thus 
 charged with an immense quantity of aqueous par- 
 ticles, was sufficient to produce forty days' rain of 
 such violence as the ordinary state of the globe can 
 give us no idea. Thus he reconciles the remainder 
 of the passage found in Genesis: "And the win- 
 dows of heaven were opened. And the rain was 
 upon the earth forty days and forty nights." 
 
 Such is the celebrated theory of "Whiston in rela- 
 tion to the manner in which the Mosaic deluge was 
 accomplished ; a theory which is remarkable from 
 the fact that it was advanced and maintained by a 
 man who was an intimate friend of the great New- 
 ton, and who must have been acquainted with the 
 labors of that distinguished philosopher. In many 
 points this theory may seem to present, at least, some 
 
EFFECT OF A C o L L i n ION. 69 
 
 show of probability, but there are so many other 
 points in which it presents obvious and glaring 
 defects, that we can regard it as nothing more than 
 the fancy of a mind devoted exclusively to similat 
 speculations. Geologists have shown that the earth 
 has indeed an exterior crust or shell, the thickness' 
 of which is very small compared with the diameter 
 of the globe, but that the interior, instead of being 
 in the state supposed by Whiston, is, on the con- 
 trary, in a state of intense heat, such, in fact, that 
 the great bulk is one vast mass of melted matter. 
 This militates strongly against the theory of Whis- 
 ton ; and were there not additional considerations 
 to counterbalance, would be sufficient to show the 
 utter fallacy of all his arguments in relation to the 
 deluge. 
 
 In order to give a greater weight to his argu- 
 ments, and at the same time to remove objections 
 which would otherwise serve to lessen materially 
 the strength of his theory, Whiston supposed that 
 the mass of the comet was at least six times greater 
 than that of the moon. It can, however, be readily 
 shown that even this enormous magnitude of the 
 comet would not be sufficient to produce any such 
 tides as he supposes to have been caused ; and that 
 the comet could not, by any possibility, have re- 
 mained more than two hours and a half so near the 
 earth as a fourth part of the moon's distance. It 
 
70 TREATISE ON COMETS. 
 
 may also be shown that it could not have remained 
 even for that length of time, unless it passed the 
 earth under a very improbable and peculiar combi- 
 nation of circumstances. Under such circumstances, 
 the production of a tide would be impossible, since 
 eleven hours at least would be necessary to enable 
 a comet to produce an effect on the waters of the 
 earth, from which the injurious consequences of a 
 deluge could follow. 
 
 Thus we see how utterly untenable are the doc- 
 trines of Whiston in relation to the deluge. They 
 are, perhaps, not less romantic than another theory 
 of the same man, of the probable object of the 
 comets in the economy of nature. He fixed on 
 these erratic worlds the residence of the damned, 
 and thus determined the nature of their punish- 
 ment. According to his theory, a comet was the 
 awful prison-house in which, as it wheeled from the 
 remotest regions of darkness and cold into the very 
 vicinity of the sun, hurrying its wretched tenants 
 to the extremes of perishing cold and devouring 
 fire, the Almighty was to dispense the severities of 
 his justice. 
 
 Remarkable, then, as the theory of Whiston is 
 found to be, it is a fact well established, that it was 
 received with a great deal of favor at the time when 
 it was promulgated, and many others following in 
 
EFFECT OF A COLLISION. 71 
 
 his footsteps have modified it, and extended it to 
 suit their own peculiar fancies. 
 
 If, however, we suppose the earth to have received 
 a shock from a collision with a comet of any con- 
 siderable magnitude, it would not he difficult to 
 conceive of a deluge such as described in Genesis. 
 Some writers have adopted this hypothesis, and 
 have carried it still further, and suppose the floods 
 of mighty waters w^hich have left their mark every- 
 where on the surface of the glohe, have been pro- 
 duced by the shock of a comet in former ages. But 
 the astronomer has shown that such is not the case ; 
 that the length of the day, which is the measure of 
 the celestial motions, is immutable, and exhibits no 
 trace of change ; and that, if the earth had ever 
 been struck' by a comet so as to change the axis 
 about which it performs its diurnal rotation, the 
 effects would still be perceptible in the variation 
 which it would have occasioned in the geographical 
 latitudes. It may be demonstrated, however, that 
 a spheroid, such as the earth is known to be, cannot 
 permanently revolve around any axis except its 
 shortest diameter, in case it was placed in any re- 
 sisting medium, such as our atmosphere. Now, if 
 by the collision of a solid comet, the earth were 
 made to revolve on any other diameter than its 
 shortest, which is the one around which observa- 
 tion proves that it now revolves, it would seem, 
 
72 TREATISE ON COMETS. 
 
 for a reason similar to that just noticed, that it could 
 not continue so to revolve, and that it ought to 
 change its axis from hour to hour, until at length it 
 would again revolve round its shortest diameter. 
 It is, however, certain that the axis of the earth's 
 rotation has never been changed, because, as the 
 ethereal fluid which is supposed to pervade space 
 offers no sensible resistance to so dense a body as 
 the earth, the libration would to this day be evident 
 in the variation of geographical latitudes. As 
 nothing is known of the primitive velocity of the 
 earth, it is possible that a comet may have given it 
 a shock without changing the axis of rotation, but 
 only destroying a part of its tangential velocity, so 
 as to diminish the size of the orbit. In this case 
 the effect would have been to increase the temper- 
 ature of the seasons ; but geologists have shown, 
 that instead of the earth having become warmer, 
 the tropical nature of the fossil remains found in 
 the most northern countries of Europe and America, 
 declare that the general climate of the earth is of a 
 lower temperature now, than it was in the extremely 
 remote ages in which these plants and animals must 
 have flourished. 
 
 We see, therefore, that the consequences of a col- 
 lision between a comet and the earth might be 
 frightful in the extreme ; yet it may be demonstrated 
 that there is but little chance that such a collision 
 
THEORY OF COMETARY INFLUENCE. 73 
 
 can ever take place. Indeed, it has been found by 
 actual calculation, from the theory of probabilities, 
 that if the nucleus of a comet having a diameter 
 equal only to one fourth part of that of the earth, 
 should come nearer the sun than the earth, its orbit 
 being otherwise unknown, the probability of the 
 earth receiving a shock from it, is only one in two 
 hundred and eighty-one millions. It is found, also, 
 that the chance of our coming in contact with its 
 nebulosity is not more than ten or twelve times 
 greater. We may rest satisfied, therefore, that, 
 although it cannot be affirmed absolutely that the 
 earth will never come in collision with a comet, there 
 is no reasonable cause to dread such an event, since, 
 by supposing the number of comets which pass near 
 enough to the sun to satisfy the required conditions 
 to be three in each year, which is perhaps too large 
 an estimate, we arrive at the conclusion that the 
 earth may receive a shock from one of these bodies 
 in the course of the next ninety-four millions of 
 years, while the probability is nearly as great that 
 such an event will never occur. This result, although 
 to those unacquainted with the mathematical theory 
 of probabilities it may seem conjectural, is worthy 
 of the very highest confidence ; and we may unhesi- 
 tatingly declare that although, in strict geometrical 
 rigor, it is not physically impossible that a comet 
 7 
 
74 TREATISE ON COMETS. 
 
 should encounter the earth, yet the moral possibility 
 of such an event is absolutely nothing. 
 
 The only instance in which any such serious 
 catastrophe has been anticipated, to any considerable 
 extent, was in the case of the return of the comet 
 of Biela to its perihelion, or point nearest the sun, 
 in the year 1832. A detailed account of this comet 
 will be given in the proper connection ; but it may 
 be well to state here, that this appearance of the 
 comet excited the liveliest interest throughout the 
 civilized world. A short time before its appearance, 
 Damoiseau created very serious apprehension in 
 France and elsewhere, by predicting that the comet 
 would pass within eighteen thousand four hundred 
 and eighty-four miles of the earth's orbit, a little 
 before midnight, on the 29th of October, 1832 ; and 
 since Olbers had computed that the radius of the 
 comet's nucleus or head which is the distance 
 from the centre of the comet to its surface would 
 be twenty-one thousand one hundred and twenty 
 miles, it was evident that its nebulosity would en- 
 velop a portion of the earth's orbit, and if, by an}- 
 cause, the comet should be retarded one month in 
 its revolution, it would undoubtedly meet the earth 
 at that point. This startling announcement, ema- 
 nating from a source which could not be questioned, 
 excited the popular apprehension to the very highest 
 degree. 
 
THEORY OF C o M E T A R Y INFLUENCE. 75 
 
 Arago, however, dispelled the fears of his coun- 
 trymen, by showing that the comet would not be 
 retarded in its course, but that it would pass nearest 
 to the earth, at a distance of fifty-five millions of 
 miles ; and, consequently, could not have the 
 slightest influence in terrestrial affairs, since it had, 
 in 1805, been ten times nearer without any sensible 
 effect, and without exciting any alarm. 
 
 We have now completed the account of cometary 
 influences, both real and imaginary, and^more espe- 
 cially the latter ; and what has been said ought to 
 be sufficient to assure every intelligent person, that 
 every superstitious notion which may have been 
 attached to them, every baneful influence which 
 they may have been supposed to exert, every sup- 
 posed influence in producing atmospheric disturb- 
 ances, and in changing or modifying the temperature 
 of our climate, and further, that all fears of a colli- 
 sion between one of these bodies and our earth, 
 must be regarded as one of the aberrations of the 
 human mind. We have been able to trace, how- 
 ever imperfectly it may have been, some distant 
 resemblance at least between these notions of come- 
 tary influence and the development of the philo- 
 sophy of a people ; and we have noticed that there 
 is even a tendency in the human mind to cling to 
 all such ideas, no matter how absurd, after *the 
 inauguration of a true philosophy has revealed their 
 
76 TREATISE ON COMETS. 
 
 glaring inconsistencies. At this very day we read 
 in the newspapers often of some comet which is 
 certainly about to destroy the present state of 
 things ; while, even in case no such apprehension is 
 announced, we are charmed with the cheerful intel- 
 ligence that in the German vineyards, in the beau- 
 tiful valleys of the Rhine and Moselle, these once 
 ill-omened bodies have been exercising a beneficial 
 influence on the ripening of the vine. 
 
 It is indeed pleasing and instructive to consider 
 the important facts which history has furnished us 
 on this subject; but the time has now come when 
 all such fears as we have noticed should be at an 
 end, and when the return of a comet, which formerly 
 spread dismay and terror throughout the world, 
 should be regarded as one of the greatest triumphs 
 of science. 
 
 " Lo ! from the dread immensity of space 
 Returning with accelerated course, 
 The rushing comet to the sun descends; 
 And as he sinks below the shading earth, 
 "With awful train projected o'er the heavens, 
 The guilty nations tremble. The enlighten'd few 
 "Whose godlike minds philosophy exalts, 
 The glorious stranger hail. They feel a joy 
 Divinely great ; they in their powers exult, 
 That wondrous force of thought, which mounting spurns 
 This dusky spot, and measures all the sky ; 
 While from his far excursion through the wilds 
 Of barren ether, faithful to his time, 
 They see the blazing wonder rise anew." 
 
APPARENT FORMS OF COMETS. 77 
 
 CHAPTER II. 
 
 APPARENT FORMS OF COMETS DESCRIPTION OF SOME OF THE HOST 
 REMARKABLE COMETS WHICH HAVE APPEARED DURING THE PERIOD 
 OF AUTHENTIC HISTORY THE GREAT COMETS OF 1556, 1680, 1769, 
 1807, 1811, 1843, 1853, AND 1858 NUMBER OF COMETS. 
 
 BEFORE proceeding to give a detailed account of 
 some of the most remarkable comets which have 
 appeared during the period which has elapsed since 
 the earliest records of the Chinese, it may not be 
 improper to explain, in this connection, some of the 
 general and more prominent characteristics by which 
 these bodies are distinguished. We shall subse- 
 quently enter into a complete exposition of the 
 theory of their form and physical constitution, and 
 we propose to notice now simply what may be 
 required to render the descriptions which follow 
 intelligible. 
 
 The comets, for the most part, present the appear- 
 ance of a globular mass of illuminated vapor or 
 nebulous matter, with a train, or brush, as it is 
 termed by the Chinese astronomers, extending in a 
 direction opposite to the sun. There are, however, 
 those which, being invisible to the naked eye, ex- 
 hibit no signs of a train or elongation opposite the 
 
78 TREATISE ON COMETS. 
 
 sun, and of which even the nebulous mass itself is 
 ill-defined and irregular. The latter are generally 
 of extreme faintness, and evince their cometary 
 nature only by their motion in their orbits. The 
 discovery of these has been much more rapid of 
 late years than formerly, owing to the perfection 
 and number of the instruments employed in search- 
 ing for them; and although, as will hereafter be 
 noticed, these apparently insignificant objects are by 
 no means uninteresting, yet it will be sufficient for 
 our present purpose to understand the phenomena 
 presented in the case of the larger and brighter 
 telescopic comets, and those visible to the naked 
 eye. 
 
 The nebulous mass of matter, independent of the 
 train, is called the head of the comet, while the 
 latter is usually termed the tail. At or near the 
 centre of the head there is almost always a point 
 of greater condensation of light than in the other 
 parts of the comet ; and this bright spot, which is 
 not invariable, is called the nucleus. The nucleus 
 appears, in the case of some of the great comets, 
 sometimes like a minute stellar point, and some- 
 times presents the appearance of a planetary disc 
 shining through a nebulous haze, which is termed 
 the coma. 
 
 These definitions will serve to illustrate the de- 
 scriptions which we shall now give of some of the 
 
DESCRIPTION OF COMETS. 79 
 
 most remarkable comets whose appearances have 
 been recorded ; and, as we have already remarked, 
 we shall subsequently explain, in the proper con- 
 nection, all the various changes which take place in 
 the figure and constitution of the nucleus and train 
 as the comet approaches the sun, and in receding 
 from it. 
 
 The Chinese have recorded the appearances of 
 comets professedly as far back as the year 2550 
 before the commencement of our era; but the 
 accounts given are so vague and unsatisfactory, and 
 the dates so uncertain, that although these bodies 
 have not failed to attract a great deal of attention 
 even in the very remotest ages of the world, yet we 
 have no authentic observations earlier than about 
 four hundred years before Christ. There can be no 
 doubt, however, but that the Chinese have actual 
 records far anterior to this date ; but so little is 
 known in regard to them, that we can glean nothing 
 from the accounts given which could interest a 
 general reader. 
 
 During the four centuries which immediately pre- 
 ceded the birth of Christ, we have reliable accounts 
 of the appearances of thirteen comets. Some of 
 these are described as having been of prodigious 
 magnitudes, and as having presented terrific aspects. 
 So little was known in those days in regard to their 
 nature and motions, that the phenomena recorded 
 
80 TREATISE ON COMETS. 
 
 give us no additional information in regard to them 
 which is of any value. As usual, they were sup- 
 posed to have caused the most dreadful disasters 
 which occurred at or near the date of their appear- 
 ance ; and if the accounts are not exaggerated, the 
 phenomena presented were fully as imposing as any 
 which have been exhibited in modern times. It is 
 related by Diodorus, that at the time of the expe- 
 dition of Timoleus of Corinth to Sicily, in the year 
 344 before our era, a burning torch was visible in 
 the heavens throughout the entire night, and that 
 it preceded the train of Timoleus until its arrival 
 in Sicily, thus connecting indirectly the appearance 
 of the comet with this particular event. Seneca 
 records the appearance of a comet in the year 156, 
 which is said to have been apparently as large as 
 the sun. At first its appearance was red and fiery, 
 emitting a suificient quantity of light to dissipate 
 the darkness of night ; but it afterwards diminished 
 gradually, and finally became invisible. 
 
 The year 136 B. C. was signalized by the appear- 
 ance of three large comets, one of which was so 
 bright that the heavens appeared to be on fire, the 
 comet being described as having occupied one-fourth 
 part of the sky. Lucian informs us that a comet, 
 which appeared in the year 48 B. C., was so brilliant 
 that the darkness of the night was illuminated by it, 
 and that the comet itself presented a terrific appear- 
 
DESCRIPTION OF COMETS. 81 
 
 ance. It is also recorded that a comet appeared in 
 the year 43 B. C., which was visible two or three 
 hours before sunset. Dion Cassius relates that, 
 under the consulate of M. Valerius Messala Barba- 
 tus and P. Sulpicius Quirinus, which corresponds 
 to the year 11 B.C., before the death of Agrippa, 
 a comet was seen for many days. It appeared as if 
 suspended over the city of Rome, and afterwards 
 resolved itself into many smaller comets. The latter 
 statement might cause us to feel inclined to reject 
 the account given by Dion Cassius, were it not that 
 we have already an example of a similar character 
 in the case of Biela's comet, which will be described 
 hereafter. 
 
 The foregoing descriptions relate to the principal 
 comets which appeared anterior to the commence- 
 ment of our era. During the five centuries imme- 
 diately following this epoch, we have records of the 
 appearances of no less than eighty-three comets, 
 some of which are described as having been truly 
 magnificent. A very beautiful one appeared in 
 China, in the year 178. It was first seen on the 30th 
 of August, and exhibited a tail only a few degrees 
 in length. It became, however, gradually brighter, 
 until the train extended over more than sixty de- 
 grees of the heavens. The nucleus was remarkable 
 for its ruddy appearance, and remained visible nearly 
 three months ; having, in the meantime, passed over 
 
 F 
 
82 TREATISE ON COMETS. 
 
 an arc of three hundred degrees in its apparent 
 course. 
 
 In the year 389, a comet of great brilliancy ap- 
 peared in the north-east, a few hours before sunrise. 
 It is described as resembling a lamp, with the flame 
 tending upwards towards the zenith. It was visible 
 during a period of forty days, and finally disappeared 
 in the constellation Ursa Major. A comet of pre- 
 cisely the same aspect, and moving in the same di- 
 rection, was visible in the following year. Claudian 
 describes a comet which was visible in the year 402, 
 as being first seen in the east, towards that part of 
 the heavens in which the constellations Cepheus 
 and Cassiopeia are situated. It subsequently moved 
 northward above Ursa Major, and was so brilliant 
 that it diminished the beauty of the bright stars of 
 that constellation. It then began to fade away, and 
 finally vanished in a very narrow flame. 
 
 On the 19th of July, 418, there was a total eclipse 
 of the sun, during which the darkness was so great 
 that the stars were plainly visible. During the time 
 of total obscuration, a light in the form of a cone 
 was perceived in the sky, which resembled the flame 
 of a lamp. This object proved to be a comet which 
 was subsequently seen in China, where it presented 
 a very beautiful appearance. When it first became 
 visible in China, it was in the constellation Cygnus, 
 and moving west and north through the Great Bear: 
 
DESCRIPTION OF COMETS. 83 
 
 it finally disappeared in the constellation Leo, having 
 been visible more than three months. 
 
 After the commencement of the sixth century, 
 the recorded appearances of comets are much more 
 frequent; and we shall attempt to notice only those 
 which were most remarkable, either on account of 
 their extreme brilliancy, or from some" peculiar phy- 
 sical characteristic. It should be remarked, how- 
 ever, that until about the year 1200, we are com- 
 pelled to rely almost entirely on the Chinese ac- 
 counts, which are often, if not generally, vague and 
 unsatisfactory. The great social and political con- 
 vulsions which were continually disturbing the na- 
 tions of Europe, were hostile to the advancement 
 of astronomical scien-ce ; and while the most en- 
 lightened nations of the globe were absorbed wholly 
 in the study of eloquence, poetry, and the fine arts, 
 and more especially in the pursuit of military fame, 
 it was reserved for the inhabitants of the East to 
 cherish science in its infancy, and to furnish the ele- 
 ments of its progress. When the affairs of Europe 
 had become settled, the study of astronomy soon 
 began to engage the attention of learned men ; and 
 henceforward we are provided with the most com- 
 plete and accurate descriptions of the comets which 
 appeared. 
 
 In the year 530, a very large and brilliant comet 
 appeared in China, which was found to move rapidly 
 
84 TREATISE ON COMETS. 
 
 toward the north, and west. It was first seen in 
 October ; and, since a comet was seen in Europe in 
 the beginning of the year 531, presenting the same 
 general appearance, we may conclude that the comet 
 was visible in China before its passage through the 
 point of its orbit nearest the sun, and in Europe 
 after its passage through that point. This comet 
 has been supposed to be identical with that which 
 appeared at the death of Julius Csesar, and which 
 Whiston supposed to have caused the Mosaic deluge, 
 as we have already noticed. It is related by Gregory 
 of Tours, that in the year 582, on Easter day, at 
 Soissons, the heavens were seen on fire, which 
 remark is supposed, in all probability, to refer to a 
 very large and terrific comet, which first made its 
 appearance in January of that year. It was sur- 
 rounded by an immense, nebulous envelope, such 
 that the comet is said to have been situated, as it 
 were, in a kind of opening. Its train extended to 
 an enormous distance from the nucleus, and is de- 
 scribed as having resembled the smoke of a great 
 conflagration, viewed from a distance. The comet 
 was seen in the west immediately after sunset. 
 
 On the 8th of July, 615, a comet was seen in China, 
 in the Great Bear. Its tail was fifty or sixty degrees 
 in length, and was remarkable for its dusky color. 
 During the night its head or nucleus appeared to 
 have a motion of libration. It moved to the north- 
 
DESCRIPTIONOF COMETS. 85 
 
 west for several days, and afterwards retrograded 
 and disappeared. A comet is mentioned by Euro- 
 pean historians about the time of the election of 
 Pope Donus, in the year 676, which had a tail five 
 degrees in length, and was remarkable from the fact 
 that its motion was such that its gradual diminution 
 from day to day, as the comet receded from the 
 earth, was distinctly noticed. This comet was also 
 observed in China, on the 4th of September, 676, in 
 the constellation G-emini. It moved toward the 
 north-east, and disappeared about the 1st of No- 
 vember in the constellation Ursa Major. 
 
 Father de Maille relates, that on the 22d of March, 
 837, a comet was seen in China in the adjacent part 
 of the constellations Aquarius and Pegasus, the tail 
 of which was seven degrees in length. It was found 
 to move toward the west, and on the 6th of April 
 the length of the tail was ten degrees. On the 10th 
 of April the length of the tail had increased to fifty 
 degrees, and was separated into two distinct parts, 
 the bifurcation commencing at the nucleus and in- 
 creasing in width to the end of the tail. On the 
 12th of April, its length was sixty degrees, and ap- 
 peared again undivided, the secondary train, which 
 was not as bright as the primary, having apparently 
 vanished. On the 14th of April, the tail extended 
 from the horizon to the zenith, and the appearance 
 of the comet was extremely grand. After this date 
 8 
 
86 TREATISE ox COMETS. 
 
 the length of the tail decreased rapidly, and on the 
 28th of April the comet was seen for the last time, 
 with a train only three degrees long. Some astrono- 
 mers have supposed this comet to have been a former 
 appearance of a comet which appeared in 1682, and 
 which is now known as Halley's comet. 
 
 One of the largest and most remarkable comets 
 on record made its appearance in the year 895. It 
 was seen in China on the 25th of June, in that year, 
 and the tail or brush is said to have been one hun- 
 dred degrees in length. When the comet first be- 
 came visible it was in the Great Bear, but it subse- 
 quently moved towards the constellation Hercules. 
 The brilliancy increased very rapidly, and, accord- 
 ing to the Chinese historians, it actually attained the 
 enormous and unexampled length of two hundred 
 degrees. This account is undoubtedly somewhat 
 exaggerated, but it is sufficiently certain that the 
 length of the train was unsurpassed by any other 
 which had ever before been recorded. Another 
 large comet appeared in the autumn of the year 
 975. In China it was seen on the 3d of August, in 
 the constellation Hydra, at which time the tail was 
 forty degrees in length. The comet was visible 
 during a period of eighty-three days, and passed 
 through the constellations Cancer, Gemini, Taurus, 
 and Aries, and a part of Pegasus and Andromeda. 
 The course pursued by the comet was such that 
 
DESCRIPTION OF COMETS. 87 
 
 Pingre supposes it to be identical with the great 
 comet which appeared in 1264 and in 1556. By 
 supposing that the comet passed nearest the sun a 
 few days before the end of July, 975, it may be 
 shown that it would have been in conjunction with 
 the sun on or about the 3d of August, but that its 
 north latitude being considerable, it would rise 
 before the sun, and might exhibit all the phenomena 
 which have been recorded. 
 
 In April, 1066, a comet of great brilliancy was 
 seen both in Europe and in China. It was supposed 
 by European historians to be the forerunner of the 
 conquest of England, by William, Duke of Nor- 
 mandy. The course of the comet seems to render 
 it probable that it is "a previous return of the comet 
 of 1677. Another comet was seen for a very short 
 time, in October, 1097. It was visible in Europe 
 only fifteen days, and is remarkable on account of 
 having two separate trains, which were directed to- 
 ward the east and south-east respectively. In China, 
 on October 6th and 9th, its tail was noted as being 
 respectively thirty and fifty degrees in length. On 
 the 6th of October it was seen in the constellation 
 Libra, and on the 16th of the same month it had 
 moved north as far as the head of Hercules. It 
 ceased to be visible in China on the 25th of October. 
 A large and brilliant comet was also seen in Pales- 
 tine and in China early in February, 1106. Its tail 
 
88 TREATISE ON COMETS. 
 
 was sixty degrees in length, and was similar in color 
 to the whiteness of snow. The comet was visible 
 during a period of fifty days. 
 
 In the year 1264, a great and celebrated comet 
 made its appearance, and its magnitude was such 
 that it is mentioned by all the historians of that day ; 
 and it is also distinctly stated that no one living at 
 the time of its appearance had ever seen one which 
 could be compared with it. It was first seen about 
 the 1st of July, and attained its maximum brilliancy 
 in the latter part of August and in the beginning 
 of September. The tail was one hundred degrees 
 in length, and appeared curved in the form of a 
 sabre. The train was visible in the east early in the 
 evening, although the nucleus did not appear above 
 the eastern horizon until near morning. The comet 
 was last seen on the evening of the 3d of October, 
 the date of the death of Pope Urban IV., of which 
 event it was considered the precursor. The course 
 pursued by the comet was such that, in more modern 
 times, it has been conjectured that this body is 
 identical with the great comets which appeared in 
 975 and in 1556. 
 
 We have here, and also in the preceding pages, 
 alluded to the supposed identity of comets which 
 have appeared at different epochs. These allusions 
 may seem premature, from the fact that, thus far, no 
 account has been given of the nature and forms of 
 
DESCRIPTION OF COMETS. 89 
 
 the paths pursued by the comets in their motions 
 through space. For reasons which will presently 
 be made obvious, it has seemed expedient to defer 
 a complete exposition of the theory of the motions 
 of the comets, until we come to treat of those comets 
 which are definitely known to have fixed periods of 
 revolution, and which have been certainly identified 
 at two or more different visits to our vicinity. 
 Under such circumstances it may be advisable to 
 state, at once, in reference to the forms of the 
 cometary orbits, what will sufl3.ce for our present 
 purpose, reserving for the proper connection a de- 
 tailed explanation of the theory of the motions of 
 these chaotic worlds. 
 
 The comets are known to move in regular orbits 
 in three different kinds of curves, in obedience to 
 the law of universal gravitation. These curves are 
 the ellipse, parabola, and hyperbola, the nature of 
 which will be subsequently explained. Those which 
 move in ellipses will return regularly to the sun at 
 stated intervals, while those which move in para- 
 bolas and hyperbolas visit our system but once, and 
 thence pass on to visit other suns and systems. Of 
 all the comets which have been discovered, a ma- 
 jority are fo-und to move in these curves, and the 
 number which move in elliptic orbits whose dimen- 
 sions are confined within the probable bounds of the 
 solar system, is much more limited. But a small 
 
90 TREATISE ON COMETS. 
 
 number, comparatively, have been observed at more 
 than one appearance, and these we propose to con- 
 sider as a separate and distinct class. There are 
 many others which the delicate refinements of 
 modern astronomy have shown to move in orbits 
 whose ellipticity is clearly indicated, but in which 
 the deviation from either the hyperbola or parabola 
 is so uncertain that it is impossible to predict, 
 within very narrow limits, the precise date of their 
 return. The comet of 1264 is one of these ; and 
 although the elements which determine the form and 
 position of its orbit in space, are found to bear a close 
 resemblance to those which have been obtained for 
 the comets of 975 and 1556, yet we are unable to 
 venture a prediction that these comets are identical, 
 and that another appearance may be expected after 
 the lapse of about 300 years from the date of its last 
 visit to the sun. The point of the orbit which is 
 nearest the sun is called the perihelion ; the point 
 which is most remote is called the aphelion ; and 
 the nature of these curves is such, that for a consi- 
 derable distance on either side of the perihelion 
 they nearly coincide. This, together with the un- 
 certainty of the observations, and the fact that the 
 comet is visible from the earth only when near the 
 sun, sometimes renders it impossible to decide in 
 regard to the exact form of the orbit. "With these 
 explanatory remarks, we will continue the descrip- 
 
DESCRIPTION OF COMETS. 91 
 
 tioD of the most remarkable comets which have 
 appeared subsequent to the one we have just 
 noticed. 
 
 In the year 1402 a very large and brilliant comet 
 made its appearance. It was seen at different 
 places on the 8th of February, and was so brilliant 
 that it could be seen at mid-day. It disappeared 
 early in March. Another comet of extraordinary 
 magnitude made its appearance in 1456. It is rep- 
 resented by historians as being grand and terrible, 
 with a tail more than sixty degrees in length. It 
 was visible during the month of June, and was very 
 remarkable on account of the rapid variations in 
 the size of the train, which at one time did not 
 exceed seven degrees in length. The first appear- 
 ance of the comet was so sudden and terrific, that a 
 belief became very prevalent, among all classes, 
 that it would destroy the earth, and that the Day 
 of Judgment was at hand. This is the comet which 
 we have already noticed as having created a most 
 profound sensation at Rome, since it was regarded 
 as having presaged the rapid success of the Turks, 
 who were then engaged in subjugating the nations 
 of Europe. The fact that the followers of Moham- 
 med had already crossed the Hellespont, and that 
 no force had been able to check their progress, 
 together with the great apprehension which the 
 appearance of the comet had already excited, seemed 
 
92 T 11 E A T I S E N C M E T S . 
 
 to add to the general gloom which pervaded all 
 classes of society ; and it was under these circum- 
 stances that Pope Calixtus II. ordered prayers to be 
 offered daily for preservation from the baneful influ- 
 ences of the comet, the devil, and the Turks, and 
 issued a bull in which all three were equally ana- 
 thematized. This comet is now recognized as a 
 previous return of the comet of 1682, known as 
 Halley's comet; and its great brilliancy resulted 
 from the favorable circumstances, in this respect, 
 of its position with reference to the earth and sun. 
 
 In February, 1490, a comet appeared in Europe 
 which had a small nucleus, but a train extending 
 to an enormous distance, and remarkable for its 
 phosphorescent appearance. It was visible at Bou- 
 logne about the middle of February, and was pro- 
 bably not seen in China and Japan. A comet is 
 recorded in the Chinese annals in the year 1491, but 
 there is hardly any reason to believe it to have been 
 identical with this. 
 
 The comet of 1456 again made its appearance in 
 July and August, 1531, but was far from the earth 
 and sun, and consequently not very brilliant. The 
 year following, 1532, a comet was seen in China and 
 also in Europe, which has been supposed to be iden- 
 tical with one which appeared in 1661. It was 
 visible in the morning before sunrise, and the nu- 
 cleus is described as being apparently three times 
 
GREAT Co M E T o v 1 o o 6 . 93 
 
 larger than Jupiter. It was visible in Europe about 
 seventy days, and in China one hundred and fifteen 
 days. The train varied in length from one to ten 
 degrees, and was very bright. 
 
 In 1556 another comet of prodigious magnitude 
 made its appearance. It began to be generally 
 visible about the end of February, at which time it 
 is said to have been more than half as large as the 
 full moon. Its tail, however, was short and vari- 
 able, and for this reason it was described as exhi- 
 biting a movement like that of a flame, or a torch 
 disturbed by the wind. The greatest length of the 
 tail did not exceed four degrees, and its color was 
 ruddy, not unlike that of the planet Mars. It 
 moved rapidly north,- and on the 12th of March was 
 distant from the earth only eight millions of miles. 
 The comet disappeared in the sun's rays on the 23d 
 of April, in the southern part of the constellation 
 Cassiopeia. It created considerable excitement in 
 Europe, and is said to have been the immediate 
 cause of the abdication of Charles V., Emperor of 
 Germany. 
 
 The orbit of this comet was computed by Dr. 
 Halley ; but since the observations on which they 
 were based were uncertain, they have not been con- 
 sidered very accurate. They were sufficient, how- 
 ever, to indicate the probable identity of this comet 
 with those which had been observed in 975 and 
 
94 TREATISE ON COMETS. 
 
 1264. He was therefore led to conjecture that the 
 comet had a period of revolution around the sun of 
 about 292 years, and that it might he expected to 
 reappear in 1848. A few years later Pingre col- 
 lected together all the observations of the comets 
 of 9T5 and 1264, and more especially the latter, 
 which could be found in both the European and 
 Chinese annals, and from a complete investigation 
 of the elements of the orbits of the comets of 1264 
 and 1556, he found that the courses of both comets, 
 at the date of their appearance, could be very satis- 
 factorily represented by the same elements. He 
 therefore confirmed the hypothesis of Halley, and 
 concurred in the opinion that the comet might be 
 expected again in 1848. Nothing further was at- 
 tempted until the time had nearly elapsed at which 
 it was to reappear. Between the years 1843 and 
 1847 the whole subject was investigated anew by 
 Mr. Hind, of London, arid M. Bomme, of Middle- 
 burg, in the Netherlands. Mr. Hind found that the 
 elements were such as to confirm their supposed 
 identity ; and by taking into account the planetary 
 perturbations, he concluded that the comet would 
 appear about the year 1858. 
 
 The computations of Bomme were more extended, 
 and he found that with Halley 's elements, at the 
 time the comet was visible in 1264, it was moving 
 in an ellipse, with a periodic time of 112,469 days, 
 
GREAT COMET OF 1556. 95 
 
 or about 308 years ; but tbat the disturbances pro- 
 duced by the attraction of the planets would accele- 
 rate its motion, and shorten its period 5903 days, so 
 that it passed its perihelion in April, 1556. He 
 found that at this return it was describing an elliptic 
 orbit which gave a period of 112,943 days; but that 
 the next revolution would occupy a period of 111,146 
 days, bringing the comet to its perihelion on the 
 22d of August, 1860. 
 
 With Hind's elements it was found that, in 1264, 
 the ellipse described by the comet corresponded to 
 a period of 110,644 days, or nearly 303 years ; and 
 that the attraction of the planets would shorten its 
 period a little more than eleven years, thus bringing 
 the comet to its perihelion in 1556. The same ele- 
 ments indicated that at the time it was visible in 
 1556, its mean motion corresponded to a period of 
 308 years, and that the period of its next return 
 would be expedited by planetary disturbances 3828 
 days, or ten and a half years. The comet would, 
 therefore, arrive at its perihelion early in August, 
 1858. Thus it appears that there is an uncertainty 
 of at least two years in the time of its next appear- 
 ance ; and since the interval included between the 
 results of Hind and Bomme has now elapsed, we 
 must conclude that those of the latter are most accu- 
 rate, although themselves perhaps very far from the 
 truth. 
 
96 TREATISE ON COMETS. 
 
 The important circumstance of an apparent iden- 
 tity between the comet of 975 and those of 1264 
 and 1556, has already been noticed ; and it is found 
 that, assuming the identity, and adopting Hind's 
 elements, the path of the former may be very closely 
 represented. This question, however, has been ela- 
 borately discussed by M. Hoek, of Leiden, and he 
 concludes that there is no reason to regard the 
 comets as identical. He finds that the path of the 
 comet cannot be satisfactorily represented by the 
 elements of either the comet of 1264 or 1556, each 
 of which he has investigated anew. He finds also 
 that there exist great discrepancies even between 
 the orbits of the latter comets, and announces that 
 their identity is extremely doubtful. It appears, 
 therefore, that it is barely possible that the comet of 
 1556 may shortly appear, although no such predic- 
 tion can be entitled to any particular confidence. 
 Should the comet really make its appearance, its 
 future movements would no longer be involved in 
 doubt, but its next and succeeding returns could be 
 predicted with the utmost precision. 
 
 The next comet of any considerable magnitude, 
 compared with those which we have already de- 
 scribed, made its appearance in 1590. It was dis- 
 covered by Tycho Brahe, on the 5th of March, and 
 observed until the 16th of the same month. It is 
 noted as being of a medium magnitude, but had a 
 
COMETS FROM 1590 TO 1668. 97 
 
 great tail, which extended as far as the zenith, the 
 nucleus being near the horizon. On the day of its 
 discovery, the comet appeared as bright as a star of 
 the second magnitude ; and shortly afterwards, on 
 the same night, it was equal in brilliancy to a star 
 of the first magnitude. Another large comet ap- 
 peared in 1618. It was discovered in Silesia on the 
 10th of November, and at Rome on the same even- 
 ing. It was also observed by the Spanish Ambassa- 
 dor at Ispahan, in Persia, for fifteen days, com- 
 mencing with the date of its discovery in Europe ; 
 and presented a magnificent appearance in the east- 
 ern sky, about two hours before sunrise. The length 
 of the tail was about 60 degrees. 
 
 In December, 1652, a comet was seen in Europe, 
 which was of a pale and livid color, and almost 
 equalled the moon in magnitude and brilliancy. No 
 mention is made of the length of its tail. Another 
 comet of considerable size was seen in Spain, on the 
 17th of November, 1664. It was subsequently ob- 
 served in France and Germany ; and was last seen 
 by Helvetius, at Dantzic, on the 18th of February, 
 1666. At its first appearance, it was noted as large 
 as a star of the first magnitude, but not so bright. 
 The length of the tail varied from five to ten de- 
 grees. Four years later, 1668, the tail of a comet 
 was seen above the southern horizon, at different 
 places in Italy. The nucleus was not visible, but 
 9 
 
98 TREATISE ON COMETS. 
 
 the comet was observed in the southern hemisphere. 
 This comet has been supposed to be identical with 
 the great comet which appeared in 1843, and is re- 
 markable for its near approach to the sun. 
 
 One of the most remarkable comets on record 
 made its appearance toward the close of the year 
 1680, and was carefully observed by the astronomers 
 of Europe. It is said to have appeared to descend, 
 as it were, from the distant regions of space with a 
 prodigious velocity, almost perpendicular to the sun, 
 and to have ascended again in the same manner, 
 with a velocity retarded as it had before been acce- 
 lerated. The comet was discovered at Coburg, in 
 Saxony, on the 4th of November, 1680 ; but the 
 name of the discoverer is unknown. It was also 
 discovered independently by Godfrey Kirch, on the 
 14th of November, while about to observe the moon 
 and Mars. It was very carefully observed at Paris 
 by Oassini, during the entire period of its visibility. 
 It was observed by Flamstead, at Greenwich, until 
 the 15th of February, 1681. The observations of 
 Cassini, Flamstead, and others, enabled Newton to 
 trace out the form of its orbit, and its position in 
 space ; and he found that the observations could be 
 satisfactorily represented by supposing the orbit to 
 be a parabola, with the sun in the focus. At its 
 perihelion, he found that the distance of the comet 
 from the surface of the sun did not exceed one-sixth 
 
GREAT COMET OF 1680. 99 
 
 of the sun's diameter, or about 147,000 miles. The 
 velocity of the comet, wheu nearest the sun, must 
 therefore have been 880,000 miles an hour. 
 
 A more complete determination of the elements 
 of the orbit of this comet was made by Dr. Halley, 
 who found that the orbit was sensibly an ellipse, 
 with the sun in one of the foci ; and that the peri- 
 helion distance was only 590,000 miles from the 
 sun's centre, while its aphelion distance was at least 
 13,000,000,000, or thirteen thousand millions of 
 miles. He determined also that, in passing the point 
 of intersection of its orbit with the plane of the 
 earth's orbit, in moving from the north to the south 
 side of the latter, or in passing through what is 
 called the descending node, the comet was distant 
 from the earth only 440,000 miles. It passed through 
 this point on the 22d of November; and it was re- 
 marked by Halley that, had the earth been then in 
 that part of its orbit nearest the descending node 
 of the comet, their mutual gravitation would have 
 caused a change in the plane of the earth's orbit, 
 and in the length of our year; and if so large a 
 body, with so rapid a motion, were to strike the 
 earth, the shock might reduce this beautiful struc- 
 ture to its original chaos. 
 
 Whistoii supposed this comet to be identical with 
 the comets which appeared in the years 1106, 531, 
 and 43, B. C. ; and under this supposition he com- 
 
100 TREATISE ON COMETS. 
 
 putes backward, and attempts to explain the phe- 
 nomena of the Mosaic deluge by a near approach 
 of the comet to the earth, as already explained. 
 The computations of Euler, Pingre, and Encke, have 
 fully confirmed the supposed ellipticity of its orbit, but 
 indicate that its period of revolution must be vastly 
 greater than 575 years ; and that, consequently, the 
 speculations of "Whiston, at least so far as this comet 
 is concerned, must be regarded as extremely absurd. 
 This comet was remarkable for its near approach to 
 the earth and sun, and also for the brilliancy of its 
 light and the length of its tail, which extended over 
 half the vault of heaven, or from the zenith to the 
 horizon. 
 
 In the month of August, 1686, a comet was visible 
 at Para, in Brazil, whose nucleus was as bright as a 
 star of the first magnitude. The tail was eighteen 
 degrees in length, and is described as having pre- 
 sented a very beautiful appearance. Three years 
 later a comet of uncommon splendor was seen in 
 China and at different places in the southern hemi- 
 sphere. It was first discovered at Pekin on the llth 
 of December, 1689, but was not visible in Europe. 
 On the evening of its discovery the nucleus was very 
 brilliant, and the tail was about twelve degrees in 
 length. In the southern hemisphere the comet was 
 much more favorably situated for observation, and 
 consequently its general appearance was correspond- 
 
COMET OF 1744. 101 
 
 iugly improved. It was equal in brilliancy to the 
 bright stars of the first magnitude, and the greatest 
 length of the tail was 60 degrees. Another comet, 
 similar in size and appearance to that of 1686, just 
 described, was seen in Brazil on the 28th of October, 
 1695. The length of the tail was 18 degrees, and 
 the nucleus was remarked to be at first almost ob- 
 scured by the atmosphere or coma by which it was 
 surrounded, and was even, at intervals, scarcely dis- 
 tinguishable. 
 
 The first part of the eighteenth century was not 
 remarkable either for the number or brilliancy of 
 the comets which appeared. There were, however, 
 several of sufficient size and brilliancy to be visible 
 to the naked eye, but not such as to present a very 
 striking appearance. It was not, therefore, till 1744 
 that a comet was visible which deserves a notice in 
 this connection. This comet was discovered at 
 Harlem on the 9th of December, 1743, and was ob- 
 served at several observatories, until about the time 
 of its perihelion passage, which took place on the 
 evening of the 1st of March, 1744. It was one of 
 the largest and most beautiful comets which had 
 appeared ,since the famous one of 1680, and there- 
 fore excited no small degree of interest throughout 
 the civilized world. The~ nucleus was very bright, 
 and although the tail was not remarkable for its 
 length, yet it was noticed by Cassini, early in Feb- 
 9* 
 
102 TREATISE ON COMETS. 
 
 ruary, that the head was separated into two distinct 
 parts, and the train exhibited decided symptoms of 
 a corresponding appearance. The tail was, indeed, 
 shortly after this observation of Cassini, observed 
 to be divided into two branches. At one time sub- 
 sequently, and just before its passage through the 
 perihelion of its orbit, it is said to have had six 
 separate and distinct tails, the lower one relatively 
 to the horizon being greatly curved near the ex- 
 tremity. The accompanying diagram represents the 
 appearance of the comet at this time. The remark- 
 able phenomenon, however, of six distinct tails was 
 of only a few days' duration, and the comet appeared 
 again as before, having but two trains. This comet 
 approached the sun to within twenty millions of 
 miles, and the elements of its orbit which have been 
 computed do not seem to indicate that it will ever 
 reappear to human vision. 
 
 The year 1748 was remarkable for the appearance 
 of two bright comets at nearly the same time. The 
 first was discovered at Paris about the end of April 
 of that year, and was also observed in South Ame- 
 rica, and in China. It is described as having been 
 a fine object, easily seen with the naked eye, and 
 having a tail twenty degrees in length. It was last 
 observed by Maraldi, at Paris, on the 30th of June. 
 The second was discovered at Harlem, on the 19th 
 of May, in a different part of the heavens from that 
 
GREAT COMET OF 1769. 103 
 
 in which the one just described was seen. Its nucleus 
 was brighter than the preceding, but there was no 
 appearance of a tail. From this date until 1769, if 
 we except a reappearance of the comet of 1682, 
 known as Halley's comet, which will be described 
 hereafter, no comet of considerable size was ob- 
 served. 
 
 The great comet of 1769 was discovered by Mes- 
 sier, at Paris, on the 8th of August, and was subse- 
 quently observed throughout Europe. It was ob- 
 served^ by La Nux at the Isle of Bourbon, in the 
 Indian Ocean, from the 26th of August to the 26th 
 of September ; and all the various changes which 
 took place in the form and size of both the nucleus 
 and the tail were carefully recorded. On the llth 
 of September the length of the tail was found to be 
 97 degrees, while on the 28th of August it did not 
 much exceed 15 degrees. On the 9th of September 
 the length of the tail was variously estimated from 
 43 to 75 degrees. It passed nearest the sun on the 
 7th of October, about midnight, at a distance of 
 eleven millions of miles. It was nearest the earth 
 about the middle of September, and was distant 
 about ten millions of miles. It was last seen on the 
 1st of December, having been visible more than 
 three months. At the time of the appearance of this 
 comet, the methods of determining the form and 
 position of the orbits of these bodies in space by 
 
104 TREATISE ON COMETS. 
 
 means of observations made during a few days, had 
 been successfully investigated ; and since the astrono- 
 mers were enabled to predict from day to day, and 
 even for several weeks in advance, the variety and 
 grandeur of the phenomena which it would present, 
 even to the unassisted eye, it very naturally excited 
 a lively interest among all classes of society. The 
 tail was not actually as long as had been exhibited 
 in the case of many other comets which had ap- 
 peared ; but the near approach of the comet to both 
 the earth and sun, and at very nearly the same time, 
 gave it a most magnificent appearance, and caused 
 it to appear even greater than it really was. The 
 researches of modern astronomers seem to indicate 
 that the comet will return again to our system after 
 the lapse of a few thousand years, but fail to indicate 
 within any very close limits the exact period of its 
 return. 
 
 During the last half of the eighteenth century, 
 no less than forty-five comets were observed ; but, 
 with the exception of Halley's comet, and that just 
 described, they were not remarkable for .their bril- 
 liancy. Some of them were bright enough to be 
 distinguished by the naked eye, and sometimes ex- 
 hibited a faint train a few degrees in length ; but 
 the great majority were round and ill-defined nebu- 
 losities, with a slight condensation of light at the 
 centre. 
 
GREAT COMET OF 1807. 105 
 
 The next comet of considerable size was disco- 
 vered by Pons, at Marseilles, on the 20th of Sep- 
 tember, 1807. This is the date of the first authentic 
 observation, although it seems to have been seen 
 eight days previously by an Augustine monk in 
 Italy. This was the finest comet which had ap- 
 peared since that of 1769. On the 30th of September 
 the nucleus was nearly as bright as a star of the 
 second magnitude, and the tail was distinctly visible. 
 On the 8th of October it was seen in the north-west, 
 soon after sunset, and not far distant from Arcturus, 
 which was then only a few degrees above the hori- 
 zon. It is described as presenting to the naked eye 
 the appearance of a dim nebulous star of the second 
 magnitude, with a beam of light on one side of it. 
 Through a telescope its tail presented a brilliant 
 appearance, and was more than a degree in length. 
 The coma, and even the nucleus, were remarkably 
 ill-defined. On the 7th of November the tail was 
 seen divided into two parts ; and, a few days later, 
 the comet ceased to be visible to the naked eye. It 
 was last observed on the 27th of March, 1808, at 
 which date it was a very faint telescopic object. 
 
 The orbit of this comet has been very carefully 
 determined by Bessel, and he finds that it is an 
 ellipse, corresponding to a period of revolution of 
 about 1500 years. The comet passed the perihelion 
 of its orbit in the forenoon of the 19th of Septem- 
 
106 TREATISE ON COMETS. 
 
 her, at a distance from the sun of about 62,000,000 
 miles. The aphelion distance is about eleven thou- 
 sand millions of miles. The diameter of the nucleus 
 was about 4600 miles, or about the size of the planet 
 Mars, and seemed to be of considerable density. 
 The diameter of the coma or envelope was estimated 
 at 120,000 miles, but appeared to vary at different 
 dates ; and the velocity of the comet in its orbit, 
 when nearest the sun, was considerably more than 
 two millions of miles daily. The tail, as already re- 
 marked, was divided into two separate branches. 
 The north side, however, was much brighter and 
 better defined than the other, and was also invariably 
 convex, while the other was concave. The most 
 remarkable feature about the tail was the rapid 
 variations which took place in its length and bril- 
 liancy. It was seen at times to exhibit coruscations 
 or flashes of light, not unlike what is exhibited in 
 the case of the aurora borealis. It is asserted that 
 in less than one second of time, streamers might be 
 seen to shoot out from the body of the tail, and 
 generally from the vicinity of the nucleus, to a dis- 
 tance of two and a half degrees, and that they as 
 rapidly disappeared and issued out again. These 
 coruscations may perhaps have been much less 
 frequent than what is here stated, since the accounts 
 given are very probably exaggerated ; but, as will 
 be evident in the case of comets which appeared 
 
GREAT COMET OF 1811. 107 
 
 subsequently, there can be no doubt but that 
 streamers were observed to reach out even as far as 
 the whole length of the tail, or to a distance of 
 nearly five millions of miles. The light of these 
 streamers was also described as being sometimes 
 whiter and clearer at the end than at the base. 
 
 We come now to consider the case of the great 
 comet of 1811, which was discovered by Flaugergues, 
 at Vivieres, on the 26th of March of that year. 
 "When first discovered it was a faint nebulous object, 
 with a slow motion as seen from the earth, and far 
 distant from both the earth and sun. It did not 
 exhibit any signs of a tail, but presented simply a 
 slight condensation of light at the centre. It con- 
 tinued visible until the 10th of June, when it was 
 lost in the approaching twilight. It had, however, 
 been already extensively observed by the astron- 
 omers of Europe ; and from the observations it was 
 found, by computing its orbit, that it would not 
 pass its perihelion until about the 12th of September, 
 and that it would again emerge from the sun's rays 
 early in August. It was found, also, that it was 
 rapidly approaching both the earth and sun, and 
 that it would certainly present a magnificent appear- 
 ance during the entire months of September and 
 October. These predictions were indeed realized 
 to the very fullest extent. The comet was again 
 detected on the 20th of August, and observed almost 
 
108 TREATISE ON COMETS. 
 
 without interruption till the llth of January, 1812, 
 when it was again lost in the twilight. It was, 
 however, detected the third time by Wisniewsky, 
 in Neu-Tscherkask, on the 31st of July, 1812, and 
 observed for the last time on the 17th of August of 
 the same year, having been visible for a period of 
 nearly seventeen months. It was visible to the 
 naked eye during a period of more than three 
 months ; and, on account of its immense size and 
 unexampled brilliancy, excited a most profound 
 sensation. To astronomers it was especially inte- 
 resting, on account of the facilities which its long 
 visibility afforded for the determination of the ele- 
 ments of its orbit, with the very greatest precision ; 
 and, also, for the opportunity of observing its aspect 
 and its physical constitution. The great perfection 
 to which theoretical and practical astronomy had 
 been carried at the date of its appearance, and the 
 great number of observers, provided with superior 
 instruments, rendered it possible to investigate with 
 great facility all the various questions which might 
 arise. Accordingly, we find by reference to scien- 
 tific periodicals, that all the varied phenomena pre- 
 sented by the comet have been carefully recorded, 
 as well as accurate determinations, from night to 
 night, of its position in the heavens. 
 
 The comet passed the perihelion of its orbit, or 
 the point nearest the sun, on the 12th of September, 
 
GREAT COMET OF 1811. 109 
 
 at a distance of 98,565,000 miles. On the 7th of 
 September the tail appeared to be bent off in two 
 branches. These branches, however, did not pro- 
 ceed from the head of the comet, but seemed to be, 
 as it were, hung together at a slight distance from 
 it, and separated from it by a dark interval. The 
 tail was then five degrees in length, but on Septem- 
 ber 20th it had increased to 10 degrees. On the llth 
 of October the length of the train was only about 
 13 degrees, while a few days previous it was found 
 to be 25 degrees long, and 6 degrees broad at the 
 extremity. The central condensation of nebulous 
 matter was 50,000 miles in diameter, or more than 
 six times the diameter of the earth. The envelope 
 was 30,000 miles in thickness, and the centre of the 
 nucleus was separated from its interior surface by a 
 space of 36,000 miles, so that the diameter of the 
 head of the comet must have been 132,000 miles. 
 In the centre of the nucleus there was a brilliant 
 point 428 miles in diameter, which was probably 
 composed of solid matter. The part of the nucleus 
 immediately surrounding this may be supposed to 
 have been fluid, and those portions still more remote 
 to have been composed of a sort of gaseous or ele- 
 mentary form of sidereal matter. It was imagined 
 by some that the comet had a sort of phosphorescent 
 light of its own, proceeding from the denser portion 
 of the nucleus. The coma was extremely rarefied, 
 10 
 
110 TREATISE ON COMETS. 
 
 and presented the appearance of a very faint whitish 
 light, scattered in separate portions. At the time 
 when the train was seen divided into two parts, this 
 apparent division of the coma was very perceptible, 
 and the part which directly encompassed the nucleus 
 was much brighter than the other portion, which 
 shone with a faint greyish light, and sweeping around 
 the former at a distance, formed the double tail. The 
 phenomena thus presented made the comet appear 
 somewhat like a very brilliant object surrounded by 
 a dense net-work of gauze. The diameter of this 
 exterior envelope was more than 500,000 miles, and 
 its appearance was such as to induce Schroeter to 
 suppose that it very clearly indicated the existence 
 of a repulsive force residing in or around the nucleus, 
 but of what character he was unable to determine. 
 Toward the latter part of November and in the first 
 week in December, great changes were observed to 
 take place in the form and magnitude of the head 
 of the comet. The nebulous matter, which had for 
 several months exhibited the appearance of extreme 
 rarefaction, was seen to be condensed or attracted 
 towards the nucleus ; and the changes were so rapid 
 that there was exhibited the most incontrovertible 
 proof of physical action on a grand scale, perhaps 
 such that no phenomena which have ever been wit- 
 nessed on our earth, will serve to give us any adequate 
 conception of its magnitude. 
 
GREAT COMET OF 1811. Ill 
 
 The position of the comet, when brightest, with 
 respect to the earth and sun, was such that although 
 the train at one time extended to a distance of 
 120,000,000 miles, it did not subtend an angle of 
 more than 20 degrees. Under more favorable cir- 
 cumstances as regards position, it might have been 
 50 degrees in length. In the case of this comet, 
 also, streamers were seen to shoot out from the con- 
 vex side of the tail to a distance of more than two 
 millions of miles, thus producing to a limited extent 
 the appearance of coruscations such as were pre- 
 sented by the comet of 1807. The breadth of the 
 tail, at its extremity, was nearly fifteen millions of 
 miles. The comet approached the earth only to 
 within about 110,000,000 miles. It was supposed 
 by Herschel, that the solid portion of the nucleus 
 was spherical, that it shone in part by its own native 
 light, and that it probably had a rotation around an 
 axis. 
 
 The elements of the orbit of this comet have been 
 investigated by several astronomers, and it has been 
 found to be elliptical, although the periods of revo- 
 lution assigned by different computers vary between 
 long limits. The most complete research, however, 
 was performed by Argelander, who found, from a 
 discussion of the entire series of observations worthy 
 of confidence, that the period of revolution of the 
 comet is 3,065 years. Bessel, by a similar process 
 
112 TREATISE ON COMETS. 
 
 of investigation, and taking into account the dis- 
 turbances produced by the attractions of the planets 
 which will subsequently be explained finds its 
 period to be 3,383 years. Its aphelion distance is, 
 therefore, more than one hundred and fifty thousand 
 millions of miles. But great as this distance is, it 
 is perfectly certain that there are many comets which 
 revolve in orbits far more extensive than the one 
 described by this comet. Indeed, there seems to be 
 no limit to the distance to which these erratic worlds 
 may sweep outward from the sun ; and their return 
 depends simply on the fact, whether they recede so 
 far as to fall within the attractive influence of some 
 other sun, toward which they begin to urge their 
 flight, and through whose system of planets they 
 carry the same apprehensions of danger which they 
 have caused in our own system, or more especially 
 to the inhabitants of our earth. Thus may the 
 comets be regarded a connecting link between the 
 systems of suns and planets which fill the immensity 
 of space. 
 
 The great comet of 1811 may justly be considered 
 as the most magnificent one, in all respects, which 
 has ever visited our sun. In the case of nearly all 
 the very bright comets, the perihelion distance has 
 been small ; but in the case here presented it was 
 considerably more than the mean distance of the 
 earth from the sun. The other bright comets have 
 
COMETS FROM 1819 TO 1830. 113 
 
 usually approached very near the earth, while this 
 one was even more remote from us than from the 
 sun. Yet under all these circumstances its brilliancy 
 has rarely, if ever, been excelled, while in magni- 
 tude it was surpassed only by the sun itself. It is 
 no wonder, therefore, that to those unacquainted 
 with the nature of such bodies, it was a source of 
 great alarm ; or that even among the more intelli- 
 gent population of Europe, it was regarded as having 
 produced a most wonderful effect on the climate of 
 the earth ; although astronomers have shown that 
 such was not the case. 
 
 For a period of thirty years immediately following 
 the disappearance of the comet of 1811, the dis- 
 covery of these mysterious bodies was of frequent 
 occurrence ; but only in a few instances were they 
 of sufficient brilliancy to be visible to the naked eye. 
 Early in July, 1819, there was one discovered in 
 Europe which became pretty bright, and had a tail 
 about 8 degrees in length. In the years 1821, 1822, 
 and 1823 there were comets visible to the naked eye, 
 one in each year, and the last one was remarkable 
 for having exhibited the phenomenon of having, in 
 addition to the usual tail, another one in a contrary 
 direction, varying from four to seven degrees in 
 length. In 1825 and 1826 there were comets of 
 considerable size ; and in 1830 there were two which 
 were distinctly visible to the naked eye, one of 
 10* H 
 
114 TREATISE ON COMETS. 
 
 which, discovered at sea on March 17, had a tail 
 about 8 degrees in length, with a very bright nucleus. 
 In 1835 the comet of Halley made its appearance, in 
 accordance with the prediction ; and, in the southern 
 hemisphere, presented a very fine appearance. On 
 the 6th of March, 1840, a comet was discovered by 
 Galle, at Berlin, which had a tail several degrees in 
 length. These comprise the principal comets, with 
 the exception of two or three periodic ones to be 
 described hereafter which had appeared from 1811 
 up to this date, and do not afford any instance of 
 one of sufficient interest to merit a particular de- 
 scription. In the case, however, of the comet which 
 appeared in March, 1843, we are permitted to con- 
 template phenomena of a somewhat different cha- 
 racter from those presented by the great comet just 
 described, but in which there is fully as much of the 
 very highest interest. 
 
 On the morning of the 28th of February, 1843, 
 soon after sunrise, a brilliant object was seen only 
 two or three degrees from the sun, by numerous 
 observers in different parts of the globe. It was 
 seen at the Cape of Good Hope a little before sun- 
 set, and at several places in the New England States 
 as early as half past seven o'clock in the morning. 
 In the early part of the afternoon it was seen by the 
 naked eye as a bright light, resembling a dagger in 
 form, and presented at once the appearance of a 
 

GREAT COMET OF 1843. 115 
 
 very brilliant comet. The length of the head and 
 tail was about equal to twice the diameter of the 
 sun, or about one degree. The head is described at 
 this time as appearing circular, when viewed with 
 the naked eye ; its light equal to that of the moon 
 at midnight, in a clear sky ; and its apparent size 
 about one-eighth the area of the full moon. The 
 train shone with a paler light, gradually diverging 
 from the nucleus, and fading away by degrees in 
 the intensely bright sky. It was observed at Wood- 
 stock, Vermont, with a telescope of small magnify- 
 ing power, and is described as presenting a distinct 
 and most beautiful appearance, exhibiting a very 
 white and bright nucleus, and a tail divided near 
 the nucleus into two separate branches. At six 
 minutes after three o'clock, on the afternoon of the 
 same day, the distance of the nucleus of the comet 
 from the sun was measured by Captain Clark, at 
 Portland, Maine. The distance of the nucleus from 
 the sun's farthest limb was found to be 4 degrees, 
 6 minutes, and 15 seconds of arc ; or 3 degrees, 50 
 minutes, and 43 seconds from the centre. The head 
 of the comet, and also every part of the tail, were 
 then described as being as well defined as the moon 
 on a clear day. The nucleus and tail presented the 
 same appearance, and resembled a perfectly pure 
 white cloud, without any variation, except a slight 
 change near the head, just sufficient to distinguish 
 
116 TREATISE ON COMETS. 
 
 the nucleus from the tail at that point. In fact, the 
 nucleus appeared so dense that it was probable that 
 it might have been visible on the sun's disc, in case 
 it had passed between this and the observer. The 
 entire length of the comet, as already remarked, 
 was at this time about twice the diameter of the 
 sun ; and since the distance of the comet and sun 
 from the earth were nearly the same, as will soon 
 be evident, we shall find that the length of the 
 nucleus and train combined was 1,700,000 miles. 
 
 The comet was observed also by means of a sex- 
 tant, in Mexico, by Mr. Bowring, from nine o'clock 
 in the morning of the 28th of February, until near 
 sunset, and its altitude repeatedly measured. It 
 was seen on the same day at Boulogne, Parma, and 
 Genoa, in Italy ; and it is reported also that it was 
 seen on the 27th of February, by Captain liay, at 
 Conception, in South America. It was then east of 
 the sun, and distant from the nearest limb about 
 one-sixth of his diameter, or about 130,000 miles. 
 It was seen at Pernambuco, in Brazil, and in Van 
 Dieman's Land, on the 1st of March ; and the fol- 
 lowing day it w^as very generally observed in the 
 tropical latitudes in the northern hemisphere. On 
 the 3d, the head was so far disengaged from the 
 rays of the sun, as to be visible above the horizon 
 for a short time after sunset ; and when seen through 
 a telescope of considerable power, it presented the 
 
GREAT COMET OF 1843. 117 
 
 appearance of a planetary disc, from which were 
 divergent rays in the direction of the tail. The tail 
 is said to have appeared double, consisting of two 
 principal lateral streamers, making a very small 
 angle with each other, and divided by a compara- 
 tively dark line, and to have been about 25 degrees 
 in length. It seemed, however, to be prolonged, on 
 the north side, by a divergent streamer, making an 
 angle of at least 5 degrees with the general direction 
 of the axis, and plainly distinguishable as far as 65 
 degrees from the head. A similar, but fainter pro- 
 longation, is said to have been visible on the south 
 side. The representations of the comet at this time, 
 show it to be highly symmetrical, and give it the 
 appearance of a vivid cone of light, with a dark 
 axis, and nearly rectilinear sides, inclosed in a fainter 
 cone, the sides of which curve slightly outwards. 
 
 On the 4th of March, the comet was seen at New 
 York ; and on the evening of the 5th, it was gene- 
 rally noticed throughout the United States. From 
 this date until the beginning of April, it presented 
 a most magnificent appearance in the western hea- 
 vens, and is said to have been so brilliant in some 
 places as to throw a strong light on the sea. The 
 nucleus, however, did not remain brilliant for any 
 considerable portion of time. On the 3d of March 
 it was as bright as a star of the first magnitude ; on 
 the llth it was not brighter than a star of the third 
 
118 TREATISE ON COMETS. 
 
 magnitude; and on the 19th and 20th it had tecome 
 so faint that it could not be discerned without the 
 aid of a telescope. The tail still preserved its bril- 
 liancy, and remained visible as a great beam of 
 nebulous light, until the first week in April. 
 
 The first regular observation of the comet, with a 
 suitable instrument, was made at the Cape of Good 
 Hope on the 3d of March, after which it was regu- 
 larly observed until its final disappearance the last 
 recorded observation having been made at Berlin, 
 on the 15th of April. It was not seen in England 
 till the 17th of March, and then only as an immense 
 stream of light, without any appearance of a nucleus. 
 The first observation of the nucleus in Europe was 
 made at Rome and Naples on the 17th of March, 
 and it was subsequently observed at all the obser- 
 vatories on the continent. The opposite diagram 
 represents the comet as it appeared to the naked 
 eye in the tropical latitudes, early in March. 
 
 The greatest length of the tail was on the 5th of 
 March, when it was about 70 degrees, and was ob- 
 served to be slightly curved. Its breadth was a 
 little more than a degree. In the tropical latitudes 
 the tail was about sixty degrees in length, and the 
 nucleus shone forth with almost unexampled lustre, 
 creating the most profound sensations of astonish- 
 ment and admiration wherever it was seen. On the 
 llth of March, as seen at Calcutta, the tail was 
 
GREAT COMET or 1843. 119 
 
 observed to have shot forth a lateral ray or train, 
 nearly twice as long as the regular one, but fainter, 
 and making an angle of about 18 degrees with its 
 direction, on the southern side. This projection of 
 a secondary train to such an enormous length it 
 having been nearly 100 degrees was not observed 
 either before or after this date. The length of the 
 tail did not vary much after it had attained its max- 
 imum, but remained nearly stationary, in this re- 
 spect, until the first week in April, when it ceased 
 to be visible to the naked eye, owing to the increased 
 distance of the comet from the earth. The appa- 
 rent length and size of the train, although enor- 
 mous, hardly serve to give a definite idea of its 
 absolute size. The greatest apparent length was at 
 the time when the comet was nearest the earth, but 
 when it was still distant more than 80,000,000 miles. 
 Let us suppose, therefore, that the length of the 
 tail was found to be 60 degrees, or about 115 times 
 the apparent diameter of the sun, and we shall find 
 its absolute length to be 86,000,000 miles. At the 
 time of the discovery of the .comet, in the daytime, 
 the tail was turned nearly toward the earth, and 
 consequently appeared brighter, yet shorter, than 
 would otherwise have been the case. Had it been 
 at right angles to the plane of the orbit of the earth, 
 supposing its apparent length to have been the same 
 as actually observed, its actual length would not 
 
120 
 
 TREATISE ON COMETS. 
 
 have exceeded 2,000,000 miles. It is not certainly 
 known whether the comet had a train of any con- 
 siderable extent previous to its perihelion passage. 
 It is probable, however, that it had ; but from what 
 is now known of the development of the tails of 
 comets, it could not have been near as large as im- 
 mediately after the perihelion passage. The length 
 of the tail for each day during which the comet was 
 visible, will serve to show the progress of its devel- 
 opment, and is exhibited in the following table : 
 
 Date. 
 
 Length of tail 
 in miles. 
 
 Date. 
 
 Length of tail 
 in miles. 
 
 Feb'y 28, . 
 
 . . 35,000,000 
 
 March 15, ..,, 
 
 . 105,000,000 
 
 March 1, . 
 
 . . 55,000,000 
 
 " 17, . . 
 
 . 106,000,000 
 
 " 2, . 
 
 . . 70,000,000 
 
 " 19, . . 
 
 . 107,000,000 
 
 3, . 
 
 . . 82,000,000 
 
 " 21, . . 
 
 . 108,000,000 
 
 4, . 
 
 . . 91,000,000 
 
 " 23, . . 
 
 . 109,000,000 
 
 " 5, . 
 
 . . 96,000,000 
 
 " 25, . . 
 
 . 108,000,000 
 
 6, . 
 
 . . 99,000,000 
 
 " 27, . . 
 
 . 107,000,000 
 
 7, . 
 
 . . 100,000,000 
 
 " 29, . . 
 
 . 105,000,000 
 
 8, . 
 
 . . 101,000,000 
 
 April 1, . . 
 
 . 102,000,000 
 
 9, . 
 
 . . 102,000,000 
 
 3, . . 
 
 . 99,000,000 
 
 " 10, . 
 
 . . 102,800,000 
 
 5, . . 
 
 . 95,000,000 
 
 " 11, . 
 
 . . 103,000,000 
 
 " 7, . . 
 
 . 89,000,000 
 
 " 13, . 
 
 . . 104,000,000 
 
 
 
 The breadth of the tail at the extremity varied 
 from one to three millions of miles. The length of 
 the streamer seen at Calcutta on the llth of March, 
 must have been about 150,000,000 miles ; and if we 
 suppose this to have been projected in a single day, 
 or perhaps in much less than 24 hours, it seems 
 
GREAT COMET OF 1843. 121 
 
 almost, if not actually impossible, to form any con- 
 ception of the forces which must have operated to 
 produce such a velocity of projection of material 
 substance through space. It is certain, at least, that 
 the velocity must have been such as no other natural 
 phenomenon is capable of exciting; and that in cases 
 similar to this we have to deal with forces incom- 
 parably superior in energy to any with which we 
 are acquainted, and with matter whose inertia must 
 be extremely insensible. 
 
 The head of the comet, when compared with the 
 dimensions of the tail, was small. It was hardly 
 perceptible even on the 28th of February, when its 
 brilliancy was greatest the comet presenting the 
 appearance of a mere point, with a divergent mass 
 of matter in the form of a tail. For this reason its 
 cometary nature was at first doubted by those unac- 
 quainted with such objects. A few days later the 
 head, although faint in comparison with the tail, 
 could be very distinctly seen, but remained visible 
 to the naked eye only for a very few days. During 
 the last days on which the comet was observed with 
 large telescopes, it was almost impossible to distin- 
 guish the nucleus, even while the train was yet 
 brilliant to the unassisted eye. It was for this rea- 
 son that Bessel, in speaking of the appearance of 
 the comet, remarked that it seemed to have ex- 
 hausted its head in the manufacture of its tail a 
 11 
 
122 TREATISE ON COMETS. 
 
 remark which, when we come to consider the theory 
 of the formation of the tails of comets, will have 
 due significance. The diameter of the head of the 
 comet was probably about 8000 miles, and the abso- 
 lute diameter of the nebulosity surrounding the 
 head was about 30,000 miles. These results, in 
 connection with the dimensions of the tail, already 
 given, will serve to exhibit the enormous magnitude 
 of this celestial visitor. 
 
 This comet did not exhibit any indication of a 
 solid nucleus, but, on the contrary, considering the 
 size and brilliancy of the head a3 compared with 
 the tail, it seems certain that the entire comet was 
 composed of nebulous matter in a state of extreme 
 tenuity a supposition which the elements of its 
 orbit will furnish the most unequivocal testimony 
 to confirm. However great, then, the volume of the 
 comet may have been, the absolute quantity of 
 matter was inconsiderable, when compared with even 
 the smallest planets of our system. Indeed, it is 
 highly probable that the very smallest members of 
 the group of small planets between Mars and Jupi- 
 ter, those whose diameters do not exceed a few 
 hundred miles, greatly exceed in weight the entire 
 mass of the comet. The question may therefore 
 arise, as to what is due the intensity of its light 
 when seen at noonday, on the 28th of February, in 
 the full blaze of the sun. For this the elements of 
 
GREAT COMET OF 1843. 123 
 
 the orbit furnish a ready and complete method of 
 solution. 
 
 The elements of the orbit of the great comet of 
 1843 may well be said to be among the most re- 
 markable of those which have hitherto been re- 
 corded. The first elements which were computed 
 actually gave the perihelion distance of the comet, 
 measured from the sun's centre, as less than the 
 semi-diameter of the sun, which is a physical im- 
 possibility, since the comet was observed after its 
 perihelion passage. A complete discussion of all 
 the observations taken during a period of forty days, 
 made the nearest approach of the centre of the 
 nucleus to the centre of the sun only 390,000 miles, 
 or about 50,000 miles below his surface. Another 
 determination made it 405,000 miles, or about 
 36,000 miles less than the radius of the sun. Were 
 these results correct, the comet would have plunged 
 directly into the sun in passing the perihelion of its 
 orbit, and would, in all probability, have become 
 absorbed in the solar atmosphere. It would, there- 
 fore, have been visible only before the perihelion 
 passage, which took place on the evening of the 
 27th of February, the day previous to its discovery 7 . 
 The average distance of the comet in its perihelion, 
 obtained by the astronomers who were engaged in 
 * computations respecting its orbit, was about 510,000 
 miles. If we adopt this distance, and there is the 
 
124 TREATISE ON COMETS. 
 
 greatest probability of its correctness, we conclude- 
 that the comet approached the sun's surface within 
 about seventy thousand miles, or within one-sixth 
 of his radius. "We perceive, therefore, that the 
 comet almost literally grazed the sun's disc a fact 
 which serves to explain the great brilliancy of the 
 nucleus at this time. 
 
 It is well known that the intensity of both the 
 light and radiant heat of the sun, at different dis- 
 tances from that luminary, increase proportionally 
 to the spherical area of the portion of the visible 
 hemisphere covered by the sun's disc. In the case 
 of the earth, this disc -has an average diameter of 
 a little more than half a degree ; but, could an ob- 
 server have been stationed on the comet, when in 
 the perihelion, the apparent angular diameter of the 
 sun. would have been nearly 125 degrees. We shall 
 therefore find, from the principle just stated in refer- 
 ence to the distribution of light and heat by the 
 sun, that the amount of light and heat received on 
 equal areas of exposed surface of the earth and 
 comet, would be in the ratio of one to forty-nine 
 thousand. Now, it has been determined by experi- 
 ment, that, by collecting the rays of the sun to such 
 an extent that the intensity of his heat and light as 
 received at the earth is increased one thousand 
 times, the heat produced is sufficient to melt any 
 mineral substance known on our earth. With this 
 
GKEAT COMET OF 1843. 125 
 
 heat iron is melted in a few seconds, and such min- 
 erals as cornelian, agate, and rock crystal. The 
 heat of the sun, as received at the comet, must have 
 been at least forty-nine times more intense than the 
 heat thus generated. The great comet of 1680 was 
 distant from the sun's surface about 150,000 miles, 
 when in its perihelion, or more than double the cor- 
 responding distance of this comet; and yet it was 
 computed by Newton that the former was subjected 
 to an intensity of heat 2000 times that of red-hot 
 iron. How much more intense, then, must have 
 been the heat received by the comet of 1843 ! Such 
 a temperature would actually have converted our 
 earth into vapor ; or, if any substance retained a 
 solid form, it would have been in a state of the most 
 intense ignition. On the morning of the 28th of 
 February, the comet must have been literally red 
 hot, although evidently in a gaseous state ; and, from 
 its appearance as observed, it retained its heat for 
 several days after its perihelion passage, having 
 presented a peculiar fiery appearance. In the tropi- 
 cal latitudes, and more especially in the vicinity of 
 the equator, it was described as resembling a stream 
 of fire from a furnace. It may be conjectured, 
 therefore, that in case the comet had originally a 
 solid nucleus, it was converted into vapor at this 
 approach to the sun; while the intensity of the heat, 
 and the effect of its operation, were such that the 
 11* 
 
126 TREATISE ON COMETS. 
 
 comet retained this gaseous form during the entire 
 period of its visibility in 1843. 
 
 The comet did not, however, remain long in a 
 position exposed to such an enormous range of tem- 
 perature. Its velocity at this time was not less than 
 370 miles in a second of time, or 1,332,000 miles" 
 per hour, which would be sufficient, if undiminished, 
 to cause the comet to make an entire revolution 
 around the sun in two hours and one-third. This 
 velocity was so great only for a small portion of 
 time, and rapidly diminished as the comet proceeded 
 in its orbit. It was still such that during the twelve 
 hours immediately preceding the perihelion passage, 
 and the twelve hours immediately following it, the 
 comet passed over an arc of its orbit amounting to 
 290 degrees, or, in other words, performed more 
 than three-quarters of its circuit around the sun. 
 During the brief period in which it was visible, it 
 had described 173 degrees of its orbit, measured 
 from the perihelion; while to describe the next 
 7 degrees will require many years, at least, and per- 
 haps many centuries. It thus appears that in about 
 two hours the .heat of the sun, as experienced by 
 the comet, would have been reduced to about one- 
 fourth its maximum amount, and that afterwards 
 the diminution was proportionally greater. In this 
 manner, then, we may explain the various changes 
 which were observed to take place in the brilliancy 
 
GREAT COMET OF 1843. 127 
 
 of the comet, and also the extreme intensity of its 
 light at the time of discovery. 
 
 In computing the elements of the orbit of this 
 comet, it was found by many astronomers to give 
 strong indications of ellipticity, while others were 
 led to conclude that the entire series of observations 
 could be represented within the limits of their pro- 
 bable errors, by supposing the orbit to be a parabola. 
 Encke obtained a hyperbolic orbit, whose eccen- 
 tricity, however, differed but little from that of the 
 parabola. But it should be remarked that before 
 any calculations had been made, it was asserted 
 that, in the year 1668, the tail of an immense comet 
 had been seen at Lisbon, at Boulogne, at several 
 points in Brazil, and elsewhere, occupying nearly 
 the same position in the heavens, and at the same 
 season of the year. Cassini observed the tail at 
 Boulogne, on the 10th of March, 1668 ; and from 
 this determined that the head was in the immediate 
 vicinity of the sun. The tail was about 50 degrees 
 in length, and was so brilliant that its reflected trace 
 was easily distinguished on the sea. This brilliancy, 
 however, lasted only for a few days ; and on one or 
 two occasions the head could be barely distinguished 
 in the twilight, but as a very dim object. The comet 
 of 1668 was not very accurately observed, and its 
 orbit was unknown ; yet the strange coincidence of 
 similar situation, season of the year, and physical 
 
128 TREATISE ON COMETS. 
 
 appearance, excited a very strong suspicion of iden- 
 tity, thus assigning to the comet a period of exactly 
 175 years. By estimating the position of the head 
 from the observed direction of the tail, Henderson 
 was enabled to determine an approximate orbit for 
 the comet of 1668 ; and he found, strangely enough, 
 that the elements closely resembled those of the 
 comet of 1843. Assuming, then, their identity, and 
 using the corrected elements of the latter comet, it 
 was found that the accordance was still better; and 
 it was, therefore, announced that these comets not 
 only exhibited the same appearance, but pursued 
 very nearly the same path, both real and apparent. 
 Again, it has been remarked that comets are re- 
 corded to have been seen in the years 268, 442, 791, 
 968, 1143, 1317, and 1494, which may have been 
 previous returns of the comet now under considera- 
 tion, since a period of 175 years would, disregard- 
 ing the planetary perturbations, which, from the 
 position of the orbit, must necessarily be small, 
 make it appear in the years 268, 443, 618, 793, 968, 
 1143, 1318, and 1493. 
 
 Such are the reasons for supposing the great 
 comet of 1843 to have a periodic time of 175 years ; 
 but it should be noticed that a similar comet was 
 also seen in 1689. It was not observed in Europe, 
 but was seen at Pekin and in the southern hemi- 
 sphere, where the tail was estimated at upwards of 
 
GH EAT COMET OF 1843. 129 
 
 60 degrees in length. It was observed from the 
 llth to the 23d of December ; and from the observa- 
 tions, rude as they are, its orbit has been computed. 
 Its perihelion distance was small, and the places 
 may be closely represented by elements similar to 
 those of the comet of 1843. Those who have felt 
 inclined to consider these comets identical, have 
 assigned a period of 21 years and 10 months, which 
 gives seven revolutions between 1689 and 1843. By 
 this means the identity of the comets of 1668, 1689, 
 and 1843, is supposed to be clearly established ; and 
 the fact that the comet has not been observed at in- 
 termediate returns, is accounted for by the position 
 of its orbit, which is such that the comet is best seen 
 in the southern hemisphere, and may pass unob- 
 served in the northern hemisphere. If we adopt 
 this hypothesis, the comet may be expected to reap- 
 pear about the end of 1864 or beginning of 1865, 
 but will be seen only in the southern hemisphere. 
 
 Some astronomers have gone still further, and 
 have supposed the comet to have a period of only 7 
 years, assigning the unfavorable situation of its orbit 
 as a reason for its not having been seen at its suc- 
 cessive returns. This, however, seems wholly in- 
 admissible, since a comparison of all the best obser- 
 vations militates against the hypothesis of an ellipse 
 of short period. They may all be represented by a 
 parabola within the usual limits of the errors of 
 
 I 
 
130 TREATISE ON COMETS. 
 
 cometary observations, although an ellipse of about 
 180 years represents them a little better. Still later 
 determinations of the orbit, in which the probable 
 errors of the observations are investigated, and the 
 planetary disturbances duly introduced, increase the 
 period ; and in one instance it is found to be an 
 ellipse corresponding to a period of revolution of 
 over 500 years. It is, therefore, extremely doubtful 
 whether the comets of 1668, 1689, and 1843, are 
 identical, and the doubt can be removed only by 
 another appearance of the comet. The greatest 
 probability is in favor of the identity of the comets 
 of 1668 and 1843 alone, thus establishing a period 
 of 175 years within a day or two, more or less ; yet 
 in this case there exists great uncertainty, and such 
 that we can hardly venture to predict its reappear- 
 ance in the year 2018. 
 
 The most then which we can say respecting the 
 great comet of 1843, is that the prodigious length 
 of its tail and its small perihelion distance, both 
 being such as have never before been observed, 
 invest it with peculiar interest ; and many years may- 
 elapse before another shall appear which, in so short 
 a space of time, will excite an interest so universal, 
 or a sensation so profound. "We have been minute 
 in the description of the details of its appearance, 
 in order that the degree and kind of interest which 
 is attached to the comets, even by astronomers, in 
 
COMETS FROM 1845 TO 1853. 131 
 
 the present state of the science, may be understood, 
 and also for the purpose of exhibiting the important 
 results which the exact calculations of modern as- 
 tronomy afford us. 
 
 The next comet which became visible to the naked 
 eye, was discovered by Colla, at Parma, on the 2d 
 of June, 1845. It had a very beautiful tail, about 
 three degrees in length. It has been supposed to 
 have a period of about 249 years, making it identical 
 with a comet which appeared in 1596. Another 
 bright comet was discovered by Hind, at London, 
 on the 6th of February, 1847. It passed its peri- 
 helion on the 30th of March, at a distance from the 
 sun of about four millions of miles, and was so 
 bright that it was observed at London at mid-day, 
 when only a few degrees from the sun. A comet 
 was also discovered by Miss Mitchell, at Kantucket, 
 Mass., on the 1st of October, 1847, in the vicinity 
 of the north pole of the heavens, which subsequently 
 increased in brilliancy, and on October 6th became 
 visible to the naked eye. It continued to increase 
 in brilliancy for several days, when it was obscured 
 by bright moonlight. The tail was very faint, and 
 did not exceed two degrees in length. It passed the 
 perihelion of its orbit on the 14th of November. 
 
 On the 10th of June, 1853, a faint telescopic object, 
 which was subsequently found to be a comet, was 
 discovered by Klinkerfues, at Gottingen. It con- 
 
132 TREATISE ON COMETS. 
 
 tinned to increase in brilliancy, and on the 7th of 
 August began to be faintly visible to the naked eye. 
 On the 20th of August it was as bright as stars of 
 the third magnitude, and ten days later it equalled 
 in brilliancy the brightest stars of the first magni- 
 tude. On the 31st of August it was observed at 
 Olmiitz in the day time, when only twelve degrees 
 from the sun ; and on the 2d, 3d, and 4th days of 
 September it was observed at noon, although only 
 seven degrees from the sun. It was also seen at 
 Liverpool on the 3d of September, about noon, by 
 means of a telescope. The first appearance of a tail 
 was about the beginning of August, when it had 
 attained a length of nearly a quarter of a degree. 
 On the 22d of August it was nearly two degrees in 
 length, and for a little more than a week continued 
 to increase, until it attained a maximum length of 
 fifteen degrees. During this time it presented a 
 very beautiful appearance in the western heavens, 
 soon after sunset, to the naked eye, and its motions 
 were watched with no small degree of interest. It 
 passed its perihelion, or point of its orbit nearest 
 the sun, on the 1st of September; but it is not known 
 to have been observed in the northern hemisphere 
 after the 4th of September. It was seen at the Cape 
 of Good Hope on September llth, and was observed 
 until the llth of January, 1854. It was also ob- 
 served at Santiago, in Chili, from the 16th of Sep- 
 
ORE AT Co MET OF 1858. 183 
 
 tember till the 7th of October, and at Kew Zealand 
 from September 14th to October 10th. About the 
 middle of September the tail was five degrees in 
 length, from which time its brightness rapidly 
 diminished, till it became invisible to the naked 
 eye. The orbit is found to be a parabola, and, con- 
 sequently, the comet will never reappear to the in- 
 habitants of our earth, but will pass on in its cease- 
 less wanderings to visit other suns and systems. 
 
 On the evening of the 2d of June, 1858, a faint 
 nebulosity was discovered by Donati, an Italian 
 astronomer, at Florence, in the constellation Leo, 
 which the observations of a few days proved to be a 
 comet. It was then very far distant from both the earth 
 and sun, and had also a very slow motion as seen 
 from the earth. The comet was seen at nearly all 
 the observatories in Europe in less than a week after 
 the announcement of its discovery. The news of 
 the discovery did not reach America until about the 
 1st of July, since it was not generally announced in 
 Europe until near the middle of June. The comet, 
 however, had already been independently discovered 
 by Tuttle, at Cambridge, Massachusetts, on the 
 28th of June ; by Parkhurst, at Perth Amboy, $"ew 
 Jersey, on the 29th of June, and by Miss Mitchell, 
 at JSTantueket, on the 1st of July, each observer 
 being at the time unaware of its previous discovery 
 in Italy. It was immediately observed at all the 
 12 
 
134 TREATISE ON COMETS. 
 
 principal observatories in the United States, and 
 henceforward observations became general on both 
 continents. As soon as a sufficient number of 
 observations had been taken, the elements of its 
 orbit were computed, and it was found that the 
 comet was distant from the earth, at the time of 
 its discovery by Donati, about 240,000,000 miles, 
 and that it was slowly approaching the sun, and 
 receding from the earth. The distance of the 
 comet from the earth attained its maximum about 
 the middle of June, when it remained stationary for 
 a few days in respect to its geocentric distance. It 
 soon began to approach the earth, and it was found 
 that its brilliancy would be subsequently increased 
 nearly three hundred times. On account of the slow 
 motion of the comet, and its great distance, it was 
 found to be difficult to determine the time of its 
 perihelion passage with any considerable precision. 
 Indeed, the computations of different astronomers 
 exhibited a great difference in this element ; and it 
 was not till near the middle of August that its future 
 course could be satisfactorily ascertained. It was 
 then found that the comet would pass the point of 
 its orbit nearest the sun on the 29th of September, 
 that it would become visible to the naked eye early 
 in September, and that during the latter part of this 
 month, and the first half of October, it would 
 present a magnificent appearance in the western 
 
GREAT COMET OF 1858. 135 
 
 heavens, soon after sunset. It was found also that 
 the comet would be so situated with respect to the 
 earth and sun, that it would not only be visible in 
 the evening after sunset, but during the greater part 
 of September it would rise several hours before the 
 sun, and, consequently, would be favorably situated 
 for observation in the morning. 
 
 The predictions were fully realized. The comet 
 was distinctly visible to the naked eye on September 
 10th, and continued to increase rapidly in brilliancy 
 until about the 5th of October, when it had attained 
 its maximum in this respect. It ceased to be visible 
 in northern latitudes, except near the equator, about 
 the 20th of October, but was observed in the south- 
 ern hemisphere from the beginning of October to 
 the middle of May, 1859. The first appearance of 
 a tail was noticed about the 20th of August, with 
 the aid of a telescope, and early in September it 
 was very distinctly noticed to be curved. Between 
 the 10th and 25th of September, the comet increased 
 rapidly in brilliancy, and the train was lengthened 
 proportionally. The comet was now rapidly ap- 
 proaching both the earth and sun, and under 
 circumstances, as we shall subsequently notice, pe- 
 culiarly favorable to the development of the tail. 
 Another fortunate circumstance to be noticed in 
 this connection, was that the moonlight, about the 
 time of the greatest brilliancy of the comet, ceased 
 
136 TREATISE ON COMETS. 
 
 to interfere with the observation of the minutest 
 details of its form, and contributed largely to en- 
 hance its general appearance. The return of the 
 moonlight just before it ceased to be visible above 
 the horizon in the south-west, and when its brilliancy 
 was speedily on the decline, caused its decrease and 
 final disappearance to be fully as sudden as had 
 been its increase and maximum intensity of light. 
 
 At the time of its perihelion passage the comet 
 was distant from the sun about 55,000,000 miles. 
 Its nearest approach to the earth took place on the 
 llth of October, at which date it was distant from 
 us about 50,000,000 miles. The following table 
 gives the distance of the comet from the earth and 
 sun, and also its hourly motion in its orbit, for differ- 
 ent dates during the period of its visibility in the 
 northern hemisphere. 
 
 Distance from gun Distance from earth Hourly velocity 
 
 UU.IV. 
 
 in miles. 
 
 in miles. 
 
 in miles. 
 
 June 2, . 
 
 . . 214,000,000 
 
 240,000,000 
 
 65,000 
 
 July 1, . 
 
 . . 175,000,000 
 
 241,000,000 
 
 72,000 
 
 Aug. 1, . 
 
 . . 128,000,000 
 
 221,000,000 
 
 84,000 
 
 Sept. 1, . 
 
 . . 82,000,000 
 
 160,000,000 
 
 105,000 
 
 " 15, . 
 
 . . 64,000,000 
 
 116,000,000 
 
 119,000 
 
 Oct. 1, . 
 
 . . 56,000,000 
 
 66,000,000 
 
 128,000 
 
 " 11, 
 
 . . 61,000,000 
 
 52,000,000 
 
 123,000 
 
 " 21, . 
 
 . . 71,000,000 
 
 67,000,000 
 
 114,000 
 
 The brilliancy of a comet, as seen from the earth, 
 varies inversely as the product of the squares of its 
 distance from the earth and sun respectively ; and 
 
GREAT COMET OF 1858. 137 
 
 it results, therefore, from the foregoing table, that this 
 comet was 200 times brighter on the 1st of October 
 than at the time of its discovery. The brightest 
 stars in the heavens are about 100 times brighter 
 than those which can be barely distinguished by the 
 naked eye. This fact may serve to give an idea of 
 the comparative brilliancy of the comet at these two 
 dates. 
 
 The tail of this comet is certainly the most re- 
 markable feature to be considered. At the time 
 when the comet was nearest the earth, it could be 
 distinctly traced to a distance from the nucleus of 
 nearly 70 degrees, and was curved in the form of a 
 sabre. On the 20th of September, it was first ob- 
 served to be bifurcated, or divided into two branches 
 or streams. The dark space between these two 
 streams of nebulous light was directly behind the 
 nucleus, and gave it the appearance of the shadow 
 of the latter obscuring the light of the tail. The 
 southern portion of the tail was much more brilliant 
 than the northern portion ; and the difference was 
 such that, in the twilight, the latter could not be 
 distinguished, so that the tail appeared single. The 
 length of the tail, at this time, was about 5 degrees. 
 On the 24th of September the length of the tail was 
 more than 7 degrees, and the curvature began to be 
 very plainly exhibited. On the 27th its length was 
 13 degrees, and presented the phenomenon of a 
 12* 
 
188 TREATISE ON COMETS. 
 
 streamer, or supplementary tail, issuing from the 
 convex side of the principal one, and nearly in the 
 direction of a straight line drawn from the sun 
 through the nucleus. From this date until the 10th 
 of October, the tail increased rapidly in brilliancy 
 and length ; and from the phenomena which it pre- 
 sented, there is every reason to believe that it was 
 directly connected with the changes which, as we 
 shall notice, were continually taking place in the 
 matter composing the envelope and nucleus. At 
 times, rays or jets of light were seen streaming in 
 different directions from the centre ; and corusca- 
 tions were seen, precisely as in the case of several 
 comets which have already been described. On the 
 5th of October there were two streamers distinctly 
 visible, one of which was more than fifty degrees in 
 length, corresponding to upwards of 50,000,000 
 miles. At this time, the comet was so bright that 
 not only the nucleus, but also a portion of the en- 
 velope and tail could be seen, by means of a tele- 
 scope, before sunset. It was visible to the naked 
 eye in the bright twilight ; and, when viewed with 
 a telescope, a secondary envelope could be distinctly 
 noticed. At the point where the curvature of the 
 tail began to be most strongly marked, several short 
 streamers could be seen ; and occasionally there 
 seemed to be transverse bands in the tail, nearly a 
 half a degree in breadth, with clear, well-defined 
 
GrREAT COMET OF 1858. 
 
 139 
 
 outlines, and closely resembling the auroral stream- 
 ers, with the exception only that they retained their 
 relative positions in the tail. On the 10th of Octo- 
 ber, the date on which the tail had attained its 
 greatest dimensions, these slightly diverging bands, 
 the point of divergence being situated between 
 the sun and the nucleus, alternating with dark 
 spaces, could be readily distinguished, and in reality 
 gave the comet a most beautiful appearance. They 
 were, on an average, five degrees in length, and 
 from a third to half a degree in breadth. The oppo- 
 site diagram represents the comet as it appeared at 
 this time. 
 
 The apparent length of the tail was necessarily 
 affected by the moonlight which prevailed at two 
 different times during the period of visibility, and 
 we are therefore unable to determine the precise 
 variations in its length from day to day. The fol- 
 lowing gives the length of the tail for successive 
 dates, as observed at the Observatory at Cambridge, 
 Massachusetts : 
 
 Date Length of tail 
 Date< in miles. 
 
 Date. 
 
 Length of tail 
 in miles. 
 
 ug. 29, 1858 . . 14,000,000 
 
 Sept. 30, 1858 . 
 
 . 26,000,000 
 
 apt. 8, 
 
 ' . . 16,000,000 
 
 Oct. 2, " . 
 
 . 27,000,000 
 
 12, 
 
 . . 19,000,000 
 
 ' 5, " . 
 
 . 33,000,000 
 
 17, 
 
 . . 10,000,000 
 
 6, " . 
 
 . 45,000,000 
 
 23, 
 
 . . 12,000,000 
 
 ' 8, " 
 
 . 43,000,000 
 
 24, 
 
 . . 12,000,000 
 
 ' 10, " . 
 
 . 51,000,000 
 
 25, 
 
 . . 17,000,000 
 
 ' 12, " . 
 
 . 39,000,000 
 
 27, 
 
 . . 18,000,000 
 
 ' 15, . 
 
 . 14,000,000 
 
 28, 
 
 . . 26,000,000 
 
 
 
140 TREATISE ON COMETS. 
 
 The discrepancy which seems to exist in the mea- 
 surements from the 17th to the 25th of September, 
 inclusive, is to be attributed to the influence of the 
 moonlight ; and any other slight variation from a 
 regular increase which may be apparent, is probably 
 due to a hazy or unfavorable state of the atmo- 
 sphere at the time when the measurements were 
 taken . 
 
 The breadth of the tail at the extremity was 
 found, on the 25th of September, to be 1,500,000 
 miles. On the 30th it was 3,000,000 miles, and on 
 the 10th of October 10,000,000 miles. The stream- 
 ers were from 10,000,000 to 50,000,000 miles in 
 length ; and in one instance, at least, their breadth 
 at the extremity was upwards of 1,000,000 miles. 
 
 The nucleus of the comet was observed to undergo 
 the most rapid changes that can be readily conceived 
 of, not only in form, but also in size. On the 19th 
 of July, its diameter was found to be 5600 miles ; 
 August 30th, it was 4660 miles ; and September 
 24th it was only 1030 miles. On the 5th of October, 
 it was certainly less than 540 miles in diameter; 
 while on October 18th it had increased to 900 miles. 
 On the 8th of September, the diameter of the nu- 
 cleus was ascertained to be 2000 miles ; and in im- 
 mediate contact with it was an intensely brilliant 
 nebulosity, having a diameter of about 3000 miles, 
 while the diffused light was estimated to extend 
 
G n B A T COMET OF 1858. 141 
 
 40,000 or 50,000 miles in the direction of the sun. 
 About two weeks later, it was noticed that there 
 was interposed between the nucleus and the sun, an 
 obscure, crescent-shaped outline, within which the 
 light was unequally distributed, and which had a 
 strangely confused and chaotic look. The nucleus 
 at this time was singularly brilliant, and appeared 
 elongated on the upper side a phenomenon which 
 was supposed to indicate the existence of some in- 
 ternal disturbing force. Outside of the nucleus 
 there was a bright envelope, whose vertex was in 
 the direction of the sun, and about 6000 miles dis- 
 tant from the nucleus. This envelope was sur- 
 rounded on its outer margin by a dark band, sepa- 
 rating it from a second and less brilliant envelope, 
 whose vertex was distant from that of the first one 
 about 7500 miles. This again was encircled by a 
 dark arch, outside of which there was an atmosphere 
 of faint and diffused nebulosity, which gradually 
 faded away, and became invisible at a distance of 
 about 40,000 miles from the nucleus. These bands 
 could be traced through an arc of more than 200 
 degrees, around the nucleus ; but extended further 
 into the train on the brighter or upper side with 
 reference to the horizon. 
 
 The phenomena which these successive bands or 
 envelopes presented, was of the most instructive 
 and interesting character. They were observed in 
 
142 TREATISE ON COMETS. 
 
 succession to disengage themselves from the nucleus, 
 then gradually to expand and recede, and finally to 
 disappear in the formation of the tail. So plainly 
 was this exhibited, that each envelope could be 
 traced through all its successive stages ; and thus 
 have the means been furnished for investigating in 
 detail the mysterious processes by which the train 
 is thrown out from the nucleus, under the action of 
 forces whose nature and mode of operation will be 
 fully explained when we come to treat of the theory 
 of the physical constitution of comets, and of the 
 formation of their tails. In this way, also, we are 
 enabled to account for the rapid decrease of the 
 diameter of the nucleus as the comet approached 
 the sun, when the matter of which it is composed 
 was driven off to form the tarl ; and, also, the in- 
 crease in the size of the nucleus as the comet again 
 receded. On the 3d of October, the nucleus ap- 
 peared to be divided into two distinct parts, one, 
 however, being much brighter than the other. 
 These were separated by a distinctly dark opening, 
 and were distant about 4000 miles. No change was 
 observed at this time in the relative positions and 
 form of the envelopes, although the tail seemed to 
 have been sensibly affected the bifurcation being 
 much more distinct, and broader in comparison with 
 the breadth of the tail. 
 
 The opposite diagram is a telescopic view of the 
 
GREAT COMET OF 1858. 143 
 
 nucleus and part of the tail of the comet on the 3d 
 of October, 1858. 
 
 The successive envelopes are here exhibited pre- 
 cisely as they appeared when the comet was seen 
 through a powerful telescope. The division of the 
 nucleus is also indicated, and the dark space sepa- 
 rating the tail into two parts. The development of 
 these envelopes, as already remarked, was extremely 
 interesting. In most cases they were observed to 
 escape from the vertex of the nucleus ; but this was 
 by no means general. They were also seen escaping 
 from the sides of the nucleus, and have been de- 
 scribed as an escape of jets of luminous gas, which 
 streamed off like light spray thrown up against an 
 opposing wind and driven before it a description 
 which is certainly very aptly conceived. The entire 
 distance between the vertex of the nucleus and the 
 inner surface of the outer envelope, varied from ten 
 to twenty thousand miles. It was noticed, also, that 
 just before an eruption took place, the brilliancy of 
 the nucleus was perceptibly greater than either im- 
 mediately before or after that event ; and the varia- 
 tions were so rapid as to indicate almost the exact 
 instant at which the eruption took place. In order 
 to show the progressive motion of the envelopes 
 from their point of origin, the following distances 
 from the nucleus to the vertex of one of these enve- 
 lopes, tor each day during the period of its trans- 
 
144 
 
 TREATISE ON COMETS. 
 
 formation, as observed by Bond, at Cambridge, 
 Mass., are given: 
 
 Date. 
 
 Distance of Vertex 
 from nucleus 
 in miles. 
 
 Date. 
 
 Distance of Vertex 
 from nucleus 
 in miles. 
 
 1858, Sept. 27, 
 
 . . . 3,500 
 
 1858, Oct. 6, 
 
 . . . 10,100 
 
 " " 29, 
 
 . . . 6,000 
 
 " 8, 
 
 . . . 12,400 
 
 " Oct. 2, 
 
 . . . 7,500 
 
 " " 9, 
 
 . . . 13,200 
 
 " 4, 
 
 . . . 8,900 
 
 " 10, 
 
 . . . 14,100 
 
 " 5, 
 
 . . . 9,550 
 
 
 
 It appears, therefore, that the daily motion of 
 the vertex of the envelope, relative to the vertex 
 of the nucleus, varied from 300 to 1200 miles. 
 This rate of motion was found to coincide very 
 closely with that of the other envelopes at the same 
 date. The motion of the envelopes, it might fur- 
 ther be remarked, varied with the position of the 
 cornet with respect to the sun. The manner in 
 which these envelopes were driven off, and the evi- 
 dent tendency of each towards a condensation 
 around a central axis, together with the fact that 
 the axes manifested a disposition to diverge from 
 the sun, has led to the conjecture that these collec- 
 tions of nebulosity were in reality a group of new 
 comets in process of formation. It is therefore 
 unfortunate, that the circumstances were such as to 
 preclude the possibility of following them to a more 
 complete development. 
 
 We may thus comprehend the peculiar interest 
 which attaches to the great comet of 1858. The 
 
GREAT COMET or 1858. 145 
 
 long period during which it was visible before it had 
 attained its most magnificent proportions, had ena- 
 bled astronomers to be prepared to observe all the 
 phenomena of the changes of its physical appear- 
 ance with the greatest facility. Other comets have 
 appeared which furnished phenomena of precisely 
 the same character, and perhaps on as grand a scale, 
 but the state of astronomical science was such that 
 their importance was not fully understood, and con- 
 sequently the observations were not so refined and 
 extensive. Under such circumstances, we may de- 
 clare, without hesitation, that the comet now under 
 consideration has been of more essential service to 
 science, and otherwise of more universal interest, 
 than any other which has ever been seen. The 
 splendor of its appearance in October elicited the 
 most profound admiration throughout the civilized 
 world; and although, like its predecessors, it was 
 assigned its share in the cause of any unusual events 
 which may have taken place simultaneously with 
 its appearance, yet it is certain that the general 
 diffusion of knowledge which characterizes the 
 present age, operated very effectually to counteract 
 all superstitious fears, which, otherwise, it would not 
 have failed to excite. 
 
 The computations which have been made respect- 
 ing its orbit, show that the period of its revolution 
 is about 2000 years. The following are some of 
 13 K 
 
146 TREATISE ON COMETS. 
 
 the results which have been obtained by different 
 astronomers : 
 
 Bruhns, 2102 years. 
 
 Watson, 2415 " 
 
 Lowy, 2495 " 
 
 Brlinnow, 2470 " 
 
 Newcomb. 1854 " 
 
 The uncertainty of the period arises from the 
 great eccentricity of the orbit ; and since the obser- 
 vations made in the southern hemisphere during 
 several months after its disappearance in the north- 
 ern, have not been employed in the determination 
 of these results, this uncertainty may be materially 
 reduced in subsequent computations. The disturb- 
 ance produced in its motion by the attraction of the 
 planets during the period of its visibility, have also 
 been neglected, the introduction of which may affect 
 the results slightly. It is probable, however, that a 
 more complete investigation will give a period dif- 
 fering but little from 2400 years. The comet must 
 therefore be distant from the sun, when in the aphe- 
 lion or most remote part of its orbit, no less than 
 34,000,000,000 of miles, or about one six-hundredth 
 part of the distance of the nearest fixed star. Great 
 as is this distance, the comet will still be obedient 
 to the controlling influence of the great central 
 body of our system ; and after having performed its 
 long journey of more than two thousand years, it 
 
NUMBER OF COMETS. 147 
 
 will again return to greet the inhabitants of our 
 earth. 
 
 We have thus briefly described some of the most 
 remarkable comets which have appeared during the 
 period of authentic history. To notice all which 
 appeared would not only have been tedious and 
 uninteresting, but also would have required more 
 space than could properly have been devoted to this 
 branch of our subject. The number of comets which 
 have appeared since the birth of Christ, in each 
 successive century, is as follows : First century, 22 ; 
 second, 23 ; third, 44 ; fourth, 27 ; fifth, 16 ; sixth, 
 25 ; seventh, 22 ; eighth, 16 ; ninth, 42 ; tenth, 26 ; 
 eleventh, 36; twelfth, 26: thirteenth, 26; four- 
 teenth, 29; fifteenth, 27; sixteenth, 31; seven- 
 teenth, 25; eighteenth, 64; nineteenth (till 1860), 
 114. Many of these have been reappearances of 
 the same comet; and although the total number 
 here given is 651, yet we are not to understand that 
 there has been 651 different comets observed during 
 this time. Of these, we have described those which 
 were remarkable for their brilliancy, and whose 
 periods of revolution have not been exactly deter- 
 mined. The periodic comets, properly so called, 
 will be described when we shall have explained the 
 manner in which the orbits of these bodies are 
 determined. 
 
 The number of comets here given comprises only 
 
148 TREATISE ON COMETS. 
 
 those whose appearances have actually been re- 
 corded, while it is perfectly certain that many have 
 visited the sun unperceived. By comparing the 
 positions of the orbits of those comets whose ele- 
 ments have been determined, it is found that, unlike 
 the planets, they are distributed over the entire 
 surface of the heavens, without preference of any 
 one region to any other. Again, the points where 
 they pass through the plane of the earth's orbit, are 
 found to be uniformly distributed in every direction 
 round the sun. The points where they pass nearest 
 the sun are also distributed uniformly round that 
 body, and their perihelion distances are such as 
 leads to the supposition of their uniform distribution 
 through space. If now we assume this to be actu- 
 ally the case, since these bodies can only be seen 
 from the earth when within the orbit of Mars, we 
 may be able to form an estimate of the probable 
 number of comets in space. From considerations 
 such as these, and by a process of reasoning which 
 it is not necessary to explain, Arago concludes that 
 at least seven millions of comets are enclosed within 
 the limits of our system. But a small portion of 
 these, comparatively, can ever be seen from the 
 earth ; and although this hypothesis may seem un- 
 reasonable, yet there is every reason to regard it as 
 a very approximate estimate of the number of these 
 wonderful bodies. 
 
ORBITS OF COMETS. 149 
 
 CHAPTER III. 
 
 ORBITS OF COMETS COMPUTATION OF AN ORBIT EPHEMERIS OP A 
 
 COMET PERIODIC COMETS HALLEY's COMET ENCKE's COMET 
 
 BIELA'S COMET FATE'S COMET DE vico's COMET BRONSEN'S 
 COMET D'ARREST'S COMET TUTTLE'S COMET WINNECKE'S COMET 
 LEXELL'S COMET OF 1770 OTHER PERIODIC COMETS RELATION 
 SUPPOSED TO EXIST BETWEEN THE COMETS OF SHORT PERIOD AND 
 
 THE ASTEROID PLANETS POSSIBILITY OF A COLLISION BETWEEN 
 
 A COMET AND PLANET PROBABLE APPULSE OF TWO COMETS. 
 
 As soon as it was ascertained that the comets have 
 no connection with the earth, but move in the region 
 of the planets, the investigation of their orbits, and 
 their relation to the other known bodies of the uni- 
 verse, became a problem of the very highest interest. 
 The illustrious Kepler endeavored to represent their 
 observed places, by supposing them to move in 
 straight lines with a variable velocity a supposition 
 which was subsequently adopted by several astron- 
 omers ; and computations were made, in accordance 
 with it, respecting the motions of some of those 
 comets which had previously appeared. This hypo- 
 thesis of Kepler, as we shall see, is now known to 
 be unfounded, yet the real paths pursued by the 
 comets are such that, for a considerable distance, 
 they very nearly coincide with a straight line ; and 
 13* 
 
150 THEATISE ON COMETS. 
 
 we may therefore understand how Cassini was ena- 
 bled to predict the places of a comet, for a few 
 weeks in advance, with considerable precision. 
 Hevelius, a few years later, found, by observing the 
 comet of 1665, that its path was curvilinear, and 
 that the orbit might be a parabola. He did not, 
 however, conjecture what would be the position of 
 the sun with respect to the curve ; and it was not 
 till the appearance of the great comet of 1680, that 
 this question was definitely settled. It^ was then 
 determined, by Dorfel and Newton, that the orbit of 
 the comet was a parabola, with the sun situated in 
 a fixed point within the curve, called the focus. It- 
 was further determined by Newton, that the comets 
 form a part of the solar system ; that they describe 
 orbits around the sun in obedience to his attractive 
 force ; and that their motions are in accordance with 
 the same laws which govern the motions of the 
 planets. He also explained a method of determining 
 the orbit from positions of the comet observed at 
 the earth. This was the first attempt to reduce the 
 motions of the comets to numerical computation, 
 in a manner which could be fully investigated by 
 mathematical analysis. 
 
 In order to effect a determination of the size and 
 position of the orbit of a comet, Newton proposed 
 two different methods. In the first he supposes 
 that a small portion of the orbit may be regarded 
 
COMPUTATION OF AN ORBIT. 151 
 
 as a straight line, described with a uniform motion, 
 and that its segments, intercepted by straight lines 
 drawn from the earth to the comet, at the times of 
 the respective observations, are proportional to the 
 intervals of time between these observations. In 
 this manner the ratio of these segments becomes 
 known ; and since the portion of the orbit included 
 between the extreme observations is a straight line, 
 its projection on the plane of the earth's orbit is 
 also a straight line, divided similarly to the orbit 
 itself by the projections of the straight lines joining 
 the places of the earth and the comet. It would 
 seem, therefore, to be a very simple process to find 
 the projected orbit; but it happens, unfortunately, 
 that this method is one which, in the case of the 
 orbit of a comet, admits of almost an infinite num- 
 ber of solutions, and, consequently, is so nearly 
 indeterminate that no definite results can be ob- 
 tained. The other method proposed by Newton 
 was more general and complete; and, as far as 
 theory is concerned, is wholly unexceptionable. It 
 is found, however, that when actually applied in the 
 determination of an orbit, it leads to computations 
 excessively complicated and laborious, and conse- 
 quently has never been generally adopted. 
 
 The theory of the determination of the orbits of 
 comets remained for many years where Newton had 
 left it ; arid astronomers were still attempting, but 
 
152 TKEATISE ON COMETS. 
 
 in vain, to discover a method by which an orbit 
 might be determined with accuracy, without making 
 it necessary to have recourse to such extended nu- 
 merical calculations. Failing in every attempt at a 
 direct solution, they were compelled to adopt a 
 method of trial and error, and thus, by successive 
 hypotheses, to approximate finally to an orbit which 
 would nearly represent the observations. Bosco- 
 vich, however, undertook to solve this difficult pro- 
 blem directly ; and the solution which he gave is 
 remarkable as being the first in \vhich the velocity 
 of the comet in its orbit was regarded as one of the 
 essential conditions. His method, as might be ex- 
 pected, is excessively complicated ; and from the 
 multiplicity of both algebraical formulae and gra- 
 phical operations which it presents, it has never 
 been employed in the actual computation of an 
 orbit. 
 
 The next attempt to solve this problem was made 
 by Lambert ; and among the formulae which he ob- 
 tained relative to the motion of a body in a parabola, 
 there is one extremely remarkable, on account of its 
 elegance, which gives the time of describing an arc 
 of the orbit in terms of its chord and the two lines 
 drawn from its extremity to the centre of the sun. 
 This formula is still employed in every method 
 which is in use for the solution of this problem, since 
 it affords a very easy means of correcting the hypo- 
 
COMPUTATION OF AN ORBIT. 153 
 
 thetical values which it is necessary to substitute in 
 the equations of the problem. He also showed how 
 it may be determined at once whether the distance 
 of the comet from the sun is greater or less than the 
 distance of the earth from the sun, thus enabling 
 the computer to abridge, very materially, the pre- 
 liminary computations. The method of Lambert 
 was subsequently modified by the researches of La- 
 grange and Laplace ; and the results finally arrived 
 at by these illustrious geometers are now adopted, 
 for the most part, as the basis of all analytical solu- 
 tions. Still another method was proposed by Gibers 
 for finding the orbit of a comet moving in a parabola, 
 in which, as in the case of those last mentioned, 
 only three different places, as seen from the earth, 
 are required. He supposes that the chord which 
 joins the places of the comet at the epoch of the 
 first and third observations, is divided by the line 
 drawn from the comet to the sun, at the time of the 
 second observation, into segments proportional to 
 the intervals of time between the observations. 
 This method is the simplest and most direct which 
 has been devised ; but gives accurate results only 
 when the intervals of time betwen the dates of the 
 observations are very nearly equal. In those cases 
 where the inequality of the intervals is considerable, 
 and especially when the comet is further from the 
 sun than the earth, it gives very unsatisfactory re- 
 
154 TREATISE ON COMETS. 
 
 suits ; and for this reason Legendre has devised a 
 method suitable to all such cases, in which instead 
 of assuming that the chord of the arc of the orbit 
 between the first and third observations is divided 
 by a line drawn from the comet to the sun at the 
 time of the second observation, in the ratio of the 
 elapsed times he finds an expression for the value 
 of each of the segments in terms of the time. This 
 last method may be employed with success in cases 
 where that of Gibers fails, but the numerical calcu- 
 lations are much more complicated and tedious. 
 
 We have thus stated, somewhat in detail, some of 
 the various methods which have been devised for the 
 solution of the problem which requires the orbit of 
 a comet to be determined by means of its successive 
 positions observed from the earth ; and what is here 
 given will be sufficient to show that it is one of the 
 most difficult problems of astronomy. If we already 
 knew, and could subject to calculation, the causes 
 which originally determined the motions of the 
 heavenly bodies, we could assign, at once, the data 
 for a complete solution of this problem without the 
 aid of observation. But as these are, and must ever 
 remain, unknown, we have no means of arriving at 
 the desired results, except by the method just ex- 
 plained. Again, if the observer could be situated 
 at the centre of the sun, the real and apparent mo- 
 tions of the comet would be identical, and the deter- 
 
COMPUTATION OF AN ORBIT. 155 
 
 urination of its orbit would be a process extremely 
 simple. But since the observer is stationed on the 
 earth, which is itself in motion in an eccentric orbit, 
 it may readily be perceived that it is necessary to 
 refer all positions, as seen from the centre of the 
 earth, to the corresponding places referred to the 
 centre of the sun, in order to arrive at the desired 
 result, and this it is which makes the problem 
 difficult. 
 
 We have already remarked that it was discovered 
 by Newton that the comets were obedient to the 
 same laws which regulate the motions of the planets 
 belonging to the solar system ; and we are, therefore, 
 enabled to adopt in this case, also, those beautiful 
 laws of planetary motion discovered by Kepler. 
 These laws are three in number, and are as follows : 
 1st. The heavenly bodies revolve around the sun in 
 conic sections whose common focus is the sun ; 2d. 
 The radius-vector, or line joining the planet or 
 comet with the sun, describes equal areas in equal 
 times, and, consequently, in unequal times areas 
 proportional to the times ; 3d. The squares of the 
 periodic times are to each other as the cubes of the 
 mean distances of the bodies from the sun. These 
 three laws, the direct result of the law of universal 
 gravitation, together with three observed positions 
 of the comet, are all the data required in finding the 
 magnitude and position of its orbit, and its position 
 
156 TREATISE ON COMETS. 
 
 in the orbit at any given time. But we have here 
 spoken of a class of curves called conic sections, and 
 it may not therefore be improper to explain their 
 character in this connection. 
 
 If we take a cone which may be conceived of as 
 being a solid formed by the revolution of a right- 
 angled triangle around the perpendicular side as a 
 fixed axis and pass planes through it at different 
 degrees of inclination with respect to the axis, the 
 intersections of these planes with the surface of the 
 cone will give us one or all of the three curves known 
 as conic sections; namely, the ellipse, parabola, and 
 hyperbola. The plane which passes through the cone 
 so as not to cut the base, but only the slant surface, 
 gives an ellipse, a curve which, next to the circle, is 
 the most simple. Those which cut the base will 
 mark out on the slant surface of the cone segments 
 of either the parabola or hyperbola, depending 
 simply on the position of the plane with reference 
 to the axis. This explains why these curves are 
 termed conic sections ; but their true character will 
 be much more readily comprehended by a more 
 familiar illustration. 
 
 If we take a fine thread and fasten its extremities 
 at two points on a plane surface, the distance be- 
 tween these points being less than the length of the 
 thread which will thus lie loosely between the 
 points and if now this thread is kept tightly 
 
COMPUTATION OF AN ORBIT. 157 
 
 stretched by means of a pencil which is made to 
 move round the entire circumference permitted by 
 the looseness of the thread, the curve thus described 
 will be an ellipse, or, as it is sometimes called, an 
 oval. Now it is easy to be seen that since the thread 
 has retained the same length, and has been kept 
 tightly drawn out during the description of the 
 curve, the sum of the lines drawn from any point 
 of the curve to the two points at which the thread 
 was fastened, will be the same for every such point 
 in the curve. The two points within the curve at 
 which the ends of the thread were fixed, are called 
 the foci of the ellipse, and in the case of the elliptic 
 orbit of a comet the sun is placed in one of these 
 points. 
 
 Let us now suppose that the distance between these 
 points is increased, the length of thread remaining 
 the same ; and, as may be readily seen by experiment, 
 we shall obtain a curve similar to the one already 
 traced, only that its form has been altered, while the 
 length of the line joining any point in the circum- 
 ference with the new foci, remains as in the previous 
 case. By continuing to increase the distance be- 
 tween the foci, using always the same thread, we 
 shall find that the successive ellipses will be more 
 and more flattened or elongated, until finally they 
 will differ but little from a straight line. If, on the 
 contrary, the distance between the foci is successively 
 14 
 
158 TREATISE ON COMETS. 
 
 shortened, the ellipses formed will gradually become 
 less and less flattened, and will scarcely differ from 
 a circle. If the foci be made to coincide, then will 
 the curve traced be a circle, and for this reason a 
 circle is sometimes said to be a species of ellipse. 
 The line which is drawn through the foci and termi- 
 nating in the curve at either extremity, is called the 
 major axis of the ellipse ; while that which bisects 
 this at right angles, terminating also in the curve at 
 opposite sides, is called the minor axis. The ratio 
 of the major axis to the line joining the foci is called 
 the eccentricity. The eccentricity will therefore be 
 greatest when the foci are at their greatest distance, 
 and least when they coincide, as in the case of the 
 circle. 
 
 Having thus explained the method of projecting 
 an ellipse, and the general properties of the curve, 
 we will proceed to illustrate the construction of a 
 parabola. For this purpose let a ruler be placed in 
 any desired position on a plane surface, and remain 
 fixed in that position. Another ruler is now placed 
 at right angles to this, but capable of being moved 
 along it. The latter must be so arranged, that in 
 moving along the former they shall always form a 
 right angle ; or, in other w r ords, one shall continue 
 perpendicular to the other. Let us now take a string 
 equal in length to the perpendicular edge of the 
 movable ruler, and fix one end of this string at any 
 
COMPUTATION OF AN ORBIT. 159 
 
 point in the plane surface on the same side of the 
 fixed ruler on which the movable one is placed, the 
 other end being attached to the latter at the outer 
 extremity of the perpendicular edge. If, then, by 
 means of a pencil, we commence at the point where 
 the string is fastened to the movable ruler, and move 
 it slowly towards the fixed ruler, always keeping the 
 string close to the edge of the former, and at the 
 same time move the latter along it in the direction 
 of the fixed end of the string passing this point, 
 and continuing until the pencil arrives again at the 
 other extremity of the string, the curve thus traced 
 will be a parabola. By varying the distance of the 
 fixed point of the string from the fixed ruler, a great 
 variety of parabolas may be traced out. The fixed 
 ruler is called the directrix of the parabola, and the 
 point within the curve at which the string is made 
 stationary, is called the focus. In the case of the 
 orbits of the comets, the sun is placed in this, the 
 only focus of the parabola. It may thus be per- 
 ceived that the parabola is constructed in such a 
 manner, that the distance between the focus and any 
 point of the curve is equal to the perpendicular dis- 
 tance between the same point and the directrix. 
 
 ]N~ext, to describe an hyperbola. On a plane sur- 
 face draw two straight lines at right angles and bi- 
 secting each other, the horizontal line, however, 
 being the longest. Then take two points on the 
 
160 TREATISE ON COMETS. 
 
 longest of these lines, one on each side of the per- 
 dicular line and at equal distances from it. Place a 
 ruler so that one end of it may revolve around one 
 of these points being kept closely pressed to the 
 plane surface and to the other end attach a string, 
 which is also to be fastened at the other extremity 
 to the fixed point taken on the opposite side of the 
 perpendicular line from that around which the ruler 
 revolves. The ruler is now revolved slowly around 
 its axis, while the string is stretched by means of a 
 pencil, commencing at the point where it is fastened 
 to the ruler, and moving it towards the other end of 
 the same, until it finally passes the horizontal line, 
 when it will move back again along the ruler to the 
 point of beginning. The string must be kept close 
 to the edge of the ruler at the point where the pen- 
 cil is placed, and in the same manner for every point 
 in the curve which will thus be traced out on the plane 
 surface, and which is called an hyperbola. The 
 fixed points which have been taken in the horizontal 
 line are called the foci of the hyperbola, and in the 
 case of the orbit of a comet moving in an hyperbola, 
 the sun is placed in the focus which is within the 
 curve. The point at which the fixed lines first drawn 
 intersect, is called the centre of the hyperbola. The 
 point where the curve crosses the horizontal line is 
 called the vertex of the hyperbola, and the distance 
 
COMPUTATION OF AN ORBIT. 161 
 
 between the vertex and the point of intersection of 
 the fixed line, is called the semi-transverse axis of the 
 hyperbola. That part of the perpendicular line 
 which is limited by an arc of a circle described 
 around the vertex of the hyperbola as a centre, with 
 a radius equal to the distance of the focus from the 
 centre, is called the conjugate axis of the hyperbola. 
 The ratio of the semi-transverse axis to the line 
 joining the centre with the focus ; or, which is the 
 same, the quotient which arises from dividing the 
 latter by the former, is called the eccentricity ; and it 
 will be perceived, from what has just been stated, 
 that this ratio will always he greater than unity. 
 In the ellipse, it follows, from the same principles, 
 that this ratio or quotient resulting from a division 
 of the line joining the foci, by the major axis, will 
 always be less than unity. The eccentricity of the 
 ellipse may, therefore, vary between the limits and 
 1, the eccentricity of the parabola is 1, while that 
 of the hyperbola may have any value greater than 
 this. 
 
 In this way these curves are found to bear an in- 
 timate relation to each other, and in order to ex- 
 hibit at once their characteristic differences, together 
 with their general resemblance, the following dia- 
 gram is given : 
 
 14* L 
 
162 
 
 TREATISE ON COMETS 
 
 The distance between the focus and vertex is the 
 same for each of the above curves ; and this dis- 
 tance, in the case of the orbit of a comet, is called 
 the perihelion distance. 
 
 The curves which we have just described we have 
 supposed to have been traced out with a short piece 
 of thread ; but curves may be imagined as being 
 constructed in accordance with the same laws, and 
 having therefore the same properties, traversing 
 vast and inaccessible regions of space, and in which 
 those dimensions which we have represented per- 
 haps by tenths of an inch, are replaced by millions 
 of miles. Such are the orbits of the comets ; and 
 in one of these curves every body which traverses 
 
COMPUTATION OF AN ORBIT. 163 
 
 our system is constrained to move, in obedience to 
 the law of universal gravitation. The planets, how- 
 ever, although permitted by that law to move in 
 similar paths, are found to revolve around the sun 
 in orbits which are purely elliptical, and in which 
 the eccentricity is generally very small. They are 
 found also to move in a series of orbits at distances 
 increasing in a regular progression, confined to 
 limits of only a few degrees on either side of the 
 orbit of the earth, and in the same direction in which 
 the earth pursues its annual course around the sun. 
 It might thus seem that an accordance so wonderful, 
 and an order so admirable, could not be fortuitous ; 
 and not being enjoined by the law of gravitation, 
 must be ascribed to some additional character apper- 
 taining to this law, with which we are unacquainted. 
 But the comets show this beautiful law exemplified 
 and obeyed in all its comprehensive features ; and 
 thus sweep around the sun in all the varied forms 
 of ellipses, parabolas, and hyperbolas, in all planes, 
 at all distances, and indifferently in both directions 
 in reference to the motion of the planets. 
 
 The comets are usually seen from the earth when 
 near the sun, and consequently when not far distant 
 from the perihelia of their orbits. It will be evi- 
 dent, therefore, by an inspection of the diagram on 
 page 162, that, since the three conic sections nearly 
 coincide for a considerable distance on either side 
 
164 TREATISE ON COMETS. 
 
 of the vertex, which corresponds to the perihelion 
 of the orbit, it will be difficult to determine, from 
 the small segment of the orbit observed, whether 
 the comet moves in an ellipse, a parabola, or an 
 hyperbola. If the centre of gravity of the comet 
 could be observed with the utmost precision, this 
 would not be difficult; but, as a general fact, the 
 observer only estimates the point of greatest con- 
 densation of light, which he assumes to be the cen- 
 tre of gravity, and the place of which in the heavens 
 he determines by reference to the fixed stars in its 
 immediate vicinity. We are thus left in doubt in 
 regard to the exact form of the orbit, unless it hap- 
 pens that the comet is favorably situated for its 
 determination. It is therefore customary among 
 astronomers, when a comet has made its appearance 
 unpredicted, to compute its orbit at first on the sup- 
 position that it is a parabola; and then, by com- 
 puting its place in advance, find from a comparison 
 of the actual observations, whether this hypothesis 
 is the correct one. Should it be found to be impos- 
 sible to represent the observed positions of the 
 comet by a parabola, an ellipse is next computed ; 
 and when this also fails, recourse is had to the 
 hyperbola, which, provided the previous computa- 
 tions are correct in every particular, will not fail to 
 represent the observations within the limits of their 
 probable errors. The results of these computations 
 
C M FIXATION OF AN O 11 B I T . 165. 
 
 give the elements of the orbit of the comet. These 
 elements are six in number of which two refer to 
 the nature and magnitude of the orbit, three to its 
 position in space, and one to the position of the 
 comet in its orbit at a given time. They are, there- 
 fore, the following : 1st. The perihelion distance, or 
 distance between the focus and vertex of the curve, 
 the sun being placed in the focus ; 2d. The eccen- 
 tricity, which in the case of the parabola becomes a 
 known quantity ; 3d. The inclination of the plane 
 of the orbit of the comet to the plane of the earth's 
 orbit, or to what is called the ecliptic; 4th. The posi- 
 tion of the line in which the orbit intersects the 
 ecliptic ; 5th. The position of the greater axis of the 
 orbit in space ; and 6th. The time at which the 
 comet was in its perihelion, or its angular distance 
 from this point at any stated epoch. In order to fix 
 the position of the line of intersection of the orbit 
 and ecliptic, which is called the line of nodes, 
 and also the position of the greater axis of the orbit, 
 it is necessary to adopt some standard point of 
 departure, to which all angular measurements may 
 be referred. The point used by astronomers is that 
 in which the sun, in its apparent annual course, 
 passes from the southern to the northern hemisphere 
 of the heavens, and is termed the vernal equinox. 
 When the comet is in the line of nodes, in the act 
 of passing from the south to the north side of the 
 
166 TREATISE ON COMETS. 
 
 ecliptic, it is said to be in its ascending node, and the 
 position of the line of nodes is determined by the 
 angular distance of the ascending node from the 
 vernal equinox. This angular distance is called the 
 longitude of the node. The position of the greater 
 axis of the orbit is determined by the angular dis- 
 tance between the vernal equinox and the perihelion 
 point, projected orthographically on the ecliptic. 
 This distance is called the longitude of the perihelion. 
 Such are the elements required for a complete 
 knowledge of all the past and future circumstances 
 of the motion of a heavenly body revolving around 
 the sun, in obedience to the law of universal gravi- 
 tation. We have already remarked, that if we knew 
 the exact conditions under which the comets were 
 first put in motion, we could assign the proper values 
 of these elements at once, without the aid of obser- 
 vations; but since these conditions are unknown, 
 we are compelled to reverse the process, and by 
 means of the effects produced to seek out the causes 
 which originally operated. To accomplish this, 
 three observed positions of the comet are required. 
 These observations should be taken at intervals of 
 a;t least one day, and should be referred to the cen- 
 tre of the earth. These three observations give us 
 the direction of the three visual rays drawn from 
 the earth to the comet at the times of the observa- 
 tions, and in the prolongation of which the latter 
 
COMPUTATION OF AN ORBIT. 167 
 
 must necessarily be found. The corresponding 
 places of the sun are known either by observation, 
 or by calculation from the solar tables. It remains 
 then to find a parabola, ellipse, or hyperbola, having 
 its focus at the centre of the sun, and cutting the 
 visual rays in points, the intersections of which cor- 
 respond to the number of days between the obser- 
 vations the motion in the orbit being in accordance 
 with Kepler's laws. In order to effect this, it is 
 necessary, in the first place, to refer the positions of 
 the comet as seen from the earth, to those in which 
 it would appear if seen from the centre of the sun ; 
 or, in other words, to transform the geocentric places 
 of the comet into heliocentric positions. This is 
 accomplished by computing a series of triangles, 
 formed by joining the successive places of the earth 
 and comet with the sun, and by joining the places 
 of the earth with those of the comet. The solution 
 of these triangles, together with Lambert's theorem, 
 in the case of a parabola, furnish the means of pass- 
 ing from the earth to the sun. As soon as the helio- 
 centric places are found corresponding to the dates 
 of the observations respectively, it becomes an easy 
 process to determine the position of the orbit in 
 space, since, at the centre of the sun, the real and 
 apparent motions of the comet are the same. The 
 transformation which gives the heliocentric longi- 
 tude and latitude of the comet, gives at the same 
 
168 TREATISE ON COMETS. 
 
 time its distance from the centre of the sun, or what 
 is called its radius-vector, corresponding to the three 
 observations. "We shall thus have three points of 
 the curve given ; and by finding the ellipse, para- 
 bola, or hyperbola, as the case may be, which shall 
 pass through these three points, a segment of the 
 orbit will be given. Now, the properties of these 
 curves have been so completely investigated, that if 
 a segment, no matter how small, is given, the 
 entire curve may be readily traced out. This ena- 
 bles us to determine the perihelion distance and the 
 eccentricity of the orbit, in the case of the ellipse 
 and hyperbola, the latter being already known in 
 the case of the parabola. The time of passing the 
 perihelion is then found, by means of Kepler's laws, 
 from the times of the observations. 
 
 Such is a concise explanation of the method of 
 determining the elements of the orbit of a comet from 
 three observations; and, although the process, as 
 here stated, may seem to be by no means difficult, 
 yet it should be understood that this problem, if 
 properly solved, is one of the most intricate and 
 laborious of all the problems which the science of 
 astronomy affords. Moreover, a detailed account 
 of the operations to be performed in making the 
 computations, would require a higher degree of 
 mathematical knowledge, in order to be fully com- 
 prehended, than is possessed by those for whom 
 
EPHEMERIS OF A COMET. 169 
 
 these explanations are intended; and what has 
 already been stated will be sufficient to exhibit, in 
 an intelligible manner, the general character of the 
 problem, and also of its solution. 
 
 When the elements of the orbit of a comet have 
 been determined, it becomes an easy process to 
 predict its position in the heavens for any future 
 date. It is customary among astronomers, as soon 
 as the orbit is known, should the comet continue 
 visible, to compute a series of places, for each suc- 
 cessive day, during the period of its visibility. 
 These places are indicated by means of the com- 
 puted right ascension and declination of the comet 
 the former being the angular distance between it 
 and the vernal equinox, measured on the ecliptic 
 towards the east ; and the latter, its angular distance 
 north or south from the celestial equator. The dis- 
 tance of the comet from the earth and sun, respect- 
 ively, is also determined at the same time. The 
 positions of the comet thus arranged constitute an 
 ephemeris ; and, in case the elements have been 
 accurately determined, all the various changes in 
 the brilliancy of the nucleus or head may be pre- 
 dicted for many months in advance. 
 
 In computing an ephemeris it is necessary to find 
 
 the position of the comet in its orbit for a given 
 
 epoch, by means of the time of perihelion passage, 
 
 the eccentricity of the orbit, and the perihelion dis- 
 
 15 
 
170 TREATISE ON COMETS. 
 
 tance. This being known, the heliocentric place 
 of the comet is readily found ; and then, by an easy 
 transformation, the geocentric right ascension and 
 declination may be determined. In practice, how- 
 ever, a course slightly different from this is pursued. 
 The position of the comet in its orbit is determined 
 as here explained ; but instead of finding the helio- 
 centric place, and from it the geocentric place, the 
 position of the comet with respect to three imagi- 
 nary planes, at right angles to each other, and 
 having their point of common intersection in the 
 centre of the sun, is investigated. These planes 
 are called co-ordinate planes, and the point of com- 
 mon intersection is called the origin. The three 
 lines of intersection of the planes are called co-ordi- 
 nate axes, and the distances of the comet from the 
 origin, measured in the direction of these lines, are 
 called its rectangular co-ordinates. The rectangular 
 co-ordinates of the earth are then found by means 
 of the solar tables, which, being subtracted from 
 those of the comet, give the rectangular co-ordinates 
 of the comet referred to the centre of the earth. 
 In making these transformations, the plane of the 
 equator is usually adopted as the fundamental plane, 
 and is supposed to coincide with one of the co-ordi- 
 nate planes. The ecliptic, also, may be adopted as 
 the fundamental plane ; but, in this case, the final 
 .places of the comet would be its latitude and Ion- 
 
EPHEMERIS OF A COMET. 171 
 
 gitude hi the heavens, instead of its right ascension 
 and declination. When the geocentric rectangular 
 co-ordinates have thus heen computed, they may be 
 transformed at once into polar co-ordinates, thus 
 giving the apparent position of the comet on the 
 celestial vault. It should be remarked, however, 
 that there are certain small corrections which must 
 be added to these results, in order to find the exact 
 place of the comet, but which are not of sufficient 
 importance to be noticed in this connection. The 
 ephemeris is now compared with the actual observa- 
 tions of the comet; and should any discrepancies be 
 exhibited, the elements are corrected so as to reduce 
 these differences within the limits of the probable 
 errors of the observations. In this way, therefore, 
 new and more accurate elements are obtained, and 
 an exact ephemeris is then computed precisely as 
 before. 
 
 When a new comet has been discovered, it is ob- 
 served for a few days, and as soon as three places 
 are obtained, separated by an interval of time of 
 sufficient extent, the elements are computed under 
 the supposition that the orbit is a parabola. An 
 ephemeris is then computed from the elements, and 
 compared with the entire series of observations, in- 
 cluding .an interval of time of the greatest possible 
 extent. Should it be found, from this comparison, 
 that the observations are not satisfactorily repre- 
 
172 TREATISE ON COMETS. 
 
 sented, the elements are corrected as above ex- 
 plained; and in case discordances are still found to 
 exist, it becomes necessary to abandon the hypo- 
 thesis of a parabolic orbit, as incompatible with the 
 motion of the comet. An ellipse is now computed, 
 and compared with the observations ; and if, after 
 having been finally corrected, it is still impossible 
 to satisfy the entire series of observations, recourse 
 is had to the hyperbola. The comparison thus 
 instituted between the observations and the com- 
 puted places resulting from the hypothesis of a 
 parabola, an ellipse, or an hyperbola, respectively, 
 will indicate the precise character of the orbit. The 
 final effect of these successive hypotheses on the 
 relation of the comet to our system, may be readily 
 conceived. A comet which moves in a parabola 
 cannot make, like one which moves in an ellipse, a 
 succession of revolutions around the sun. It enters 
 the system in some definite direction, but as pro- 
 ceeding from almost an indefinite distance. As 
 soon as it arrives within the sensible influence of 
 solar gravitation, the effects of this attraction are 
 manifested in the curvilinear form which its path 
 begins to assume, and which gradually increases as 
 its distance from the sun decreases. When it- 
 reaches its perihelion, the attractive force, and con- 
 sequently the curvature of its path, have attained 
 their maxima, and the comet has now also acquired 
 
PERIODIC COMETS. 173 
 
 its greatest velocity of translation. This extreme 
 velocity, by virtue of the inertia of the moving 
 mass, produces a centrifugal force which counteracts 
 gravitation ; and the body, having passed its peri- 
 helion, begins to retreat, and pursuing a similar 
 path to that in which it previously moved, only in a 
 reversed order, it finally passes out of the system 
 in nearly a straight line, and parallel to that in 
 which it entered. It is evident from this, that a 
 comet which moves in such an orbit, can visit the 
 solar system but once, unless it may happen that in 
 passing through some other adjoining system, its 
 course is so changed by the attractive influences 
 which may there be in operation, that it again 
 retraces its way, in its majestic flight, towards our 
 system, but in a direction entirely different from 
 that in which it first entered. 
 
 In the case of an hyperbolic orbit, the circum- 
 stances of the motion of the comet with respect to 
 the sun would be nearly the same as results from 
 parabolic motion. The only difference would be, 
 that in the parabola the comet enters the system in 
 nearly a straight line, and issues from it in a straight 
 line parallel to this ; while in the hyperbola these 
 straight lines are divergent, and not parallel. The 
 comet, in either of these cases, would not have a 
 periodic character. It might be said to be analogous, 
 in this respect, to one of those occasional meteors 
 15* 
 
174 TREATISE ON COMETS. 
 
 which are seen to shoot across the firmament never 
 again to reappear. The body, arriving from some 
 distant region, and coming, as would appear, for- 
 tuitously within the solar attraction, is drawn from 
 its course into an hyperbolic or parabolic path, which 
 it is seen to pursue ; and escapes from the solar 
 attraction, issuing from the system, and, for aught 
 we know, never to return. The phenomenon, in 
 each case, might be said to be occasional, and, in a 
 certain sense, accidental, and the body could not be 
 said to belong properly to the system. So far as 
 relates to the comet itself, it may be said that the 
 phenomena consists in a change of the direction of 
 its course through the universe, influenced by the 
 temporary action of solar gravitation upon it. 
 
 Such are the circumstances under which a great 
 majority of the comets which have visited our sys- 
 tem, have been found to move ; and they may on this 
 account be regarded as a connecting link between 
 the various systems of the universe. But in the case 
 presented by a comet moving in an elliptic orbit, the 
 relation between the comet and our system is much 
 more intimate, and the interest and physical import- 
 ance of the body transcendently greater. In this 
 case the comet possesses a periodic character, making 
 successive revolutions like the planets, and return- 
 ing to its perihelion at regular intervals of time. 
 Unlike those comets which move in parabolas or 
 
PERIODIC COMETS. 175 
 
 hyperbolas, it would not be an occasional visitor to 
 our system, connected with it by no permanent re- 
 lation, and subject to solar gravitation only acci- 
 dentally and temporarily. On the contrary, it is to 
 be regarded as being as permanent, if not as strictly 
 regular, a member of the system, as any of the 
 planets, although invested with an extremely differ- 
 ent physical character. This being the most plausi- 
 ble view of the cometary motions in general, and 
 one which would seem to have impressed, at once, 
 every one who has studied the phenomena of these 
 wonderful bodies, it may perhaps seem strange that 
 no attempt was made until recently to determine 
 the orbits of the comets under the direct supposition 
 of elliptic motion. But it should be remembered 
 that those which are known to be periodic move in 
 ellipses of great eccentricity ; and since they are ob- 
 served only when near their perihelion, it becomes 
 almost impossible to decide in reference to the 
 nature of the orbit. As already remarked, the ob- 
 servations are not absolutely exact ; and making due 
 allowance for the probable errors of observation, 
 even when the comet has been visible for a long 
 period of time, it is generally found that its visible 
 path may be represented with almost equal fidelity 
 by either of the three curves in which these bodies 
 move. In all such cases, it is impossible to infer, 
 from the observations alone, whether the comet 
 
176 TREATISE ON COMETS. 
 
 belongs to the class of hyperbolic or parabolic 
 bodies, which have no periodic character, or to the 
 elliptic, which has. 
 
 When the period of revolution does not exceed 
 100 years, the periodic time may be found within 
 a year of the true time from the observations made 
 during two or three months. In this case the ellip- 
 ticity is so marked that the observations will not 
 fail to indicate it. "When the period is only about 
 five or fifteen years, the observations of three months, 
 provided they are numerous, will give the orbit so 
 approximately that the next appearance of the comet 
 may be predicted within a few days of the observed 
 time. The method of determining the elements of 
 the orbits of the heavenly bodies has been carried 
 to so high a degree of perfection, that by combining 
 many observations made at different places nearly 
 simultaneously, or within a few days of each other, 
 thus forming what are called normal places of the 
 comet, almost the exact form and magnitude of the 
 orbit can be found directly. But although, in the 
 case of a periodic comet, the ellipticity is plainly 
 exhibited, and its value very closely approximated, 
 yet the unavoidable errors of observation are still 
 sufficient to render the period of revolution to a 
 limited extent uncertain. Sometimes this uncer- 
 tainty may amount to several weeks, and again it 
 may not exceed a few days or perhaps a few hours 
 
PKHIODIC COMETS. 177 
 
 in the time of the return of the comet to its perihe- 
 lion. It depends necessarily on the character and 
 number of the observations employed, and also upon 
 the interval of time during which the comet was 
 accurately observed, or in a position favorable for 
 good observations. 
 
 It often happens that the character of periodicity 
 itself, which of course belongs only to elliptic orbits, 
 supplies the means of surmounting the difficulties 
 just enumerated as affecting the exact determination 
 of the periodic time of a comet, from the observa- 
 tions made during a brief interval. If any observed 
 comet has an elliptic orbit, it must return again to 
 its perihelion after having completed one revolution, 
 and it must also have been visible on former returns 
 to that position. It is evident, therefore, that not- 
 only ought the comet to be expected to reappear in 
 future at regular intervals, but that its previous re- 
 turns to its perihelion would be found, in case it had 
 been observed, by searching among the recorded 
 appearances of such objects for any, the dates of 
 whose appearances might correspond with the sup- 
 posed period, and whose apparent motions might 
 indicate a real motion in an orbit, identical, or nearly 
 so, with that of the comet which may be under con- 
 sideration. ~Now it is known that, in most cases, 
 the elements of the orbit undergo comparatively 
 slight changes during one, or even two or three suc- 
 
 M 
 
178 T K E A T I S E ON C M E T S . 
 
 cessive revolutions. If the force of gravity of the 
 sun operated alone, and the comet was not subjected 
 to the attractions of the various planets of the sys- 
 tem, the elements would continue invariable. The 
 comet would reappear after each successive revolu- 
 tion at exactly the same point as seen from the sun ; 
 would follow, while visible, exactly the same arc of 
 its orbit ; would move in the same plane, inclined at 
 the same angle to the ecliptic, the nodes retaining 
 the same places ; and would arrive at its perihelion 
 at exactly the same point and after exactly equal 
 intervals of time. The disturbing action of the 
 planets is usually small, especially when the inclina- 
 tion of the plane of the orbit of the comet to the 
 plane of the ecliptic is considerable. In general, 
 this disturbance, which is called the planetary per- 
 turbations, will not change any of the elements more 
 than one or two degrees ; while in many instances 
 the variations in the position of the orbit in space 
 with respect to the equinox and ecliptic these being 
 the elements which are most affected will not 
 exceed half a degree. It may happen, however, 
 the orbit of the comet being very eccentric, and on 
 that account nearly intersecting the paths of several 
 planets that the disturbing action of these planets 
 may, in the aggregate, entirely, or nearly overcome 
 the principal effect of the predominant force of the 
 sun ; and the orbit of the comet at two different re- 
 
PERIODIC COMETS. 179 
 
 turns will be so completely changed that it would 
 be impossible, from a mere inspection of the elements 
 corresponding to each appearance, to identify it. 
 While, therefore, we may be prepared for the possi- 
 bility, and even the probability, that the same 
 periodic comet on the occasion of its successive re- 
 appearances, may, in passing to and from its peri- 
 helion, follow a path differing to some extent from 
 that which it had followed at previous returns ; yet, 
 as we have already remarked, such differences can- 
 not, except in rare and exceptional cases, be very 
 considerable. For the same reason the intervals 
 between its successive periods, though they may 
 differ, cannot be subject to any great variation. 
 
 The elements of the orbits of all the comets which 
 have appeared during the last century, and also of 
 many comets which had previously appeared, have 
 been computed and recorded. As soon, therefore, 
 as an unexpected comet has made its appearance, 
 and the elements of its orbit have been determined, 
 an examination of these recorded elements is made, 
 in order to detect, if possible, any which have a 
 sufficient resemblance to those of the comet in ques- 
 tion, to warrant the supposition of their identity. 
 Should it be found, on making such a comparison, 
 that there are no elements on record which satisfy 
 the required conditions of identity, we may conclude 
 that the comet is a new one. and must have recourse 
 
180 TREATISE ON COMETS. 
 
 to actual observations for the determination of the 
 precise character of its motion. The approximate 
 elements already known will enable us to conjecture 
 at least the probable influence of the principal 
 planets, and we are thus enabled to make due allow- 
 ance, in the comparison, for any discrepancies in the 
 elements which might result from this cause. In 
 the case of a parabola, there are five elements to be 
 compared ; and since, considering a limited number 
 of comets, the chances are so small that a near re- 
 semblance of all the elements may be accidental, we 
 may conclude without hesitation, in all cases where 
 such a resemblance is detected, that the comets are 
 identical, and that whatever differences are found 
 to exist, must be due to the attraction of the planets 
 and to the unavoidable errors of the observations. 
 
 The identity of a comet thus determined does not 
 always furnish the period of its revolution. It is 
 not known whether the comet has made but one 
 revolution during the interval which has elapsed, or 
 whether it may not have returned several times un- 
 perceived. This question must be decided by actual 
 computations from the observations. If it is found 
 that, by assuming an eccentricity less than that 
 which would result from the supposition of a single 
 revolution, the observations may be more completely 
 represented, it will be evident that the comet has 
 made more than one revolution between the epochs 
 
PERIODIC COMETS. 181 
 
 of the two sets of elements. By successive approxi- 
 mations, conducted in the same manner, or perhaps 
 by direct computation of elliptic elements from the 
 observations, this question may be definitely settled, 
 and the periodic time may thus be accurately deter- 
 mined. 
 
 Nor is the fact that no identity can be found by a 
 comparison of the elements of a comet which may 
 be under consideration, with those which have been 
 recorded, in itself sufficient evidence of its not 
 having a fixed period of revolution. It may have 
 returned always unperceived, or it may have been 
 seen but not observed sufficiently for the determi- 
 nation of its orbit. Prior to the time of Kepler and 
 Galileo, these bodies were often visible for a long 
 period of time, but were not observed, except as 
 objects of curiosity, or as portending the greatest 
 imaginable disasters. Indeed, we find that historians 
 have mentioned, and even described, their appear- 
 ances, and in some cases have indicated the principal 
 constellations through which such bodies passed, 
 although no observations of their apparent places 
 have been transmitted, by which any close approxi- 
 mation to their actual paths could be made. If, 
 therefore, the elements of a comet are known so 
 approximately as to give its period of revolution 
 within limits of a few years, and it is found that 
 comets have been thus described at epochs which 
 16 
 
182 TREATISE ON COMETS. 
 
 might correspond with, its previous appearances, the 
 identity is possible. In such cases it is only neces- 
 sary to reduce the modern elements to the ancient 
 epoch, and by computing the path of the comet, 
 find whether it would have presented the phenomena, 
 both in respect to brilliancy and apparent motion, 
 which were then observed. Should they be found 
 to agree, it is certain that the comets are identical. 
 The resemblance in magnitude, form, and splen- 
 dor, which may be found to exist between comets 
 which have appeared at different epochs, does not 
 afford any reasonable basis for supposing them to 
 have been successive appearances of the same comet. 
 In modern times more reliable data are furnished 
 for the solution of this problem ; while in more 
 ancient times, the only circumstance generally re- 
 corded was the appearance of the object, accom- 
 panied, in many instances, with details bearing 
 evident marks of exaggeration. It is only, there- 
 
 
 
 fore, in cases where the computations of the elements, 
 from modern observations, indicate unequivocally 
 the elliptic form of the orbit, and its approximate 
 periodic time ; or, where a distinct and close resem- 
 blance between two different systems of elements 
 for comets at two different epochs, respectively, is 
 found to exist ; that the precise period of revolution 
 of a comet can be made known, except in the case 
 already referred to, in which the period does not 
 
PERIODIC COMETS. 183 
 
 exceed fifteen years. It may thus be perceived how 
 astronomers are enabled to make such wonderful 
 predictions in regard to the future movements of 
 some of these objects, as have been made from time 
 to time. In no other science can be found so mar- 
 vellous a series of phenomena foretold. The interval 
 between the prediction and its fulfillment has some- 
 times exceeded, and will often exceed, the limits of 
 human life ; and one generation bequeathes its pre- 
 dictions to another, which is to be filled with aston- 
 ishment and admiration at witnessing their literal 
 accomplishment. 
 
 We have thus explained in a concise, yet suffi- 
 ciently extended manner, the process by which 
 astronomers are enabled to predict the past and 
 future movements of a comet, from observations 
 made only during a brief period in which it is 
 visible. In the explanations here given, it was, of 
 course, impossible to enter into a detailed account 
 of all the various methods which may be employed 
 in the solution of this problem. We have given 
 that which is most simple and readily understood, 
 and which is sufficient to demonstrate to those unac- 
 quainted with the abstruse and intricate mathe- 
 matical investigations which are effected by the 
 professional astronomer in making these calcula- 
 tions, the theory of the solution, thus showing con- 
 clusively that the final results arrived at, though 
 
184 TREATISE ON COMETS. 
 
 sometimes startling, are of the most positive 
 character. 
 
 In the preceding chapter we gave a description 
 of some of the most remarkable comets which have 
 appeared reserving, however, until now, a de- 
 scription of the phenomena connected with the 
 periodic comets, or at least those which have either 
 been identified at more than one appearance, or are 
 known to have periods of revolution not exceeding 
 one or two hundred years. We shall therefore pro- 
 ceed at once to give an account of this latter class 
 of comets, in the order in which their periodic char- 
 acter has been discovered. 
 
 It might, indeed, be expected that comets moving 
 in elliptic orbits of small dimensions, and conse- 
 quently having short periods, would have been the 
 first in which the character of periodicity would be 
 discovered. The comparative frequency of their 
 returns to their perihelia, and the consequent possi- 
 bility of verifying the fact of their periodic charac- 
 ter, together with the distinctly elliptic form of their 
 orbits, which would be supposed to be made evident 
 by computation, would afford a firm basis for such 
 an expectation. But it is found in this case, as has 
 happened in so many others in the progress of phy- 
 sical science, that the actual results of observation 
 and research have been directly contrary to such an 
 anticipation ; the most remarkable case of a comet 
 
PERIODIC COMETS. 185 
 
 of large orbit, long period, and consequently rare 
 returns, being the first, and those of small orbits, 
 short periods, and frequent returns, the last, gene- 
 rally speaking, whose periodicity has been disco- 
 vered. 
 
 Soon after the discovery of the law of gravitation 
 by Newton, it was found to confirm completely the 
 laws of Kepler, and also to be constant and univer- 
 sal in every situation in which it could be traced, 
 whether on the earth or in the solar system. This 
 fact gave to the physical sciences generally, and to 
 that of astronomy in particular, a generalization and 
 harmony which had not previously been known. 
 The general law of the motions of the heavenly 
 bodies through space, in connection with the obser- 
 vations of the great comet of 1680, led Newton to 
 conclude that the orbits of the comets must, like 
 those of the planets, be ellipses having the sun in 
 one focus, but far more eccentric ; and having their 
 aphelia, or greatest distances from the sun, far re- 
 mote in the regions of space. He adds, however, 
 that he leaves it to others to determine the greater 
 axes of their orbits and their periods of revolution, 
 by comparing comets which return after long inter- 
 vals of time, with the same orbits. The idea thus 
 advanced by Newton, extraordinary as it may ap- 
 pear, was soon to be strictly realized. Twenty years 
 later, Halley collected all the observations of comets 
 16* 
 
186 TREATISE ON COMETS. 
 
 which he could procure ; and, by means of the 
 method invented by Newton, he determined with 
 great difficulty the elements of the orbits of twenty- 
 four, which, out of 425 of these bodies recorded 
 prior to the beginning of the eighteenth century, 
 were all that had been observed with sufficient 
 accuracy to admit of the determination of their 
 orbits by the method which he employed. 
 
 On comparing the elements thus obtained, Halley 
 found that the orbits of the comets of 1531, 1607, 
 and 1682, were nearly the same. He also found 
 that one which had appeared in 1661, followed 
 nearly the same path with one which was visible in 
 1532. He was, therefore, led to conclude that the 
 former comets were reappearances of the same 
 comet, which would thus have a period of revolution 
 of about 75 or 76 years, and that the latter had a 
 period of 129 years. It may perhaps be proper to 
 add that the latter conjecture has failed to be real- 
 ized, while the former has been found to be literally 
 true. His belief of the identity of the comets of 
 1607 and 1682, was still further confirmed by records 
 of more ancient comets, among which he found 
 three nearly corresponding in their periods with the 
 former. These were the comets of 1305, 1378, and 
 1456. He now declared his opinion that the same 
 comet had appeared at these six epochs ; and that, 
 since its period of revolution was a little more than 
 
HALLEY'S COMET. 187 
 
 seventy-five years, it might be expected to return in 
 the year 1758. The following table shows the re- 
 semblance of the periods which led to this important 
 announcement : 
 
 Perihelion Passage. Interral. 
 
 November 9, 1378, -- - 
 
 ' ' 77 years, 7 months. 
 
 June 9, 1456, 75 " 9* " 
 
 August 26, 1531, 76 2 * 
 
 October 27, 1607, ?4 u 1(U tt 
 September 15, 1682, 
 
 Here was exhibited an inequality in the periodic 
 time amounting to more than two years a circum- 
 stance which, in connection with the fact that there 
 was found to be a variation in the inclination of the 
 orbit of the comet to the plane of the ecliptic, might 
 seem to cast a doubt on the conclusion at which 
 Halley had arrived. It was known, however, that 
 the comet might have been sensibly disturbed in its 
 orbit by the attractions of Jupiter and Saturn at 
 its successive returns ; and that this would perhaps 
 account for the discrepancies in its periodic time, 
 which would otherwise appear to exist. The attrac- 
 tion of Jupiter on Saturn was known to affect the 
 velocity of the latter planet; sometimes retarding 
 and sometimes accelerating it, according to their 
 relative position, so as to affect its period to the 
 extent of thirteen days. The influence of Saturn 
 on Jupiter was also known to be of precisely a sim- 
 ilar character, although its effect was not more than 
 
f 
 
 188 THEATISE ON COMETS. 
 
 half as great. Under such circumstances, Halley 
 did not hesitate to declare that, if such heavy bodies 
 as the planets could be disturbed in their motions 
 by their mutual attractions, a comet, which he 
 considered a mass of vapor, would be still more 
 disturbed in passing near such large and solid bodies, 
 especially if far removed from the influence of the 
 sun. He concluded, also, that since the motion of 
 the comet in its orbit is so rapid, a very small in- 
 crease of velocity from any disturbing cause would 
 change the form of its ellipse. 
 
 For these and similar reasons, he did not hesitate 
 to attribute the inequality observed in the interval 
 between its successive returns, and also the variation 
 in the inclination of its orbit, to the attractive influ- 
 ence of planets. Moreover, he noticed that in the 
 interval between 1607 and 1682, the comet passed 
 so near Jupiter, that his attraction must have aug- 
 mented its velocity, and consequently shortened its 
 period. In the same manner he found that, after 
 1682, its period would be again increased by the 
 influence of Jupiter and Saturn ; and he therefore 
 predicted, finally, that the comet would reappear 
 toward the latter part of the year 1758, or about 
 the beginning of 1759. It was certainly a bold 
 analogy, in those days, to attribute a difference of 
 more than a year, in the motion of a comet, to the 
 same cause that occasioned a change of only a few 
 
II A L L E Y ' s COMET. 189 
 
 days in the motions of the planets ; and this confi- 
 dent prediction by Halley may therefore be regarded 
 as one of the most remarkable events in the history 
 of astronomy. But, great as was this achievement, 
 the imperfect state of mathematical science rendered 
 it impossible for Halley to exhibit to the world an 
 absolute demonstration of the event which he fore- 
 told. It was, therefore, only possible for him to 
 announce these felicitous conceptions of a sagacious 
 mind as mere intuitive perceptions, which must be 
 received as uncertain by the world, however 
 strongly he himself may have been impressed with 
 them, until they could be verified by the process 
 of a rigorous analysis. 
 
 Such was the prediction of Halley a prediction 
 which, considering the state of astronomical science 
 at the time it was made, must certainly be regarded 
 as one of the most extraordinary and wonderful 
 which has ever been made. The distinguished phi- 
 losopher, sensible of the fact that he could not live 
 to witness the verification of his prophetic announce- 
 ment, patriotically expressed the hope that, should 
 the comet return in accordance with his prediction, 
 posterity would do him the justice to acknowledge 
 that it was announced by an English astronomer. 
 
 Before the comet had accomplished another revo- 
 lution, at the completion of which Halley's predic- 
 tion was to be fully realized, mathematical science 
 
190 TREATISE ON COMETS. 
 
 had advanced rapidly to a very high degree of per- 
 fection, and men of sublime genius had arisen, who 
 were able to follow out Newton's theory of gravita- 
 tion to its legitimate results and consequences. Of 
 the problems thus presented for investigation there 
 was one of peculiar importance, and yet of the most 
 difficult character. One of the first and simplest 
 results of the theory of gravitation was, that if a 
 single planet revolved around the sun in free space 
 its mass being supposed to be insignificant as com- 
 pared with that of the sun it must revolve in an 
 ellipse, the focus of which must be occupied by the 
 centre of the sun. But if we now suppose a second 
 planet to be admitted into the system, then the 
 ellipticity of their orbits can be preserved only on 
 the supposition that the two planets have no attrac- 
 tion for each other, and that no physical force is in 
 operation except the solar attraction. This will 
 necessarily require that their masses shall be so small, 
 compared with that of the sun, as to be entirely 
 neglected. The law of universal gravitation, how- 
 ever, is based on the principle that every particle of 
 matter, every body in nature, must be mutually 
 attracted. This may seem to indicate that the elliptic 
 character of the orbit would be effaced ; but it should 
 be remembered, that in the cases of the planets of 
 the solar system, their masses are so small compared 
 with that of the sun, that for any limited time their 
 
H ALLEY'S COMET. 191 
 
 orbits may be regarded as purely elliptic. But in 
 course of time their orbits must be slightly changed, 
 depending on their relative masses, those whose 
 masses are least being most sensibly affected. These 
 slight deviations are called the perturbations of the 
 planets and comets, or the disturbances due to their 
 reciprocal attraction. The problem thus presented 
 is known as the problem of the three bodies, and its 
 extension embraces the effects of the mutual gravi- 
 tation of all the planets of the system upon each 
 other. We must, however, defer, until a subsequent 
 connection, a complete explanation of the method 
 of solving this problem, and of its more extended 
 applications ; stating here only what may be required 
 in order to understand the manner in which the 
 exact time of the return of a comet may be predicted. 
 The problem of the three bodies, involving the 
 sun, earth, and moon, had already engaged the 
 attention of Euler, D'Alembert, and Clairaut; and 
 from the comparative smallness of the third body 
 considered, namely, the moon, the solution was 
 effected with considerable facility. But in order to 
 apply the problem to the case of the sun, a planet, 
 and a comet, it was necessary in the first place to 
 investigate the analytical formulae which would 
 represent the respective motions of the planet and 
 comet, before the actual place of the latter, at any 
 given time, could be determined, supposing the ele- 
 
192 TREATISE ON COMETS. 
 
 ments of the orbit at a fixed epoch to be known. 
 This was attempted by Clairaut ? one of the most 
 profound geometers then living ; and he endeavored 
 to determine the path of the comet of 1682, when 
 attracted by the sun and disturbed by a planet. In 
 this complicated problem, the disturbing action of 
 one planet only can be estimated at a time, and on 
 this account it becomes necessary to repeat the 
 numerical computations for each disturbing body. 
 Moreover, the disturbing action of a planet can only 
 be computed for an extremely small portion of the 
 orbit at a time, and the sum of these is the whole 
 effect. 
 
 It was enough for Clairaut to investigate the com- 
 plicated formulae by which these computations were 
 to be accomplished. He was indeed eminently 
 qualified to conduct such an investigation, and the 
 manner in which it was accomplished may be wit- 
 nessed by the accuracy of the final results. When 
 the analytical part of the problem had been solved, 
 the laborious numerical computations were under- 
 taken by Lalande, an accomplished practical as- 
 tronomer. The fidelity with which he performed 
 his part will soon be made evident. In this pro- 
 digious labor he was assisted by Madame Lepaute, 
 an accomplished lady of Paris, whose name al- 
 though, for some unaccountable reason, wholly 
 omitted in Clairaut' s Memoir giving an account of 
 
HALLEY'S COMET. 193 
 
 the computations is thus deservedly registered in 
 the annals of astronomical science. 
 
 When we consider that the period of the comet is 
 about seventy-five years, and that for two successive 
 periods it was necessary that every portion of its 
 orbit should he calculated separately, as above ex- 
 plained, a general idea of the enormous labor per- 
 formed by Lalande and Madame Lepaute may be 
 formed. They computed from morning till night, 
 and even late in the evening, without intermission, 
 for a period of six months ; having computed the 
 distance of each of the two planets, Jupiter and 
 Saturn, from the comet, and their attraction upon 
 that body, separately for each degree of the orbit, 
 during a period of 150 years. The result was, that 
 in consequence of the attractive influence of these 
 two planets, the period of revolution of the comet 
 would be lengthened nearly two years. It was found 
 that it would be delayed 518 days by the action of 
 Jupiter, and 100 days by that of Saturn, so that it 
 would arrive at its perihelion on the 18th of April, 
 1759. It was afterwards found, on a revision of the 
 calculations, which had been, toward the latter part, 
 performed too rapidly in some instances neglecting 
 quantities which were by no means inconsiderable 
 from fear of being anticipated by the arrival of 
 the comet, that the perihelion passage would take 
 place on the 4th of April, 1759. 
 17 N 
 
194 TREATISE ON COMETS. 
 
 The Memoir which contained an elaborate account 
 of these investigations was presented to the French 
 Academy on the 14th of November, 1758, and a 
 supplementary one, containing an account of the 
 revision of the latter part of the numerical com- 
 putations, was presented a few weeks later. In this 
 Memoir Clairaut explained the unfavorable circum- 
 stances under which the computations were made. 
 He had no observations on which to base the ele- 
 ments of the orbit but those of Apian, which were 
 far from being accurate, having been made at a time 
 when little attention was paid to comets. It is pro- 
 bable, however, that if Apian had been sensible of 
 their future importance, he would have observed 
 with much greater accuracy. The mass of Saturn 
 was unknown, and Clairaut was compelled to adopt 
 a value which has since been found to be erroneous. 
 He also neglected the disturbing action of the earth, 
 which was not altogether inconsiderable, since the 
 comet had passed near it in 1682, and could not take 
 into account the action of Uranus and Neptune, 
 which were then undiscovered. The effects of the 
 action of Mercury, Yenus, and Mars, being insig- 
 nificant, were not computed. 
 
 It is not strange, therefore, that Clairaut admitted 
 that his results might be liable to a small error, 
 enough, perhaps, to leave an uncertainty of one 
 month in the date of the return of the comet to its 
 
H A L L K Y ' s COMET. 195 
 
 perihelion. He stated, also, that there might be very 
 many circumstances which, independent of any 
 error either in the methods or process of calculation, 
 might cause the event to deviate more or less from 
 its predicted occurrence. He was well aware that a 
 body which passes to such distant regions of space, 
 and which is invisible for so long a time, may be 
 subjected to forces wholly unknown ; such, for ex- 
 ample, as the action of other comets, or even of some 
 planet so distant from the sun that it has not yet 
 been seen a fact which has since been established 
 by the discovery of the planets Uranus and Neptune. 
 Again, Newton and many of his followers believed in 
 the existence of an ethereal fluid throughout all space, 
 although there was no proof of it at that time, and 
 conjectured that it would accelerate the motions of 
 the heavenly bodies without altering the position of 
 their orbits. No one, however, had attempted to 
 estimate its effects till Clairaut endeavored to find 
 what influence it would have on the motions of 
 Halley's comet, which was the name assigned to 
 the comet of 1682, in honor of the discoverer of its 
 periodic time and he found that the acceleration 
 would not amount to more than seven and a half 
 minutes. 
 
 Such were the circumstances attending the pre- 
 diction of the exact time of the comet's return by 
 Clairaut; and so positively were his results an- 
 
196 TKEATISE ON COMETS. 
 
 nounced, and so firmly was he convinced of their 
 general accuracy, that he felt certain that the comet 
 would first be visible about the end of 1758, or early 
 in January, 1759. There was, however, one circum- 
 stance connected with the previous returns of the 
 comet, which seemed to Lalande to indicate the 
 possibility of the return of the comet in 1759, with- 
 out being seen. In 1456 it occupied a space nearly 
 seventy degrees in length, and spread terror through- 
 out Europe. In 1607, its appearance, as described 
 by Kepler, was that of a star of the first magnitude, 
 and so trifling was its tail that it was at first doubted 
 whether it had any. In 1682 it excited very little 
 attention, except among astronomers; and supposing 
 this decrease of magnitude and brilliancy to be pro- 
 gressive, Lalande entertained serious apprehensions 
 that on its expected return it might escape the ob- 
 servation even of astronomers ; and that thus, this 
 splendid example of the power of science and un- 
 answerable proof of the principle of universal gravi- 
 tation, would be lost to the world. It is indeed true, 
 that, in 1607 and in 1682, the comet was not favor- 
 ably situated in respect to the earth and sun to ap- 
 pear with great brilliancy, while in 1456 its position 
 was exactly the reverse of this. But still it did not 
 seem hardly possible that so great a change in phy- 
 sical appearance could be caused by simply the 
 difference of position with respect to the earth and 
 
HALLE Y ' s COMET. 197 
 
 sun at the time of its visibility ; and it may, there- 
 fore, be said, that it affords the very greatest interest 
 to observe the misgivings of this distinguished as- 
 tronomer, with respect to the appearance of the body 
 according to the prediction, in connection with his 
 unshaken faith in the results obtained. He asserted 
 that it would, without doubt, return ; and that, even 
 if astronomers failed to see it, they would not, on 
 that account, be less convinced of its presence. 
 They would know that the faintness of its light, its 
 great distance, and, perhaps, even an unfavorable 
 state of the weather, might prevent its being seen. 
 The world, however, would find it difficult to be- 
 lieve in his results, and would place this discovery, 
 which had done so much honor to modern philoso- 
 phy, among the number of chance predictions. 
 
 Under such circumstances, the return of the comet 
 was awaited by astronomers with the greatest im- 
 patience and anxiety. They were anticipated, how- 
 ever, in the discovery, by George Palitzch, a peasant, 
 residing in the neighborhood of Dresden. He first 
 saw the comet on the evening of the 25th of De- 
 cember, 1758, through a small telescope, and on the 
 next day communicated the discovery to Hoffman, 
 who observed it on the 27th and 28th of December. 
 A few days later it was independently discovered by 
 an astronomer at Leipsic, who, jealous of his dis- 
 covery, did not announce it, but gave himself up to 
 17* 
 
198 TREATISE ON COMETS. 
 
 the solitary pleasure of following the body in its 
 course from day to day, while his cotemporaries 
 throughout Europe were vainly directing their 
 anxious search for it to other quarters of the heavens. 
 Delisle, a French astronomer, and Messier, his as- 
 sistant, had been constantly engaged in searching 
 for it, but an error in the computation of the 
 ephemeris, by the former, had diverted their atten- 
 tion to a different part of the heavens from that in 
 which the comet was to be seen ; and the consequence 
 was, that it was not seen by Messier until the 21st 
 of January, 1759, nearly a month after its discovery 
 by Palitzch, but without knowing that it had been 
 already observed. 
 
 The news of the discovery now became generally 
 known, and the comet was observed at various 
 places in Europe. On its first appearance the nu- 
 cleus w r as round and brilliant, surrounded by a vapor 
 or nebulous atmosphere, but did not exhibit any 
 indications of a tail. It was rapidly approaching 
 the sun, and about the middle of February was lost 
 in the approaching twilight. It passed its perihelion 
 soon after midnight at Paris, on the 12th of March, 
 1759, only twenty-three days before the time pre- 
 dicted by Clairaut and Lalande, or within the limits 
 of probable error which they had assigned to their 
 results. Laplace has since shown, that if the mass 
 of Saturn had been accurately known at the time 
 
H ALLEY'S COMET. 199 
 
 when the computations were made, the error of the 
 final result would not have exceeded nine days. 
 This, together with the fact that the planets Uranus 
 and Neptune have since been discovered, and that 
 the existence of an ethereal fluid pervading the 
 regions of space has been established by its action 
 on the motions of the comets, enables us to form a 
 true conception of the importance of Clairaut's 
 labors, which had revealed the place of the long- 
 expected wanderer, while yet invisible to the naked 
 eye, on its return after an absence of three-quarters 
 of a century, to crown with triumph the illustrious 
 astronomer who had first foretold its period. What 
 greater exemplification of the transcendant powers 
 of the human understanding could be desired, or 
 what greater exhibition of the achievement of 
 genius? An object which had once caused the 
 greatest alarm throughout all Europe, and which 
 had been supposed, in former times, to have caused 
 some of the greatest disasters which have ever be- 
 fallen the human race, was now divested of its 
 terrific aspect and nature ; and hereafter, as it re- 
 turned successively in its long journey through 
 space, to be hailed with increasing delight, as 
 affording new and conclusive proofs of the harmony 
 of those beautiful laws which govern the celestial 
 motions. 
 
 The comet emerged from the sun's rays toward 
 
200 TREATISE ON COMETS. 
 
 the end of March, 1759, and was visible in the 
 morning just before sunrise; and, had it not been 
 iii the bright twilight, was in the most favorable 
 position, with reference to the earth and sun, for 
 being seen in all its splendor. On the 1st of April, 
 Messier was able to distinguish a tail with the aid 
 of a telescope ; but in no other instance was it cer- 
 tainly observed in Europe. On the 17th of April it 
 ceased to be visible in the morning ; on the evening 
 of the 29th of the same month it appeared about 
 the size of one of the largest stars, and from the 3d 
 of June ceased to be visible. The apparent dimi- 
 nution of the comet in magnitude and brilliancy, 
 already noticed by Lalande, may here seem to be 
 positively confirmed as a .physical fact, since its 
 splendor at this return was even less than in 1682. 
 But it should be remembered that it was seen in 
 1759 under the most disadvantageous circumstances. 
 In April, when the tail ought to have been longest, 
 the comet was far from the earth, and rapidly re- 
 ceding from the sun ; and, moreover, was almost 
 always obscured by the effect of the twilight. 
 Again, it may be readily understood why a comet 
 may, at successive returns to our system, sometimes 
 appear to have a tail, and sometimes to be without 
 one, according to its position with regard to the 
 earth and sun. It may happen that, at one return, 
 the earth may be near the comet at or near the time 
 
H A L L E Y ' S XJ M E T. 201 
 
 of its perihelion passage, when its light is neces- 
 sarily greatest and its train most extended, and thus 
 a most favorable opportunity will be afforded for 
 witnessing its physical appearance ; while, at its 
 next return, the earth being at a remote part of its 
 orbit while the comet is passing the sun, it may be 
 seen only with great difficulty, or even become 
 quite invisible. It is evident, therefore, that no 
 expectation of a uniform physical appearance of a 
 comet at successive returns, can be indulged ; and 
 that there is scarcely the slightest chance of ever 
 being able to recognize even a single one among 
 the many thousands which are sweeping through 
 the regions of space, by means of any apparent 
 similarity of form and brilliancy. 
 
 In the southern hemisphere, Halley's comet was 
 much more favorably situated for observation, in 
 1759, than in the higher latitudes of the northern 
 hemisphere. It was observed at the Isle of Bourbon 
 and at Pondicherry, and the tail was distinctly visi- 
 ble to the naked eye at both places, its length 
 varying from ten to forty-seven degrees, which 
 seems to accord fully with the former appearances 
 of the same body. 
 
 Such were the circumstances connected with the 
 first predicted return of Halley's comet; and its 
 appearance, in accordance with the prediction, ex- 
 cited the curiosity and Avonder of both the learned 
 
202 TREATISE ON COMETS. 
 
 and unlearned throughout all Europe. Another 
 interval of seventy-six years has elapsed since its 
 appearance in 1759, the same computations have 
 been performed, only that this time the planet Ura- 
 nus was known, and the mass of Saturn had been ac- 
 curately determined; and on the 16th of November, 
 1835, the comet, true to its predicted course, passed 
 the perihelion of its orbit. This revolution, how- 
 ever, was performed under circumstances far more 
 favorable than had ever before occurred. During 
 the interval between 1759 and 1835, the science of 
 analysis, as applied to physical astronomy, had 
 made rapid advances. The methods of investigation 
 had acquired greater simplicity, and had also been 
 made more general and comprehensive. Learned 
 societies had likewise been established in several 
 cities in Europe and elsewhere ; and had stimulated 
 the spirit of inquiry by a succession of prizes 
 offered for the solution of problems arising out of 
 the difficulties which were progressively developed 
 by the advancement of astronomical science. Among 
 these questions, the determination of the elliptic 
 orbits of comets, and the perturbations which they 
 experience in their course, by the action of the 
 planets near which they happen to pass, were con- 
 sidered of primary importance. The French Aca- 
 demy, accordingly, offered, in the year 1778, a high 
 mathematical prize for an essay on this subject. 
 
H ALLEY'S COMET. 203 
 
 which called forth a Memoir from Lagrange, in 
 which he formed at once a complete solution and a 
 model for all future investigations of the same kind. 
 This investigation by Lagrange was, however, of a 
 general character; and it remained to apply it to the 
 particular case of Halley's comet, the only one then 
 known whose periodicity had been definitely deter- 
 mined. In 1820, the Academy of Sciences at Turin 
 offered a prize for this application of Lagrange's 
 theory, which was awarded to Damoiseau, a French 
 astronomer ; and, in 1826, the French Institute pro- 
 posed a similar prize, having offered it twice before 
 Avithout calling forth any claimant. This prize was 
 awarded to Pontecoulant, who determined the per- 
 turbations of Halley's comet by taking into account 
 the simultaneous action of Jupiter, Saturn, Uranus, 
 and the earth the comet having, in 1759, passed 
 sufficiently near the latter to experience from it sen- 
 sible disturbances. He found, finally, that it would 
 pass its perihelion on the 7th of November, 1835 ; 
 and afterwards, by a revision of the calculations, he 
 predicted that its arrival at that point of its orbit 
 nearest the sun, would take place on the morning 
 of the 14th of the same month. 
 
 The same problem was undertaken by Lubbock, 
 Rosenberger, and Lehmann. Damoiseau and Ponte- 
 coulant had assumed the orbit in which the comet 
 was moving in 1759 as the basis of their investiga- 
 
204 TREATISE ON COMETS. 
 
 tions, and had calculated simply the alterations 
 which would be produced upon it by the action of 
 the planets from 1759 to 1835. In his second solu- 
 tion, Pontecoulant corrected the elements by means 
 of the observations in 1759, and consequently ob- 
 tained more satisfactory results. Lubbock, Rosen- 
 berger, and Lehmann, undertook in the first place 
 to ascertain the orbit which it followed in 1759, by 
 means of such observations as had been recorded at 
 that time ; and then, by making due allowance for 
 the planetary perturbations, to predict the exact 
 date of its return to its perihelion. It results, there- 
 fore, as might be expected, that the time of passing 
 the perihelion in 1835, as found by these astrono- 
 mers, exhibits a considerable uncertainty in the 
 period, arising from their having adopted different 
 systems of elliptic elements of the orbit of the comet 
 at its former appearance. The time of perihelion 
 passage obtained by the astronomers above named, 
 are as follows : 
 
 Lubbock .... October 31, 1835. 
 Damoiseau , November 4, " 
 
 Rosenberger . , . T v 11, " 
 
 Pont6coulant . . . " 14, " 
 
 Lehmann . . .' * ; " 26, " 
 
 The computations of Rosenberger and Lehmann 
 were published only a short time before the comet 
 made its appearance, those of Lehmann having 
 been announced on the 25th of July, 1835. They 
 
HALLE Y'S COMET. 205 
 
 published also an ephemeris of the comet for the 
 period of its visibility, in which its exact route in 
 the heavens was accurately designated, and the date 
 at which it would probably be visible with the aid 
 of a telescope. On the 5th of August, 1835, Du- 
 mouchel, Director of the Observatory of the Roman 
 College, at Rome, directed his telescope to the point 
 indicated by Rosenberger's ephemeris ; and saw the 
 comet, as a faint and almost invisible stain of light 
 on the deep blue of the heavens, within a degree of 
 its predicted place. On the 20th of August it was 
 generally observed in Europe, and was subsequently 
 followed until about the time of its perihelion pass- 
 age, on the 16th of November, when it ceased to be 
 visible, on account of its having moved toward the 
 south. It was observed in the southern hemisphere 
 throughout February, March, and April, 1836; and 
 disappeared finally on the 17th of May of that year. 
 When the comet first became visible, it presented 
 the appearance of a small, round, or somewhat ellip- 
 tic nebula, without any indications of a tail, and 
 having a point in which the light seemed to be 
 strongly condensed, situated eccentrically within it. 
 On the 2d of October the first indications of a tail 
 were exhibited, which increased rapidly, and on the 
 5th had attained a length of five degrees. On the 
 15th of October, nearly a month before its perihelion 
 passage, the tail had attained its maximum length, 
 18 
 
206 TREATISE ON COMETS. 
 
 which was upwards of twenty degrees. From that 
 date it decreased rapidly, and on the 29th of Octo- 
 ber it was only three degrees in length. It con- 
 tinued to decrease ; and on the 16th of November, 
 the date of its perihelion passage, the comet was 
 observed at Pulkova, in Russia, where no tail was 
 visible. 
 
 Previous to the return of the comet, although its 
 physical appearance was not expected to be such as 
 to create sensations of terror, even among those 
 inclined to be superstitious, yet the expectation was 
 very general among astronomers that, at its return 
 to its perihelion in 1835, it would afford an oppor- 
 tunity for obtaining new data, from which to derive 
 some satisfactory theory respecting the physical con- 
 stitution of the class of bodies of which it is so 
 striking an example. It is perhaps unnecessary to 
 state, that it no sooner became visible than pheno- 
 mena began to be exhibited, preceding and accom- 
 panying the gradual formation of the tail, the obser- 
 vation of which has been most justly regarded as 
 forming a memorable era in the history of astrono- 
 my, or more especially in that relating to the 
 cometary bodies. All those strange and important 
 appearances which the comet presented were ob- 
 served with the greatest zeal, and delineated with 
 the utmost fidelity, by several different astronomers, 
 among whom were Bessel, Schwabe, and Struve, in 
 
H ALLEY'S COMET. 207 
 
 the northern hemisphere, mid Maclear and Herschel 
 at the Cape of Good Hope ; each of whom have 
 recorded the successive transformations presented 
 hy the comet, under the physical influence of a con- 
 stantly varying temperature, in its approach to and 
 departure from the sun. 
 
 The most striking phenomena which the comet 
 presented early in October, were those which, com- 
 mencing simultaneously with the growth of the tail, 
 evidently connected themselves with the production 
 of that appendage, and its projection from the head 
 of the comet. On the 2d of October, the nucleus, 
 which had hitherto been faint and small, was ob- 
 served to have become suddenly brighter, and clearly 
 indicated the commencement of the formation of 
 the tail, by the appearance of being in the act of 
 throwing out a jet or stream of light from that part 
 presented toward the sun. This ejection was by 
 no means either uniform or continuous, but appeared 
 to take place at intervals, like the issuing of the 
 fiery matter from the crater of a volcano. On the 
 4th of October, as seen through a powerful telescope, 
 the central part of the comet presented a very curi- 
 ous appearance. The nucleus was elliptical, at least 
 five or six times longer in one direction than in the 
 other; and is said to have resembled a burning coal, 
 from which there issued, in a direction nearly oppo- 
 site to that of the tail, a divergent flame, varying in 
 
208 TREATISE ON COMETS. 
 
 intensity, in form, and in direction. Occasionally it 
 appeared doubled, and seemed to indicate that a 
 luminous gas was issuing from the nucleus. This 
 ejection, after ceasing for a brief period, was re- 
 sumed on the 8th of October, and continued, much 
 more violently than before, with occasional inter- 
 ruptions, until the tail ceased to be visible. At this 
 time, Schwabe noticed what he considered to be a 
 secondary train, extending for a short distance in a 
 direction opposite to that of the original tail, and, 
 therefore, toward the sun. Bessel, however, re- 
 garded this appearance merely as the renewed ejec- 
 tion of nebulous matter which was afterwards turned 
 back from the sun, as smoke would be by a current 
 of air blowing from the sun in the direction of the 
 real tail. Both the form of this luminous ejection, 
 and the direction in which it issued from the nu- 
 cleus, were observed to undergo, from time to time, 
 singular and capricious alterations the different 
 phases so rapidly succeeding each other, that on no 
 two successive evenings were the appearances simi- 
 lar. Sometimes there were two, and often three, 
 nebulous emanations observed to issue from the 
 nucleus in divergent directions. The directions of 
 tjiese emanations of nebulous matter, and also their 
 comparative brightness, appeared to be subject to 
 continual variations. At one time the emitted jet 
 was observed to be single, as above described, and 
 
II A L L E Y ' s COMET. 209 
 
 confined within narrow limits of divergence from 
 the nucleus. At other times, it presented a fan- 
 shaped or swallow-tailed form, not unlike that of 
 the flame issuing from a gas-burner with a flattened 
 orifice. 
 
 Appearances similar to these were noticed by 
 Arago, at Paris, and also by other astronomers at 
 several different places. On the 15th of October, 
 Arago saw a luminous sector, or diverging light, 
 issuing from the head of the comet, a little to the 
 south of the point immediately opposite the tail, 
 which was much more brilliant than the rest of the 
 nebulosity, and was bounded by two lines of fainter 
 light directed toward the centre of the head. On 
 the following evening no trace of this sector ex- 
 isted ; but at a point diametrically opposite the axis 
 .of the tail, a new sector was formed, of more than 
 a quarter of a circumference, in angular extent, 
 which was much more elongated and brilliant than 
 the one first seen, and which was bounded by two 
 very bright lines tending to the centre of the head. 
 On the 17th this appearance had diminished in 
 splendor, and on the evening of the 18th the 
 atmosphere being extremely clear the entire 
 comet, including the tail, had very sensibly de- 
 creased in brilliancy. On the 21st, there were three 
 luminous sectors or brushes of light seen in the 
 nebulosity of the head, the most feeble and dilated 
 18* o 
 
210 TREATISE ox COMETS. 
 
 of which was exactly in the prolongation of the tail. 
 Two days later, these had totally vanished, but the 
 whole aspect of the comet was completely and 
 suddenly changed. The nucleus, hitherto so bril- 
 liant and well-defined, had become large and dif- 
 fused ; and although it still occupied the centre of 
 the head, yet the brilliancy of the nebulosity on the 
 eastern side far surpassed that on the western. 
 
 Another remarkable circumstance connected with 
 the emission of the luminous jets already described, 
 was that the direction of the principal jet was ob- 
 served to oscillate to and fro, on either side of a line 
 drawn from the sun through the centre of the head 
 of the comet, just as the needle of a compass oscil- 
 lates on either side of the magnetic meridian. This 
 vibratory motion was so rapid that the direction of 
 the jets was observed to be visibly changed from 
 hour to hour, under the eye of the observer. These 
 jets, though very bright at their point of emanation 
 from the nucleus, are described as fading rapidly 
 away, and becoming diffused as they expanded into 
 the coma, at the same time curving backwards like 
 streams of steam or smoke, thrown out more or less 
 obliquely from narrow orifices, in opposition to a 
 powerful wind, against which they are unable to 
 make way, and ultimately yielding to its force, are 
 drifted back and confounded in a vaporous train, 
 following the general direction of the current. At 
 
II ALLEY'S COMET. 211 
 
 one time, Struve saw the comet thus attended by 
 two delicately-shaped appendages of light, of a most 
 graceful form, one of which preceded, and the other 
 followed the nucleus. Subsequently it appeared to 
 be surrounded by a sort of semicircular veil, which, 
 extending backward, was lost in a double train of 
 light, stretching out to a vast distance from the 
 body of the comet. On the 5th of November, 1835, 
 when seen through the powerful telescope at Pul- 
 kova, two flames were seen issuing from the nucleus 
 in nearly opposite directions, both of which were 
 curved toward the same side. The brighter flame, 
 directed toward the north, was marked by strongly 
 defined edges; while the other, directed toward 
 the south, was more feeble and ill-defined. The 
 opposite diagram represents the comet as seen at 
 this time. 
 
 The comet was not observed in Europe, with but 
 few exceptions, after the perihelion passage. For 
 upwards of two months it was lost in the sun's 
 rays, but again reappeared on or about the 24th of 
 January, 1836, when it was observed in the southern 
 hemisphere. Its aspect, however, was altogether 
 different from that under which it had been seen 
 previous to its perihelion passage having, during 
 the interval in which it was invisible, undergone 
 some great physical change, .which had operated to 
 produce an entire transformation of its physical 
 
212 TREATISE ox COMETS. 
 
 appearance. It is described as no longer presenting 
 any vestige of a tail, but as appearing to the naked 
 eye about as bright as a star of the fifth magnitude, 
 shining through a haze ; while, with the aid of a 
 powerful telescope, it was seen as a planetary 
 nebula, with a small, round, and well-defined disc, 
 surrounded by a nebulous envelope or coma of con- 
 siderable extent. Within the well-defined head or 
 disc, and somewhat eccentrically placed, was a vivid 
 nucleus, resembling a miniature comet, with a head 
 and tail of its own, perfectly distinct from and 
 greatly exceeding, in the intensity of its light, the 
 other portions of the comet. A minute bright 
 point, resembling a small star, was distinctly per- 
 ceived within it, but which was never sufficiently 
 well defined to give any positive assurance of the 
 existence of a solid sphere, nor could any indication 
 of a gibbous form be detected. 
 
 The phenomena and changes which the comet 
 presented subsequent to its reappearance in the 
 southern hemisphere, and until its final disappear- 
 ance, were carefully observed at the Cape of Good 
 Hope. As it receded from the sun, the coma or 
 envelope speedily disappeared, as if absorbed into 
 the disc, which, on the contrary, is said to have in- 
 creased continually in dimensions, and with such 
 rapidity, that in the week ending February 1st, 
 1836, the actual volume or real solid contents of the 
 
II ALLEY'S Co M E T. 213 
 
 illuminated space had expanded at least forty times. 
 This increase of the size of the nucleus, or rather 
 the head, continued with undiminished rapidity 
 until the final disappearance of the comet. The 
 brilliancy of the comet was found to decrease in 
 proportion as the magnitude of the head increased. 
 The nucleus of the comet, properly so called, was 
 not, however, observed to undergo any very con- 
 siderable change, while the envelope was continually 
 dilating, and becoming fainter and fainter. More- 
 over, the ray proceeding from it is said to have in- 
 creased in length and comparative brightness, pre- 
 serving its direction along the greater diameter of 
 the head, which had acquired the form of a 
 parabola, and exhibiting none of those irregular 
 phenomena which characterized the jets emitted 
 previous to its perihelion passage. This ray of 
 nebulous light, however, faded away by degrees; 
 and on the 5th of May the comet appeared precisely 
 as at the time of its first appearance in August, 
 1835, or as a small round nebula, with a condensa- 
 tion of light near its centre. This was the last 
 observation of the comet in the southern hemi- 
 sphere ; but it was seen twelve days later by Lamont, 
 at Munich, in Bavaria, when it disappeared not to 
 return again until about the end of February, 1911. 
 The position of the comet in the heavens from 
 day to day was also observed with great precision ; and 
 
214 TEE-ATISE ON COMETS. 
 
 from these observations, combined with those made 
 in 1759, and at previous returns, it will now be pos- 
 sible to investigate all the circumstances of its mo- 
 tion, and to predict the date of its next perihelion 
 passage within a few hours, or, perhaps, within less 
 than an hour. To accomplish this will require im- 
 mense labor, being sufficient, when carefully per- 
 formed, to occupy the entire time of one computer 
 for several years. But great as this task may seem 
 to be, it is nevertheless certain that some one will 
 undertake it if it has not been commenced already 
 and that the time of the next return of the comet 
 will be so accurately predicted, that the exact place 
 in the heavens to which the telescope must be 
 directed will be announced, and the comet will be 
 seen in strict accordance with the announcement. 
 It is indeed true, that there may be bodies of a 
 planetary or cometary nature in the far distant 
 regions of the heavens in which the comet wanders, 
 which may disturb its motions and change the period 
 of its revolution ; but so accurately can the disturb- 
 ing influence of all the known bodies of our system 
 be calculated, that the existence of these unknown 
 sources of perturbation will be made evident to future 
 astronomers, should any difference be hereafter ex- 
 hibited between the computed and actual orbit of 
 the comet. 
 
 The accuracy of Pontecoulant's prediction of the 
 
II A L L E Y ' s COMET. 215 
 
 perihelion passage of the comet in 1835, affords a 
 striking instance of the precision of the methods of 
 calculation employed by him, when we consider the 
 long period of seventy-five years during which the 
 comet had been invisible. Moreover, the small dis- 
 crepancy of only two days can be readily accounted 
 for by the fact that the existence of the planet Nep- 
 tune was then hardly suspected, and the masses of 
 Jupiter and Uranus were not then as well deter- 
 mined as at the present time. And, further, the ex- 
 istence of a medium of resistance has since, as we 
 shall see, been conclusively established. The influ- 
 ence of this ethereal medium on the motion of this 
 comet cannot be determined, with great certainty, 
 until it has performed another revolution. Its effect 
 on the duration of the last period of the comet has 
 indeed been computed ; but this is a problem which, 
 in the present state of our knowledge, can be solved 
 only by experience, since it is evident that the effect 
 of such a medium of resistance as is here supposed 
 to exist, must depend upon the magnitude and 
 density of the comet, and upon the law by which 
 the density of the ethereal fluid varies with the dis- 
 tance from the sun. It is supposed that this medium 
 has acquired a rotary motion from east to west 
 especially within the limits of the solar system and 
 it will therefore be necessary, in order to arrive at 
 results of the very greatest accuracy, to decide what 
 
216 TREATISE ON COMETS. 
 
 part of the variations in the period of its revolution 
 may be due to that cause. These and similar ques- 
 tions are to be carefully discussed before a precise 
 knowledge of the motions of the comet can be ob- 
 tained; but that all this, and even more, will be 
 accomplished, the history of astronomy during the 
 last fifty years furnishes ample reason to expect. 
 
 The influence of the attraction of the planets on 
 the motions of Halley's comet, can be computed with 
 extreme precision ; and were there no other disturb- 
 ances to be experienced during the entire course of 
 its revolution around the sun, it would be possible 
 to predict its future movements with the same pre- 
 cision in which the motions of the planets are de- 
 termined. It is possible that the orbit of Neptune 
 does not mark the confines of the planetary system, 
 but that there may be other planets still more 
 remote, which, in due course of time, may change 
 the motions of the comet, and which, in case they 
 are not sooner revealed, may finally cause it to fail 
 to appear in accordance with prediction. The time 
 will indeed come, when the action of Neptune will 
 greatly affect the orbit of the comet ; and had this 
 planet not been discovered, it would at some future 
 day have falsified the predictions of astronomers, 
 and would, perhaps, have again involved the entire 
 theory of cometary motions in impenetrable mystery. 
 It is with no small degree of pleasure, therefore, 
 
HALLEY'S COMET. 217 
 
 that the corresponding advancement of every de- 
 partment of physical science can be contemplated. 
 The discovery of Neptune was itself one of the most 
 brilliant achievements of the human intellect which 
 has been recorded ; but it is not until its entire influ- 
 ence in the system is considered, that we can form 
 a true conception of its ultimate importance. 
 
 Some idea may be formed of the vast size of the 
 orbit of Halley's comet, by comparing it with the 
 dimensions of that of our earth. The mean distance 
 of the earth from the sun is about 95,000,000 miles, 
 while the length of the orbit of the comet is about 
 thirty-six times this distance, and its breadth about 
 one-fourth of its length. It should be remembered, 
 however, that the eccentricity of the orbit of the 
 earth is very slight, while that of the comet is very 
 great ; and, consequently, the distance of the comet 
 from the sun is subject to great variations. In 
 approaching the sun, its velocity continually in- 
 creases, until it darts around him with astonishing 
 rapidity, at a distance, when in the perihelion, of 
 only 47,000,000 miles from its centre. Its velocity 
 then gradually diminishes, after leaving the sun, till 
 it reaches the most remote point, or aphelion of 
 its orbit, where its distance from him is about 
 3,373,000,000 miles a distance which exceeds that 
 of the remotest planet of our system, but at which 
 the attraction of the sun is still sufficient to recall 
 19 
 
218 TREATISE ON COMETS. 
 
 the comet toward him. When, therefore, the comet 
 is in its perihelion, the amount of heat and light 
 which it will receive from the sun, will be four times 
 greater than what is received at the earth. When 
 the comet is in the aphelion of its orbit, the light 
 and heat will be nearly 6000 times less than at the 
 perihelion ; and could the sun be viewed from the 
 comet at this enormous distance, it would not appear 
 larger than a star. It is evident, therefore, that the 
 vicissitudes of temperature experienced by the comet 
 must be almost beyond our conception. If the earth 
 could be transported to the aphelion of the comet's 
 orbit, every liquid substance would become solid by 
 congelation, even if the atmospheric air and other 
 permanent gases did not become liquids by the same 
 process. But if, on the other hand, the earth were 
 placed at the perihelion of the orbit of the comet, 
 all the liquids upon it would be converted into 
 vapor, either mixing with the atmospheric air, or 
 arranging themselves in regular strata, one above 
 the other, according to their relative weights. 
 
 We have thus described, somewhat in detail, the 
 most important events connected with the discovery 
 of the periodic character of Halley's comet, and also 
 the most interesting phenomena which it presented. 
 We have seen that it exhibited to a high degree 
 those physical phenomena which seem to belong 
 almost exclusively to this class of bodies, and that it 
 
E x c K E ' s COMET. 219 
 
 affords an unparalleled example of the result of those 
 powers of calculation by which we are enabled to 
 follow it, in the mind, through the depths of space, 
 far beyond the extreme limits of our system, and. 
 notwithstanding disturbances which render each 
 succeeding period of its return different from the 
 last, to foretell that return with the utmost precision. 
 The next comet, whose period of revolution has 
 been definitely determined, which we shall consider, 
 is that known as Encke's comet, and which, although 
 one of the most interesting in its phenomena, is 
 still, in almost every respect, unlike that which we 
 have just described. This comet was discovered by 
 Pons, at Marseilles, on the 26th of November, 1818. 
 It was not visible to the naked eye ; and when seen 
 through a telescope, presented the appearance of an 
 ill-defined nebulosity, without a tail, but evidently 
 having a point in which the light was strongly con- 
 densed, situated eccentrically within it. As soon as 
 it had been observed to a sufficient extent, the ele- 
 ments of its orbit were computed ; and on comparison 
 of the elements of those comets which had previously 
 appeared, it was immediately noticed that this comet 
 was identical with one which was visible in 1805. 
 It was uncertain, however, whether the period was 
 thirteen years, or whether the comet had, in the 
 mean time, returned unperceived to its perihelion. 
 Subsequently, Encke undertook to remove this 
 
220 TREATISE ON COMETS. 
 
 doubt, by computing an elliptic orbit directly from 
 the observations made in 1818 and 1819, during the 
 entire period in which the comet was visible ; and he 
 found that its period of revolution was only about 
 1200 days, and that, consequently, the comet had 
 returned three times between the years 1805 and 
 1819. It was found, also, upon further examination, 
 that the same comet had been observed in 1786 and 
 in 1795. Having thus identified the comet at four 
 different returns to its perihelion, En eke was enabled 
 to ascertain the period of its revolution with great 
 precision ; the result being 1208 days. This, how- 
 ever, occasioned great astonishment, and some de- 
 gree of doubt, since it was generally believed that 
 the periods of comets must necessarily be very long. 
 The identification of the comet with those of 
 1786, 1795, and 1805, enabled Encke to use observa- 
 tions including an interval of nearly thirty-three 
 years in making his computations ; and having com- 
 puted its motion from the year 1819, he predicted 
 that, as its period would be reduced to 1203 days by 
 the disturbing action of the planets, it might be 
 expected to appear about the beginning of June, 
 1822, but that, on account of its position in the 
 heavens, it would only be visible in the southern 
 hemisphere. The return of the comet was therefore 
 looked for by the astronomers who happened to be 
 in that part of the globe ; and on the 2d of June, 
 
E x c K E ' s COMET. 221 
 
 1822, it was actually discovered by Rumker, at Para- 
 mata, in New South Wales, and observed until the 
 end of the same month. The next return was pre- 
 dicted to take place in 1825 ; and on the 13th of 
 July true to its appointed course the comet was 
 observed by Yalz, at Msmes. The next return took 
 place in 1828. It was first seen by Strove, at Dor- 
 pat, in Russia, on the 13th of October of that year, 
 and was observed in the European observatories till 
 December 25th. 
 
 In 1832, the comet again returned to its perihelion ; 
 but being unfavorably situated for observation in 
 the northern hemisphere, it was only seen by Hard- 
 ing, at Gottingen, on the 21st of August. It was 
 observed, however, by Henderson, at the Cape of 
 Good Hope, during the entire month of June, and 
 was also seen at Buenos Ayres. In 1835 it was ob- 
 served from July 22d till August 6th, and in 1838 it 
 was seen at Breslau, on the 14th of August, as a 
 very faint, ill-defined object. It subsequently in- 
 creased in brilliancy, and continued visible until the 
 middle of December. In combining all the observa- 
 tions which have been made from 1786 to 1838, in- 
 clusive, Encke found it necessary to change the 
 value which he had hitherto employed for the mass 
 of the planet Mercury, in computing the perturba- 
 tions of the comet, and also to make allowance for 
 the supposed influence of a resisting ethereal fluid, 
 19* 
 
222 TREATISE ON Co M E T s . 
 
 pervading the regions of space, as already alluded 
 to in the case of Halley's comet. It should be re- 
 marked, however, that even before this, the comet 
 had indicated that a correction to the adopted value 
 of the mass of Jupiter was necessary. The plane 
 of the orbit of the comet being inclined to that of 
 the orbit of Jupiter by a small angle, it necessarily 
 approaches very near the planet, and, consequently, 
 is subject to considerable disturbance, in its motions, 
 from this source. Under these circumstances it may 
 be perceived that, since the action of Jupiter on the 
 comet depends on their distance and relative quan- 
 tities of matter, any error in the assumed value of 
 the mass of the former would be exhibited by the 
 excess or deficiency of the resulting values of the 
 perturbations. But Laplace had computed, from 
 the theory of probabilities, that there was hardly 
 any chance that the mass of the planet, which was 
 generally used, could be in error even by the one- 
 hundredth part of its value. 
 
 The discrepancies, however, which were plainly 
 exhibited in the motions of the comet, were such as 
 to excite a strong suspicion of its inaccuracy ; and 
 accordingly a new determination of this important 
 quantity was attempted. The result was that it was 
 found by three different computers, each by an en- 
 tirely different process, that the mass of Jupiter, 
 hitherto assumed, was in error more than four times 
 
ENCKE'S COMET. 223 
 
 the amount which Laplace had assigned as the limits 
 of its probable uncertainty. "With this new value 
 of the mass of Jupiter, and admitting the existence 
 of a resisting medium, it seemed possible to an- 
 nounce the future movements of the comet with 
 great precision. The next return was therefore pre- 
 dicted to take place in 1842, and on the 8th of 
 February of that year the comet was detected by 
 Galle, at Berlin. It was observed in Europe and in 
 the United States until near the middle of April, 
 and was found to follow the computed path very 
 closely. In 1848 it was observed for a period of 
 three months, extending from August to November. 
 When first detected it was very faint, but towards 
 November it had increased in brilliancy so as to be 
 barely perceptible to the naked eye as a dull hazy 
 star of the sixth magnitude. The observations 
 made at this return of the comet have not only con- 
 firmed the supposed existence of an ethereal medium 
 of resistance, but have also been the means of de- 
 riving some very important results in relation to the 
 changes which its magnitude or volume underwent 
 in approaching the sun, and also in receding from it. 
 The next return of the comet, which has received 
 the name of Enckes Comet, in honor of the disco- 
 verer of its periodic character, took place in 1852. 
 It passed its perihelion on the 14th of March of that 
 year, but was first seen on the 2d of January. The 
 
224 TREATISE ON COMETS. 
 
 next return occurred in 1855 ; but the comet was 
 not favorably situated for observation in the north- 
 ern hemisphere. It was observed at the Cape of 
 Good Hope, and also in Santiago, in Chili. The 
 comet again appeared in 1858, almost exactly in the 
 place assigned by computation. It came to its 
 perihelion on the 18th of October of that year ; but 
 was first observed, at Berlin, on the 7th of August. 
 From the observations made at the Cape of Good 
 Hope in 1855, in combination with those made pre- 
 viously, at the successive returns of the comet, Pro- 
 fessor Encke determined its orbit anew ; and having 
 made due allowance for the planetary perturbations, 
 he constructed an ephemeris, and on the first clear 
 evening after the comet was computed to be in a 
 position for observation, the telescope was directed 
 to the spot indicated, and the comet was barely dis- 
 tinguishable as a dim nebulosity, without any dis- 
 tinct or regular outline. In a few weeks, however, 
 the brilliancy had increased to such an extent, that 
 when seen through a telescope, the comet appeared 
 as a round, well-defined nebula, without a tail, but 
 having decided indications of a point near the centre, 
 in which the light was strongly condensed. About 
 the middle of September it became visible to the 
 naked eye, but only as a star of the sixth or seventh 
 magnitude, shining through a thin haze. At this 
 time the comet was seen in the east several hours 
 
E N c K E ' s COMET. 225 
 
 before sunrise ; and being in a position very favor- 
 able for observation, it was possible to detect a very 
 slight elongation of the head of the comet in a 
 direction opposite the sun. The nucleus was almost 
 a stellar point, with a coma or envelope of very 
 dense nebulous matter. The comet was not ob- 
 served after the first week in October, having been 
 lost in the approaching twilight. The next return 
 of the comet will take place in March, 1862. 
 
 The orbit of Encke's comet is an ellipse of great 
 eccentricity, whose length is about double its 
 breadth. When at its perihelion it is distant from 
 the sun only 32,000,000 miles, or a little less than 
 the distance of Mercury. When in the aphelion of 
 its orbit the comet is distant from the sun about 
 400,000,000 miles, or a little more than four times 
 the mean distance of the earth. One extremity of 
 the orbit, therefore, reaches a little beyond the orbit 
 of the asteroid Pallas, and the other extends to that 
 of Mercury. The length of the orbit, however, is 
 found to be gradually decreasing; and, conse- 
 quently, the period of revolution is becoming shorter 
 and shorter. This phenomenon has been attributed 
 to the existence of a resisting medium, consisting 
 of a subtle ethereal fluid, pervading all space. The 
 decrease of the period of revolution of the comet, 
 after making due allowance for the perturbations 
 whish have been caused by the action of the planets, 
 
226 
 
 TREATISE ON COMETS. 
 
 is exhibited in the following table, from 1786, the 
 date of its first appearance, till 1858, the time of its 
 last return to its perihelion : 
 
 Date. 
 
 Period of revolu- 
 
 
 tion in days. 
 
 1789, 
 
 .... '1212.79 
 
 1792, 
 
 .... 1212.67 
 
 1795, 
 
 .... 1212.55 
 
 1799, 
 
 .... 1212.44 
 
 1802, 
 
 .... 1212.33 
 
 1805, 
 
 I*:-;, . . 1212.22 
 
 1809, 
 
 . ... . . 1212.10 
 
 1812 
 
 . . . . 1212.00 
 
 1815, 
 
 .... 1211.89 
 
 1819 
 
 .... 1211.78 
 
 1822, 
 
 .... 1211.66 
 
 Date. 
 
 Period of revolu- 
 tion in days. 
 
 1825, .... 1211.55 
 
 1829, .... 1211.44 
 
 1832, .... 1211.32 
 
 1835, .... 1211.22 
 
 1838, .... 1211.11 
 
 1842, .... 1210.98 
 
 1845, .... 1210.88 
 
 1848, .... 1210.77 
 
 1852, .... 1210.65 
 
 1855, .... 1210.55 
 
 1858, 1210.44 
 
 The existence of some retarding force is here very 
 strongly exhibited. The action of the planets has 
 been computed with the greatest possible precision ; 
 and yet the period is decreasing, by slow and regu- 
 lar degrees, at the rate of about eleven hundredths 
 of a day, or about two hours and thirty-eight min- 
 utes, at each successive revolution. This is evi- 
 dently the result which would be produced by a 
 resisting medium, such as is supposed to exist, since 
 the effect of the resistance is to diminish the velocity 
 of the comet in its orbit, thus lessening the force 
 resulting from the orbital motion, or what is called 
 the centrifugal force, and consequently compelling 
 the comet to fall towards the sun, in obedience to 
 the now preponderant force of gravity, until the 
 
ENCKE'S COMET. 227 
 
 equilibrium is again restored. The comet will there- 
 fore be continually describing a new orbit, smaller 
 than the one in which it had previously moved, 
 and with an increased velocity. Thus it may be 
 perceived that the medium of resistance actually 
 accelerates the motions of the comet ; but since the 
 force of resistance is confined to the plane of the 
 orbit, it can have no influence whatever in producing 
 any additional changes in the elements. In com- 
 puting the effect of the resisting medium, it is 
 assumed to increase inversely as the squares of the 
 distances, and that its resistance acts as a force tan- 
 gent to the orbit, and proportional to the squares 
 of the comet's actual velocity in each point of its 
 orbit. We shall subsequently give a complete ex- 
 planation of the theory of this ethereal medium, 
 and also consider its ultimate effect on the motions 
 of the heavenly bodies, and more especially on those 
 of the comets. 
 
 This comet, as we have already stated, is a small 
 telescopic object, of extreme tenuity, very pale and 
 diffused in its appearance, even at the point of 
 greatest condensation, and without a tail. It has, 
 however, although apparently an insignificant ob- 
 ject, opened a wider field of discovery, and more 
 new and interesting facts, than any other on record. 
 It has, as above noticed, furnished the means of 
 proving the existence of an ethereal fluid, pervading 
 
228 TREATISE ON COMETS. 
 
 all space ; and has afforded a striking instance of 
 the changes which take place in the nebulous matter 
 on approaching the sun, and in retreating from him ; 
 and also, an opportunity of measuring these changes. 
 Stars which have been seen through it more than 
 once, have attested the high degree of its attenua- 
 tion ; while the smallness of its orbit, and the short- 
 ness of its period, give it almost the characteristics 
 of a planet. The constant decrease in the size of 
 its orbit, arising from the resistance of the ethereal 
 fluid, may possibly precipitate it upon the surface 
 of the sun, if, before that event can take place, the 
 perpetual diminution of its mass does not offer an 
 example of condensation to the solid state, or of 
 annihilation ; and its perturbation by the planets 
 have been the means of indicating errors in the 
 assumed masses of Mercury and Jupiter. 
 
 At each successive return, this comet furnishes 
 additional evidence in support of the hypothesis of 
 a resisting medium. Indeed, it has already incon- 
 trovertibly established the fact of the existence of 
 such a medium, which, if there be not other consi- 
 derations to counterbalance, may result ultimately, 
 though at a time almost infinitely distant, in the 
 total annihilation of the planetary system. It is a 
 result so startling in its consequences, and so repug- 
 nant to our preconceived ideas of the stability of 
 our system, that the mind almost shrinks in horror 
 
BIELA'S COMET. 229 
 
 from its contemplation. If it were not that the cal- 
 culations from which such a conclusion has been 
 derived, have heen repeated by different individuals, 
 each one using different methods of computation, 
 but all arriving at the same general results, we 
 might perhaps hesitate to receive it. But when evi- 
 dence of the most unequivocal character has been 
 adduced, we are compelled to admit the fact, and 
 look to other forces to prevent the inevitable conse- 
 quences which must ensue. There can be but little 
 doubt that there are other forces in nature than 
 those now known, which time alone will unfold; and 
 which will enable the astronomers of some future 
 day to explain all these mysterious questions, to 
 dispel all such dismal forebodings as the existence 
 of a resisting medium in space, with our present 
 knowledge, will excite ; and to furnish finally the 
 most sublime proofs of the fixed design of perpe- 
 tuity which the Almighty Creator has enstamped on 
 his universe of creation. Let us, therefore, be con- 
 tent with our present knowledge of laws and forces ; 
 and, if it be not permitted to our day and genera- 
 tion, leave it to those who shall come after us to 
 solve these difficult problems. 
 
 The discovery of this comet's periodic character was 
 
 soon followed by the announcement of a second comet 
 
 of short period. This comet was detected by Biela, 
 
 an Austrian officer, at Josephstadt, in Bohemia, on 
 
 20 
 
230 TREATISE ON COMET?. 
 
 the 27th of February, 1826 ; and was subsequently 
 independently discovered by Gambart, at Marseilles, 
 on the 9th of March of the same year. It was a 
 faint, nebulous object, not unlike Encke's comet in 
 general appearance, and was observed only about 
 two months. As soon, however, as the elements of 
 its orbit had been computed, it was found that the 
 comet was probably identical with one which had 
 appeared in 1805, and for which Gauss had obtained 
 an elliptic orbit, corresponding to a period of about 
 six years. A more complete investigation of the 
 orbit showed that it arrived at its perihelion a little 
 before midnight, at Paris, on the 18th of March, 
 1826 ; and that the orbit in which it was then moving 
 was an ellipse, giving a period of revolution of 2460 
 days, or about six years and nine months. It was 
 also found, in tracing its path backwards, that the 
 comet was observed in 1772 and in 1805 ; and that, 
 between the former date and 1826, it had performed 
 eight revolutions. It was therefore predicted that it 
 would return again in 1832 ; and on the 26th of 
 November, of that year, it passed its perihelion in 
 almost exact accordance with the prediction. It was 
 observed during the months of October, November, 
 and December ; and was last seen, at the Cape of 
 Good Hope, on the 3d of January, 1833. At this 
 return it was very faint, and was observed with the 
 greatest difficulty. 
 

 
 
BIE LA'S COMET. 231 
 
 The orbit of this comet is an ellipse whose eccen- 
 tricity is a little less than that of Eucke's comet, or 
 about 0.75. Its perihelion distance is nearly 84,000,- 
 000 miles, and its aphelion distance 581,000,000 
 miles. It therefore comes within the orbit of the 
 earth, and recedes beyond the orbit of Jupiter; and 
 since the plane of its orbit is inclined to the plane 
 of the ecliptic by a small angle, it is evident that 
 the comet may approach very near these planets, 
 and thereby be subject to considerable disturbance 
 in its motions. Moreover, it happens that one of 
 the points in which Biela's comet intersects the 
 plane of the ecliptic, is at a distance from the earth's 
 orbit less than the sum of the semi-diameters of the 
 earth and comet. It follows, therefore, that in case 
 the earth should ever arrive at this point of its orbit 
 at the same moment at which the comet passes 
 through the point of its orbit which is nearest to it, 
 a portion of the head of the latter would envelop 
 the earth. It was, indeed, found, on the occasion 
 of the return of this comet in 1832, that if, by any 
 unforeseen circumstance, the comet should be de- 
 layed one month, it would come in contact with the 
 earth on the 30th of November. This announce- 
 ment created a profound sensation, and no small 
 degree of alarm among those unacquainted with the 
 character and motions of such bodies. But these 
 apprehensions were wholly unfounded the comet 
 
232 TREATISE ON COMETS. 
 
 having passed that point of its orbit on the 29th of 
 October, at a distance from the earth of more than 
 60,000,000 miles. 
 
 At its return in 1839, the comet was most unfa- 
 vorably situated for observation, being in the imme- 
 diate vicinity of the sun ; and consequently is not 
 known to have been seen from any part of the globe. 
 The next return, however, occurred under very 
 favorable circumstances. From the observations 
 made at its previous returns to its perihelion, San- 
 tini determined the elements of its orbit with great 
 care ; and having computed the perturbations of 
 the planets during the period in which it was invisi- 
 ble between 1832 and 1845, he computed an ephem- 
 eris, by means of which it was discovered by De 
 Yico, at Rome, on the 26th of November, 1845. It 
 was seen at Berlin on the 28th of November; at 
 Cambridge, England, on the 1st of December ; and 
 subsequently at all the principal observatories in 
 Europe. It continued to be visible until the end of 
 April, 1846, having passed its perihelion on the llth 
 of February of that year. 
 
 This comet, as already stated, is a small, round, 
 nebulous object, not visible to the naked eye even 
 when brightest, and without a tail. It presents no 
 solid, or even well-defined nucleus ; and appears to 
 be a mere mass of vapor, so extremely attenuated 
 that minute stars can be seen through it, although 
 
B i EL A 's COMET. 233 
 
 its diameter probably exceeds 50,000 miles. The 
 deficiency, however, in brilliancy and magnitude, is 
 fully compensated by the anomalous phenomena 
 which it presents, of two distinct comets, each com- 
 plete in itself, and revolving around the sun in such 
 close proximity as to be almost always visible simul- 
 taneously in the field of the telescope. In 1832, 
 and at its previous returns, it appeared as a single 
 body, with a point of condensed light near its cen- 
 tre; and when first observed in 1845, there was 
 nothing unusual noticed in its appearance. About 
 the middle of December, 1845, it was noticed that 
 the comet gave positive indications that some vio- 
 lent change of its form was about to be effected. 
 On the 19th of December, it is described as being 
 pear-shaped ; the nebulosity being unduly elongated 
 in a direction different from that in which a tail 
 would be seen. On the 29th of December, it was 
 seen attended by a faint, nebulous spot, which gave 
 it the appearance of having a short tail connected 
 with the envelope of the nucleus by a denser mass 
 of nebulous matter. But on the 13th of January it 
 was noticed at Washington, that instead of being, 
 as usual, a single comet, it apparently consisted of 
 two comets, moving through space side by side. 
 Each body is said to have exhibited all the charac- 
 teristics of a telescopic comet, being gradually con- 
 densed towards the centre, without any distinct and 
 20* 
 
284 T u i; A T i s E ON COMETS. 
 
 well-defined disc ; and each elongated on the side 
 opposite the sun. They were, however, of unequal 
 size ; one being at least eight times larger than the 
 other, and about four times brighter. 
 
 The separation of the comet into two distinct 
 parts, was observed at several places in Europe, on 
 the 15th of January, 1846, and, subsequently, at all 
 the principal observatories. The smaller comet was 
 supposed to have been thrown off from the nucleus 
 of the larger one, which was regarded as the original 
 comet. The latter was, as above noticed, at first a 
 globular mass of nebulous matter, semi-transparent 
 at its very centre, and without any tail ; while, after 
 the separation, both comets showed decided indica- 
 tions of short trains, parallel in their direction, and 
 at right angles to the line joining the centres of the 
 nuclei. Again, it was observed that from the day 
 of their separation, the original comet decreased 
 both in magnitude and brilliancy, and the companion 
 increased until near the middle of February, when 
 they were sensibly equal. After this the companion 
 still increased, and from the 14th to the 16th of 
 February, was not only much brighter than the 
 original comet, but had a distinct stellar nucleus. 
 The change of brilliancy was again reversed, the 
 original comet recovering its superiority in this re- 
 spect, and acquiring, on the 18th of February, the 
 same appearance which was presented by the com- 
 
BIELA'S COMET. 235 
 
 panioii from the 14th to the 16th of the same month. 
 After this the companion gradually faded away, and 
 on the 1st of March could only be seen with great 
 difficulty. It was last observed on the 15th of 
 March, and subsequently the comet again appeared 
 to be single. 
 
 While these anomalous changes were taking 
 place, there were indications of a connection between 
 the two nuclei, a faint arc of light being noticed, 
 which extended as a kind of bridge from the one to 
 the other. This line of light, however, did not 
 occupy any fixed position, but was observed to have 
 at times a slight angular motion, sometimes in one 
 direction and sometimes in the opposite direction, 
 and thus the comets were seen to oscillate about 
 each other, according to some mysterious law which 
 remains yet to be fully revealed. 
 
 The opposite diagram represents the appearance 
 of the comet as seen through a telescope about the 
 middle of February, 1846. 
 
 The orbits of both the original comet and its com- 
 panion, have been computed as if they were separate 
 and independent bodies, by Plantamour, of Genoa ; 
 and he finds that the real distance between their 
 centres was, during the greater portion of the period 
 of their visibility, subject only to very slight varia- 
 tions, being about 150,000 miles. The same com- 
 putations were performed by Hubbard, of the !NTa- 
 
236 
 
 TREATISE UN COMETS. 
 
 tional Observatory, at Washington, who finds that 
 the entire series of observations may be satisfactorily 
 represented by supposing each nucleus to describe 
 an independent ellipse around the sun. It is found 
 also that the comets moved on side by side, without 
 manifesting any reciprocal disturbing action ; or 
 that, at least, if any such disturbance did exist, it 
 was too minute to be indicated by the observations 
 a circumstance which shows conclusively that the 
 masses of such bodies are almost infinitely small. 
 The following table gives the distance of one nucleus 
 from the other, as determined by means of Hub- 
 bard's elements of the orbits : 
 
 Distance, in miles. 
 17, . . 226,000 
 
 25, . . 216,000 
 
 5, . . 202,000 
 
 21, . . 170,000 
 
 The next return of Biela's comet was predicted 
 to take place in 1852 ; and on the morning of the 
 26th of August, of that year, in searching for the 
 comet, Professor Secchi, at Rome, detected a small 
 nebulous object, which was very far from the position 
 of the cornet given by the ephemeris. It was soon 
 found that the faint nebulosity observed was really 
 a comet, and still later that it was the one sought 
 for. "No companion, however, was visible ; but on 
 the morning of the 16th of September following, 
 
 Date. 
 
 Distance, in miles. 
 
 Date. 
 
 1845, Dec. 1, 
 
 . . . 117,000 
 
 1846, Feb. 
 
 1846, Jan. 20, 
 
 . . . 198,000 
 
 
 
 " 28, 
 
 . . . 213,000 
 
 " Marc 
 
 " Feb. 9, 
 
 . . . 227,000 
 
 a n 
 
BIELA'S COMET. 
 
 237 
 
 this was also detected. It was very faint, and of an 
 elongated ovoid form, the apex being turned away 
 from the sun. It was distant from the other part 
 which was supposed to he the original comet, in 
 1846 a little more than half a degree. The latter, 
 at the same time, appeared quite irregular, and had 
 two very faint streaks of light issuing from it, but 
 did not exhibit a distinct nucleus, although more 
 luminous in the centre than at the edges. The 
 original comet was last seen on the 25th of Septem- 
 ber, and the companion on the 28th of the same 
 month. The distances of the two comets from each 
 other were as follows : 
 
 Date. Distance, in miles. 
 
 1852, Sept. 16, . . 1,616,000 
 
 " 20, . . 1,624,000 
 
 " 24, . . 1,625,000 
 
 " 28, . . 1,619,000 
 
 It is evident from this table of the distances of 
 the nuclei from each other, that in 1852 the two 
 comets moved nearly as in 1846, with the exception 
 only that, at this return, the distance of the two 
 bodies was nearly seven times greater than in 1846. 
 We might expect, therefore, that it would be easy 
 to determine the orbit of each nucleus with great 
 precision, from the observations made in 1846 and 
 in 1852. But it is found, unfortunately, that it is 
 
 Date. 
 
 Distance, in miles. 
 
 1852, Aug. 27, 
 
 . . 1,501,000 
 
 " 31, 
 
 . . 1,531,000 
 
 " Sept. 4, 
 
 . . 1,559,000 
 
 8, 
 
 . . 1,582,000 
 
 " 12, 
 
 . . 1,601,000 
 
238 TREATISE ON COMETS. 
 
 nearly impossible to decide which of the two comets 
 observed in 1852 is the original comet of 1846, since 
 all the observations are represented almost equally 
 well, by either the hypothesis that the one disco- 
 vered by Secchi, on the 25th of August, was the 
 original comet of 1846, or that the one found on the 
 16th of September is the identical one. The first 
 supposition has been most generally adopted, and 
 the time of the next return of the comet, in 1859, 
 was computed in accordance with this assumption. 
 It was predicted that the original comet would 
 arrive at its perihelion on the 24th of May, 1859, a 
 little before midnight at Greenwich, and that the 
 companion would arrive at the point of its orbit 
 nearest the sun about thirty-five hours earlier. 
 They were, however, situated in a position very 
 unfavorable for observation, being in the immediate 
 vicinity of the sun ; and although there can be no 
 doubt but that they returned very nearly in accord- 
 ance with 'the prediction, yet for the reasons here 
 stated, no observation is known to have been made. 
 This is much to be regretted, since a few observa- 
 tions in 1859 would have definitely decided the 
 question of identity of the two comets at the pre- 
 vious returns, and would have greatly facilitated all 
 future investigations respecting their motions. The 
 comets will again return in 1866, and will then be 
 in a position favorable for observation; and we 
 
BIELA'S COMET. 239 
 
 must, therefore, await this appearance in order to 
 settle the important question of identity. 
 
 Such are the most prominent characteristics of 
 the phenomena presented by Biela's comet, at its 
 successive returns to its perihelion ; and on account 
 of the anomalous appearance of a double comet 
 here exhibited, it may not be improper to introduce, 
 in this connection, a statement of the various hypo- 
 theses which have been devised, in order to explain 
 such an unexampled departure from the general 
 form of such bodies. In the first place, then, it is 
 found, by comparing the distances of the comets 
 from each other in 1846 and in 1852, that the sepa- 
 ration must have taken place about 500 days before 
 the perihelion passage in 1846, or about the middle 
 of July, 1843. This date, however, is still quite 
 uncertain; and it will not be possible, until this has 
 been definitely determined, to decide under what 
 circumstances the separation of the nucleus oc- 
 curred, and by what means it was effected in such 
 a way as not to change or influence the motions of 
 the primary nucleus. Again, the extraordinary 
 changes in brilliancy which the two comets exhi- 
 bited, both in 1846 and 1852, clearly indicate that 
 the brightness of these objects does not depend 
 merely on their distance from the earth and sun, 
 but upon other unknown causes. These causes 
 may, indeed, have been sufficient to develop the 
 
240 TREATISE ON COMETS. 
 
 brilliancy of the companion of Biela's comet, at its 
 last two returns to the sun, to such an extent as to 
 render it visible from the earth ; while, at its former 
 returns, on account of its unfavorable position, and 
 the inefficient operation of these causes, it might 
 have been too faint to be noticed. The great change, 
 however, in the distances of the nuclei from each 
 other from 1846 to 1852, seems to be sufficient in 
 itself to show that the separation did not take place 
 until after the return of the comet in 1832. 
 
 The most plausible view of the cause of this per- 
 manent separation of a comet into two distinct parts, 
 is that which assumes a repulsive force emanating 
 from the sun, the nature and effect of which we 
 shall fully consider when we come to treat of the 
 theory of the physical constitution of comets, and 
 of the formation of their tails ; it being only neces- 
 sary to state here, that the phenomena presented by 
 Halley's comet in 1835, and by the great comet of 
 1858, have demonstrated the existence of such a 
 force. The effect of a repulsive force upon the 
 nebulous envelope of the comet, would be to distort 
 it from the spherical form, which it would assume 
 if the force of gravity alone acted, causing it to 
 become an ellipse, the length of which, compared 
 with its breadth, would be greater as the repulsive 
 force increases. The repulsive force may now be 
 conceived to become so great as to drive the remoter 
 
F A y E ' s COMET. 
 
 particles of the envelope beyond the influence of 
 the nucleus, and carry them off into space. The 
 entire mass of the comet will therefore be urged 
 toward the sun by the difference of the total attract- 
 ive and repulsive forces. So long as the repulsive 
 is insufficient to separate the matter composing the 
 comet, it will revolve as a single body, elongated in 
 a direction opposite the sun; but if the repulsive 
 force should be so great as to overcome the gravi- 
 tation of the nucleus, the exterior portions of the 
 envelope will be successively driven off into space, 
 until finally a new comet is formed, which will 
 henceforth revolve as a companion to the original 
 one, as in the case of Biela's comet. 
 
 The successive returns of this comet have also 
 indicated the existence of a resisting medium, the 
 period being always less than at the preceding peri- 
 helion passage. In 1852, it came to the perihelion 
 nearly two days in advance of the predicted time, 
 after due allowance had been made for the action 
 of the planets on the comet during the entire inter- 
 val elapsed since 1846. 
 
 On the 22d of November, 1843, a telescopic comet 
 was discovered by M. Faye, of the Royal Observa- 
 tory, at Paris, which was subsequently found to 
 move in an elliptic orbit, with a period of revolution 
 of about seven and a half years. When first disco- 
 vered, the comet had a brilliant nude/us, and a short 
 21 Q 
 
242f TREATISE ON COMETS. 
 
 fan-shaped tail, about one-tenth of a degree in 
 length. It was last seen at Pulkova, in Russia, on 
 the 10th of April, 1844. It passed its perihelion on 
 the 17th of October, 1844, at a distance from the 
 sun of 161,000,000 miles. Its aphelion distance is 
 563,000,000 miles. The comet, therefore, does not 
 come within the orbit of the planet Mars, and re- 
 cedes, in its aphelion, beyond the orbit of Jupiter. 
 But the most remarkable feature connected with the 
 orbit of this cornet, is the eccentricity, which is only 
 a little more than that of the orbits of some of the 
 asteroid planets, thus forming a connecting link 
 between the cometary and planetary systems. 
 
 As soon as the entire series of observations of the 
 comet, made in 1843 and 1844, had been published, 
 a complete investigation of the elements of its orbit 
 was accomplished; and the result was, that its period 
 of revolution was found to be 2718 days, or a little 
 less than seven and a half years. It was also no- 
 ticed that the position of its orbit is such that the 
 comet is liable to excessive perturbations, on account 
 of its close proximity, when near the aphelion, to 
 the orbit of Jupiter. In 1840 it was so near this 
 planet, that his attraction was about one-tenth part 
 of that of the sun, which must necessarily have 
 caused a great alteration of the form of its orbit. 
 In 1815, also, it probably came still nearer to Jupiter, 
 and the perturbations must have been much greater 
 
F AYE' s COME T. 243 
 
 than in 1840. Under such circumstances, M. Yalz 
 was led to conclude that, prior to 1840, the comet 
 moved in an orbit entirely different from that in 
 which it is now moving; and that, possibly, this 
 comet might be identical with a comet which ap- 
 peared in 1770, and which, although at that time 
 moving in an ellipse of only five years, has, by the 
 action of Jupiter, been so excessively disturbed in 
 its motions that it has not since been seen. 
 
 A complete investigation of this problem was 
 subsequently undertaken by Le Yerrier, who finds, 
 after computing the perturbations of Faye's comet, 
 by Jupiter, that it cannot be identical with the 
 comet of 1770. He also computed the perturbations 
 of the comet arising from the attraction of the 
 planets during the interval from 1843 to 1851, and 
 predicted that it would return to its perihelion on 
 the 3d of April, 1851. With the aid of Le Yerrier s 
 elements, an ephemeris was computed for the latter 
 part of 1850, and beginning of 1851, by means of 
 which observers might be enabled more easily to 
 detect the comet at its return ; and more especially 
 because it was foreseen that it would be extremely 
 faint and small, and not capable of being seen ex- 
 cept through the most powerful telescopes. By 
 means of this ephemeris the comet was re-discovered 
 at the observatory at Cambridge, England, on the 
 25th of December, 1850, and was followed until the 
 
244 TKEATISE ON COMETS. 
 
 4th of March, 1851. It was subsequently observed 
 at Cambridge, Mass., and at Pulkova, in Russia; and 
 the positions actually observed scarcely differed from 
 those predicted by means of Le Verrier's elements. 
 The comet was described as an extremely faint object: 
 so much so as to be barely visible through the large 
 telescopes of those observatories. It appeared to be 
 slightly elongated in the direction of the sun, but no 
 well-defined nucleus or point of condensation could 
 be perceived. It came to its perihelion on the morning 
 of the 2d of April, 1851, or within one day of the 
 predicted time. This difference is undoubtedly due, 
 to a certain extent, to the influence of the resisting 
 medium. 
 
 The next return of this comet took place in 1858. 
 It was observed during the months of September 
 and October, and passed its perihelion on the 12th 
 of the former month. After making due allowance 
 for the action of the planets from 1851 to 1858, it is 
 found that Le Verrier's elements still represent the 
 motion of the comet with great precision. At its 
 last return, in 1858, the comet was very faint and 
 ill-defined, and without either a nucleus or a tail, 
 being, apparently, a diffused mass of nebulous 
 matter. The next return to the perihelion will occur 
 in March, 1866. 
 
 A fourth comet of short period was discovered by 
 De Vico, at Rome, on the 22d of August, 1844, 
 
IS EAT (D(MIET (DF 
 
DE Vice's COMET. 245 
 
 which, in the moonlight, was almost visible to the 
 naked eye. It was also discovered independently 
 by Encke, at Berlin, on the 5th of September, and 
 on the following evening it was seen at Hamburgh, 
 by M. Melhop, an amateur astronomer. In America 
 it was first seen by Mr. H. L. Smith, at Cleveland, 
 Ohio, on the 10th of September. Soon after the 
 middle of this month it became distinctly visible to 
 the naked eye, and when seen through a telescope, 
 the nucleus presented a very beautiful appearance, 
 being bright and strongly condensed. The tail was 
 about one degree in length, extending in a direction 
 opposite to the sun. It subsequently decreased 
 rapidly in brilliancy, and was observed for the last 
 time at Pulkova, on the 31st of December. The 
 orbit was computed, as usual, on the supposition that 
 it was a parabola since this is more easily computed 
 than an elliptic orbit but it was soon found that it 
 was impossible to represent the observations by such 
 an orbit, and accordingly an ellipse was computed, 
 which gave a periodic time of about five and a half 
 years. It passed its perihelion on the 2d of Sep- 
 tember, 1844, at a distance from the sun of about 
 114,000,000 miles. When in the aphelion of its 
 orbit, its distance from the sun is 475,000,000 miles. 
 The inclination of the plane of its orbit to the plane 
 of the ecliptic, is less than three degrees ; and, con- 
 sequently, it may come so near the planet Jupiter 
 21* 
 
246 TREATISE ON COMETS. 
 
 that its orbit will eventually be entirely changed by 
 the perturbations caused by this planet. It also 
 comes very near the earth, and is considerably dis- 
 turbed by its action. 
 
 The interest which the appearance of the comet, 
 in connection with its short period, it being less than 
 that of any other which had hitherto been dis- 
 covered, with the exception of Encke's come.t, in- 
 duced the Academy of Sciences, at Amsterdam, to 
 make the investigation of its orbit the subject of a 
 prize. Accordingly, a thorough determination of 
 the elements of its orbit from the entire series of 
 observations, embracing a period of more than four 
 months, was undertaken by Briinnow, at that time 
 assistant in the Royal Observatory at Berlin. He 
 computed the perturbations of the comet by all the 
 planets within the orbit of Uranus, excepting, of 
 course, the asteroid planets between Mars and Jupi- 
 ter, and finally obtained an orbit which satisfied all 
 the observations with extreme precision. The period 
 of revolution was thus found to be 1996J days, or 
 very nearly 5| years. The comet would, therefore, 
 return again to its perihelion about the middle of 
 February, 1850 ; but it happened, very unfortunately, 
 that it was so near the sun during the entire period 
 in which it remained sufficiently near the earth to 
 be visible, if seen under more favorable circum- 
 
D K V i c o ' s COMET. 247 
 
 stances, with respect to its apparent position in the 
 heavens, that it could not he observed. 
 
 The next return of the comet to its perihelion oc- 
 curred in the summer of 1855, and since it would 
 then he in a position favorable for observation, its 
 reappearance was contemplated with no small degree 
 of interest. The perturbations were computed from 
 1844 to 1855, and Dr. Briinnow announced that it 
 would be in its perihelion on the evening of the 6th 
 of August, 1855. As soon as the comet was com- 
 puted to be near enough to be visible from the earth, 
 it was sought for in the position indicated by the 
 ephemeris, but without success. The search was 
 continued until the comet was probably out of reach 
 of the most powerful instruments, and it is not cer- 
 tainly known that it was seen at any part of the 
 globe. On the 16th of May, 1855, Goldschmidt, of 
 Paris, while searching for the comet, found a faint 
 nebulous body, with an ill-defined and irregular out- 
 line, and without a tail, in a position which did not 
 differ much from the computed place of De Yico's 
 comet, if it be assumed that the perihelion passage 
 took place on the llth of August. The comet, how- 
 ever, must have been much nearer the earth on the 
 1st of August than at the date of this observation, 
 while its position in the heavens must have been 
 much more favorable for observation. It seems 
 certain, therefore, that, since the search continued 
 
248 TREATISE ON COMETS. 
 
 until after the beginning of August, this nebulous 
 object could not have been the comet; and we are 
 compelled to conclude that it was nowhere seen in 
 1855 a failure to realize the predictions, which may 
 be accounted for, not by supposing the methods of 
 computation to have been inaccurate, but rather by 
 the uncertainty of the ephemeris, owing to the great 
 difficulty of determining the time of perihelion 
 passage in 1855 from the observations made in 1844. 
 The comet may be expected to return again toward 
 the latter part of 1860, or in the beginning of 1861 ; 
 but it will not probably be in a position favorable 
 for observation. This is to be regretted, since the 
 comet, before it has accomplished the succeeding 
 revolution, will be subject to great perturbations by 
 Jupiter, which may so completely change its orbit, 
 that it will be difficult to identify it at any future 
 return. 
 
 Another important circumstance to be noticed in 
 
 connection with this comet, is that its orbit is very 
 
 % 
 
 similar to that of a comet which appeared in 1585. 
 Some astronomers have, for this reason, supposed 
 that De Yico's comet is identical with the comet of 
 1585 ; and possibly, also, with those of 1743, 1766, 
 and 1819. Le Yerrier and Briinnow have under- 
 taken to decide this question by direct computation ; 
 and they conclude, finally, that this comet cannot 
 be identical with either of those just mentioned, nor 
 
BRORSEN'S COMET. 249 
 
 with any other on record, unless it be with one 
 which appeared in 1678. They find that there is a 
 strong probability of identity in this case, although 
 the perturbations in the meantime have very mate- 
 rially changed the elements of its orbit. 
 
 Another periodic comet was discovered by Bror- 
 sen, at Kiel, in Denmark, on the 26th of February, 
 1846. It was a faint, telescopic object, and was 
 found to move in an elliptic orbit, with a periodic 
 time of about five and a half years ; and has been 
 supposed to be identical with the comets of 1532 
 and 1661. It was observed until the 22d of April ; 
 and from the entire series of observations, its exact 
 period was ascertained to be 2039 days. Its peri- 
 helion distance is about 62,000,000 miles. When in 
 the aphelion, its distance from the sun is 532,000,000 
 miles. The next return to the perihelion took place 
 in September or October, 1851 ; but it was in a posi- 
 tion very unfavorable for observation, and conse- 
 quently was not seen. Moreover, the small number 
 of observations which were made in 1846, rendered 
 it extremely difficult to determine the precise period 
 of its return, otherwise it would probably have been 
 observed in 1851. The next and last return took 
 place in 1857 ; but the comet was accidentally dis- 
 covered by Bruhns, at Berlin, on the 18th of March, 
 and its identity established only after parabolic ele- 
 ments had been computed which were very similar 
 
250 TREATISE ON COMETS. 
 
 to those of Brorsen's comet. The supposed identity 
 which resulted from these approximate elements, 
 was subsequently fully confirmed ; and the comet is 
 now ranked among those of short period whose 
 future movements can be foretold with the utmost 
 precision. It passed its perihelion on the 29th of 
 March, 1857 ; and may be expected to return again 
 in October, 1862. 
 
 A sixth comet of short period was discovered by 
 D'Arrest, at Leipsic, on the 27th of June, 1851. It 
 was a faint, telescopic object, without a tail, but 
 having a distinct and well-defined outline, with a 
 point of greater condensation near its centre. It 
 passed its perihelion on the 8th of July, 1851 ; and 
 was found to move in an elliptic orbit corresponding 
 to a period of revolution of 2353 days, or nearly six 
 and a half years. The next. return was therefore 
 predicted to take place about the end of 1857, or 
 beginning of 1858 ; and the comet was observed at 
 the Cape of Good Hope in December, 1857, and in 
 January, 1858. The next return will occur in June, 
 1864. 
 
 The seventh comet of short period, whose orbit is 
 definitely know r n, was discovered in the constella- 
 tion Andromeda, by Mr. Horace P. Tuttle, at the 
 observatory of Harvard College, Cambridge, Mass., 
 on the evening of the 4th of January, 1858 ; and was 
 subsequently independently discovered by Bruhns, 
 
TUTTLE'S COMET. 251 
 
 at Berlin, on the llth of the same month. At 
 the time of its discovery it appeared like a faint and 
 diffused nebula, without any indications either of a 
 tail, or of an elongation in a direction opposite to 
 the sun. At the beginning of February it had in- 
 creased very considerably in brilliancy, and pre- 
 sented the appearance of a well-defined elliptical 
 nebula, about three minutes of an arc in diameter, 
 with a distinct increase of light from the borders 
 toward the centre. It continued visible until near 
 the end of March the last observation reported 
 from any part of the world being that made at 
 Cambridge, Mass., on the 23d of this month. When 
 last observed, although then very far south, and 
 apparently enveloped in the vapors near the horizon, 
 it nevertheless appeared distinct, and nearly as bril- 
 liant as when it was first discovered; so that, in 
 case it had been detected at some observatory in the 
 southern hemisphere, it might have been followed 
 at least one month longer. The observations, how- 
 ever, which were made in Europe and in the United 
 States during the period of its visibility in the north- 
 ern hemisphere, have enabled astronomers to deter- 
 mine the elements of its orbit with considerable 
 precision. 
 
 The first approximate elements of the orbit of this 
 comet were found to bear a striking resemblance to 
 those of the second comet of 1790, which was dis- 
 
252 TREATISE ON COMETS. 
 
 covered by Mechain, at Paris, on the 9th of Janu- 
 ary of that year. Indeed, so close was this resem- 
 blance, that the identity of these two comets was 
 placed beyond a doubt; but it still remained to 
 determine whether the comet had not previously 
 returned to its perihelion unperceived, during the 
 interval of sixty-eight years which had elapsed be- 
 tween 1790 and 1858. It was found by direct cal- 
 culation of elliptic elements, that the period of 
 revolution is about thirteen and a half years ; and, 
 consequently, that the comet had returned to the 
 perihelion no less than five times between 1790 and 
 1858. It might be expected, however, that since 
 the orbit of the comet is so situated that the per- 
 turbations can never be very considerable, the comet 
 would have been observed at one or more of its 
 intermediate returns. In the latter part of 1830, 
 and in the beginning of 1831, the comet was favor- 
 ably situated for observation, both in respect to its 
 apparent angular distance from the sun, and its bril- 
 liancy. It might at this time have been easily ob- 
 served in the east before sunrise ; and it must be 
 supposed, therefore, that no search was made for 
 comets in that portion of the heavens in December, 
 1830, and in January, 1831. At the other three 
 returns of the comet to its perihelion, it was in such 
 close proximity to the sun, and on account of its 
 greater distance from the earth so diminished in 
 
TUTTLE'S COMET. 253 
 
 brilliancy, as not to come within the range of the 
 optical power of such instruments as were usually 
 employed in searching for cometary hodies. It 
 might likewise be supposed that the comet would 
 have been observed at some return to its perihelion 
 previous to 1790 ; but it should be remembered that, 
 until near the commencement of the present cen- 
 tury, no systematic search for faint, telescopic comets, 
 had been instituted; and that its discovery at a date 
 prior to 1790, could only have been the result of a 
 very fortunate accident. 
 
 The opposite diagram represents the appearance 
 of this comet on the 31st of January, 1858, and may 
 be regarded as a general delineation of the physical 
 appearance of all those comets which have no tails, 
 or distinct traces of any considerable elongation in 
 the direction of the sun. 
 
 The orbit of Turtle's comet is an ellipse, corre- 
 sponding to a periodic time of 5005 days, or a little 
 more than thirteen years and eight months. When 
 in the perihelion, its distance from the sun is 
 100,000,000 miles. Its aphelion distance is 987,- 
 000,000 miles. It passed its perihelion on the 28th 
 of February, 1858, and will return again to the 
 same point about the end of November, 1871. 
 
 Another comet of short period is that of Win- 
 necke, which was first observed by Pons, at Mar- 
 seilles, on the 12th of June, 1819. It was observed 
 22 
 
254 TREATISE ox COMETS. 
 
 during a period of only forty days, and from the 
 observations thus obtained, Encke found that its 
 orbit was an ellipse, with a period of only five and 
 a half years. It was so uncertain, however, that no 
 attempt was made to predict its future returns to its 
 perihelion ; and, consequently, it was not again seen 
 until the 8th of March, 1858, when it was disco- 
 vered by Winnecke, at Bonn, and identified by a 
 comparison of its elements with those of the comet 
 of 1819. It was observed until the beginning of 
 June, 1858, and from the series of observations it 
 was found to have a periodic time of 1830 days, or 
 almost exactly five years. It has, therefore, next to 
 Encke's comet, the shortest period which has yet 
 been discovered as belonging to such bodies. Its 
 distance from the sun, when in its perihelion, is 
 about 73,000,000 miles. Its aphelion distance is 
 about 480,000,000 miles. It must, therefore, ap- 
 proach very near the orbit of Jupiter, and in course 
 of time will be subject to excessive perturbations. 
 
 This comet is a telescopic object, not unlike 
 Tuttle's comet in its general appearance. It is pro- 
 bably much larger and brighter, but in form bears 
 a close resemblance to it. It passed its perihelion 
 on the 2d of May, 1858, and may be expected to 
 return again in April, 1863. 
 
 We have now described those periodic comets 
 whose orbits are supposed to be known with great 
 
LEXELL'S COMET. 255 
 
 precision ; but there are others, whose periodic 
 character is fully as decidedly indicated, but which 
 have not been observed to a sufficient extent for a 
 complete investigation of their elements. A revi- 
 sion of the recorded observations of former comets 
 by modern astronomers, with the aid of the refined 
 operations of modern mathematical analysis, has led 
 to the discovery of the great probability of several 
 among them having revolved in elliptic orbits, with 
 periods not differing to any considerable extent 
 from those of the periodic comets already mentioned. 
 The fact that these comets have not been re-observed 
 at their successive returns to their perihelia, may be 
 explained, either by the difficulty of observing them, 
 owing to their unfavorable positions, and the cir- 
 cumstance of observers not expecting their reap- 
 pearance, their periodic character not being then 
 suspected ; or because their orbits may have been so 
 completely changed, by the disturbing action of the 
 planets, as to keep them continually beyond the 
 limits of visibility from the earth. 
 
 A singular example of this kind is found in the 
 case of the comet which appeared in 1770, and 
 which is known as LexeWs comet, from the discoverer 
 of its periodic character. It is, perhaps, the only 
 instance in which a particular comet is certainly 
 known to have appeared in the system, making two 
 revolutions around the sun in an elliptic orbit of 
 
256 TREATISE ON COMETS. 
 
 short period, and then disappearing, without fur- 
 nishing any means of identifying it at any future or 
 previous return. This comet was discovered by 
 Messier, at Paris, on the 14th of June, 1770, and 
 observed till the 2d of October following. It passed 
 its perihelion on the 14th of August, and on the 1st 
 of July was distant from the earth only 363 terres- 
 trial radii, or about six times the distance of the 
 moon. 
 
 On computing the elements of the orbit of this 
 comet, it was found to be impossible to represent 
 the observations by a parabola. Six years after its 
 disappearance, Lexell showed that the observations 
 could be satisfactorily represented by an ellipse, 
 corresponding to a period of about five and a half 
 years. This was certainly a remarkable result, 
 since there was no other example of a comet of less 
 period than about seventy-five years ; and, more- 
 over, the fact that the comet had not been seen at 
 previous returns, since it was a brilliant object in 
 1770, seemed to militate strongly against Lexell's 
 conclusions in regard to its periodic character. The 
 aphelion distance of the comet was found to be 
 about 520,000,000 miles, and its perihelion distance, 
 65,000,000 miles. Its orbit is inclined to the orbit 
 of the earth by an angle of only a degree and a 
 half; and it follows, therefore, that it almost lite- 
 rally intersects the orbits of the earth, Mars, and 
 
LEXELL'S COMET. 257 
 
 Jupiter. This fact enabled Lexell to remove all 
 doubts in regard to the accuracy of his results, since 
 it could be easily shown that at its previous aphe- 
 lion, in 1767, the comet must have passed Jupiter 
 at a distance fifty-eight times less than the planet's 
 distance from the sun ; and that, consequently, it 
 must then have sustained an attraction from the 
 great mass of the planet, more than three times 
 more energetic than that of the sun, at the same 
 time. It follows, also, that in this manner its former 
 orbit was changed into an ellipse of only five and a 
 half years, in which it actually moved in 1770. It 
 is highly probable that, prior to 1767, its orbit was 
 a parabola, and that, although moving in an elliptic 
 orbit from 1767 to 1770, and having the periodicity 
 consequent on such motion, it nevertheless moved 
 then for the first time in its new orbit, and had 
 never come within the sphere of the sun's sensible 
 attraction anterior to this epoch. 
 
 The return of the comet in 1776 could not be 
 observed, on account of the relative situations of 
 the earth and comet at that time the latter being 
 wholly enveloped in the sun's rays or in the strong 
 twilight; and before another revolution could be 
 accomplished, its orbit was again completely 
 changed. Lexell had further determined that, 
 since the comet passed through its aphelion, which, 
 as already stated, nearly intersected the orbit of 
 22* R 
 
258 TREATISE ON COMETS. 
 
 Jupiter, at intervals of a little more than five and a 
 half years, it having been in that position in 1767, 
 and the period of the planet being less than twelve 
 years, the planet, after a single revolution, and the 
 comet after two revolutions, must necessarily again 
 encounter each other in 1779. He found that the 
 comet would approach Jupiter, about the 23d of 
 August, 1779, within a distance ninety-one times 
 less than the distance of the latter from the 
 sun. It would, therefore, be attracted towards the 
 planet by a force which was at least 200 times 
 greater than the sun's attraction at the same point 
 of its orbit ; and, consequently, its orbit would, in 
 all probability, be again changed into a parabola or 
 an hyperbola. It would thus depart for ever, for 
 aught we know, from our system, to visit other 
 suns and systems. Lexell, therefore, announced 
 that in 1770 it was actually describing an elliptic 
 orbit with a period of five and a half years; but 
 that, owing to the excessive perturbations produced 
 by Jupiter in 1779, it might be expected to have 
 already made its final disappearance. 
 
 At the time when these computations were made 
 by Lexell, the exact methods of Lagrange and La- 
 place, for the computation of perturbations, had not 
 yet been developed. Subsequently, the question 
 here propounded was resumed by the latter geom- 
 
LEXELL'S COMET. 259 
 
 eter, and a general solution was obtained for the 
 following definite problem: 
 
 The actual orbit of a comet being given, what was its 
 orbit before, and what will be its orbit after being sub- 
 mitted to any given disturbing action of a planet near 
 which it passes f 
 
 Having obtained a general solution of this prob- 
 lem, he applied his formula to the particular case 
 of Lexell's comet, and showed that, before sustain- 
 ing the disturbing action of Jupiter in 1767, the 
 comet must have moved in an ellipse whose greater 
 axis was about 2,526,000,000 miles; and, conse- 
 quently, that its period, instead of being five and a 
 half years, must have been forty-eight and a half 
 years. He showed also that the eccentricity of the 
 orbit of the comet was such that its perihelion dis- 
 tance would be nearly equal to the mean distance 
 of Jupiter, so that it could never have been visible. 
 We perceive, therefore, that previous to 1767, the 
 comet experienced excessive perturbations only when 
 near its perihelion. It was further demonstrated by 
 Laplace, that after being subjected to the disturbing 
 action of Jupiter in 1779, the orbit of five and a half 
 years was changed into an ellipse whose greater axis 
 was about 1,400,000,000 miles, thus indicating a pe- 
 riod of twenty years. The eccentricity of the orbit 
 of the comet was found to be such that its perihelion 
 distance was more than double that of the planet 
 
260 TREATISE ON COMETS. 
 
 Mars, so that even in such an orbit the comet could 
 not become visible. 
 
 The question rested thus for nearly half a century, 
 when the investigations of Laplace were thoroughly 
 revised by Le Verrier. He found that the observa- 
 tions of 1770 were not sufficiently accurate and 
 numerous to warrant such absolute conclusions ; and 
 showed that the orbit of 1770 is subject to an un- 
 certainty comprised between certain definite limits ; 
 and also that, tracing the consequences of this to the 
 positions of the comet in 1767 and 1779, these places 
 are subject to still wider limits of uncertainty. 
 Thus he finds that the observations of 1770 can be 
 almost equally well represented, no matter whether 
 the comet is supposed, in 1779, to have passed con- 
 siderably outside, or considerably inside, of Jupiter's 
 orbit; or whether, as it was supposed to have done, 
 it actually passes within and among the orbits of the 
 satellites. Finally, he deduces the following gene- 
 ral conclusions : 
 
 1. That if the comet had passed within the orbits 
 of the satellites of Jupiter, it must have fallen down 
 upon the planet and coalesced with it an incident 
 which must be considered highly improbable, though 
 not absolutely impossible. 
 
 2. The action of Jupiter may have thrown the 
 comet into a parabolic or hyperbolic orbit, in which 
 case it must have taken its final departure from our 
 
L K x E L L ' s COMET. 261 
 
 system, never again to return ; except it may be by 
 the consequence of some disturbance of its motion 
 produced in another sphere of attraction. 
 
 3. It may have been thrown into an elliptic orbit, 
 having a great axis and long period, and so placed 
 and formed that the comet would never become 
 visible a supposition which agrees with the solution 
 of Laplace. 
 
 4. It may have had merely its elliptic elements 
 more or less modified by the action of the planet, 
 without losing its character of short periodicity a 
 result which is perhaps the most probable, and which 
 would render it possible that this comet may still 
 be identified with some one of the many comets of 
 short period, which the activity and sagacity of ob- 
 servers are continually bringing to notice. 
 
 In order to facilitate all such comparisons, Le 
 Verrier has given a table, which includes all the pos- 
 sible systems of elliptic elements of short period 
 which the comet could have assumed, subject to the 
 disturbing action of Jupiter in 1779, and taking the 
 observations of 1770 within the limits of their pro- 
 bable errors, but which it will be unnecessary to add 
 in this connection. He then proceeds to demonstrate 
 that the orbit in which the comet moved prior to the 
 disturbing action of Jupiter upon it in 1767, not 
 only could not have been an ellipse or hyperbola, 
 but must have been an ellipse whose periodic 
 
262 TREATISE ON COMETS. 
 
 time was considerably less than had been deduced 
 by Laplace, from the insufficient observations of 
 Messier. His calculations show, that before that 
 epoch, the perihelion distance of the comet could 
 not, under any possible supposition, have exceeded 
 three times the earth's mean distance from the sun, 
 and was, most probably, included between one and 
 a half and two times that distance ; and that the ap- 
 helion distance could not have exceeded six times 
 the same distance, a magnitude three times less than 
 that assigned to it by the calculations of Laplace. 
 
 Having thus thoroughly investigated the path of 
 Lexell's comet, both before and after its successive 
 changes, from the action of Jupiter, Le Yerrier went 
 still further, and endeavored to find whether any of 
 the periodic comets now known can be supposed to 
 be identical with this comet. In accomplishing this 
 it became necessary to trace the paths of each comet 
 back through all its previous revolutions since 1779, 
 making due allowance for the disturbances to which 
 it was subjected by the action of the planets in the 
 interval which had elapsed, and then to compare 
 the elements thus obtained for 1779, with the table 
 of possible orbits of Lexell's comet which had 
 already been determined. Should one of these 
 orbits be found to be identical with that of the comet 
 in question, its identity with Lexell's comet would 
 be inferred with the highest degree of probability; 
 
COMETS OF SHORT PERIOD. 263 
 
 but if, on the other hand, such discrepancies were 
 found to prevail as must exceed all possible errors 
 of observation or calculation, the identity could not 
 be assumed. It was found in this way that none 
 of the periodic comets hitherto observed, and more 
 especially those of short period, can be identical 
 with Lexell's comet, however strongly some of the 
 elements of their present orbits might, at first, seem 
 to indicate such a presumption. 
 
 Another comet of short period was discovered by 
 Blainplan, at Marseilles, on the 28th of November, 
 
 1819, and was observed until the 25th of January, 
 
 1820. From the observations which were made 
 during this period of two months, Encke found that 
 its orbit was an ellipse, corresponding to a period 
 of revolution of about five years. Clausen considers 
 this comet to be identical with one which appeared 
 in January, 1743. It passed its perihelion on the 
 20th of November, 1819, but has not been observed 
 at any succeeding return.* 
 
 A comet which was discovered by Pigott, at York, 
 in 1783, was shown by Burckhardt to move in an 
 elliptic orbit, with a periodic time of five and a half 
 years. It passed its perihelion on the 20th of No- 
 vember of that year, but has not been seen since 
 the 21st of December following. 
 
 On the 26th of June, a telescopic comet was dis- 
 covered by Peters, at Naples, and observed till the 
 
264 TREATISE ON COMETS. 
 
 21st of July. Its orbit has been calculated by 
 D'Arrest, and found to be an ellipse with a period 
 of 5804 days, or nearly sixteen years. Later com- 
 putations by Peters have assigned to it a periodic 
 time of a little less than thirteen years. It passed 
 its perihelion on the 1st of June, 1846, and was pre- 
 dicted to return again in the first part of 1859. It 
 was, however, in a position very unfavorable for ob- 
 servation in the northern hemisphere, and no obser- 
 vation of it since 1846 has yet been announced. 
 
 These are the only additional comets whose period 
 is less than sixteen years, which have hitherto been 
 detected ; but there are several with periods of seventy 
 years and upward, whose orbits are supposed to be 
 known with considerable accuracy, although the 
 comets have only been observed at one return to the 
 perihelion. Among these might be noticed a comet 
 discovered by Pons, at Marseilles, on the 20th of 
 July, 1812, for which Encke found a periodic time 
 of about seventy-five and a half years ; also, one 
 discovered by Olbers, at Bremen, on the 6th of March, 
 1815, whose orbit, calculated by Bessel, proved to 
 be an ellipse with a period of seventy four years. 
 The next perihelion passage of this comet is pre- 
 dicted to take place on the 9th of February, 1887, 
 its time of revolution between 1815 and 1887 being 
 diminished about two years by the action of the 
 planets. A comet, discovered byDe Vico, at Rome, 
 
COMETS OF LONG PERIOD. 265 
 
 on the 28th of February, 1846, is found to have a 
 period of between seventy-two and seventy-three 
 years ; and one discovered at Altona, in Denmark, 
 by Brorsen, on the 20th of July, 1847, has been sup- 
 posed to move in an elliptic orbit with a period of 
 seventy-five years. Still another comet, belonging 
 to the same class, was discovered by "Westphal, at 
 Gottingen, on the 27th of June, 1852, and subse- 
 quently by Peters, at Constantinople. Its orbit is 
 found to be an ellipse, corresponding to a period of 
 about seventy years. 
 
 On the 25th of July, 1857, a faint telescopic comet 
 was discocered by Peters, at Albany, N. Y., which 
 was observed until the 20th of October, following. 
 Soon after its discovery, it was found that its orbit was 
 an ellipse, with a period of about 250 years. Its aphe- 
 lion distance is about 7,700,000,000 miles, or nearly 
 5,000,000,000 miles beyond the orbit of Neptune. 
 
 These are the only comets of long period whose 
 orbits are known to be contained within the present 
 limits of the planetary system. There are, however, 
 many on record, whose periodicity, although not yet 
 in any instance established by observations made at 
 successive returns to their perihelia, is, nevertheless, 
 so unequivocally indicated, that it is possible to ob- 
 tain a very approximate value of the dimensions of 
 their orbits, from the observations made during the 
 time of their visibility at a single perihelion passage. 
 23 
 
266 TREATISE ox COMETS. 
 
 We have already given several examples of this class 
 of cometary bodies in the preceding chapter. 
 
 Of all the comets whose orbits have been com- 
 puted, only twenty-seven have been definitely ascer- 
 tained to move in elliptic orbits, and thirteen have 
 been found to move in hyperbolas. The others 
 must, therefore, be supposed to have moved in orbits 
 which were either parabolic, or in which the eccen- 
 tricity differed so little from that of a parabola as 
 not to be indicated by the observations which have 
 been made. Those which move in elliptic orbits 
 have periods of revolution varying from 3500 to 
 6000 years, and of these only the following have 
 been identified and observed at more than one return 
 to their perihelia. 
 
 XT ,. n Period of Time of next return 
 
 revolution in days. to the perihelion. 
 
 Halley's .... 27877 .... February, 1912 
 
 Encke's 1207 . , . . " 1862 
 
 Biela's 2421 .... " 1866 
 
 Faye's 2727 .... March, 1866 
 
 Brorsen's . . . 2031 .... October, 1862 
 
 V Arrest's .... 2340 .... July, 1864 
 
 Turtle's 5004 .... November, 1871 
 
 Winnecke's . . . 1830 .... April, 1863 
 
 A very curious circumstance, \vhich has often been 
 noticed in connection with the periodic comets, is 
 that those already known may be arranged, for the 
 most part, into two classes one including those 
 comets whose mean distances from the sun are all 
 
D i v i s i o N OF -COMETS. 267 
 
 nearly the same with those of the small planets be- 
 tween Mars and Jupiter; and the other class, in- 
 cluding those whose mean distance is, in every case, 
 yery nearly equal to that of Uranus. It has, there- 
 fore, been conjectured that the comets composing 
 the first class may have formerly constituted one, or 
 at most two, whose orbit or orbits were contracted 
 by the disturbing action of Jupiter, and that after- 
 ward they may have been sub-divided forming the 
 different comets which belong to this class by a 
 process yet unknown, but probably similar to that 
 by which Biela's comet has been separated into two 
 distinct parts almost under the eyes of the observers. 
 This is the theory advanced by Professor Stephen 
 Alexander, who imagines that the catastrophe or suc- 
 cession of catastrophes which divided these comets, 
 must have been of a very ancient date. The comets 
 of Peters (1846) and Tuttle, whose periods are a 
 little more than thirteen years, may perhaps be an 
 exception to any general law or order of distance 
 which may thus be supposed to exist. 
 
 The existence of a group of small planets between 
 Mars and Jupiter has long since been accounted for 
 by the supposition that they at one time, although, 
 perhaps, almost infinitely remote, constituted a single 
 planet which revolved around the sun at about the 
 average of their mean distances. This original 
 planet is supposed to have been broken into almost 
 
268 T K E A T I S E ON C M E T S . 
 
 innumerable fragments by some internal convulsion 
 of nature, thus forming a group of small planets or 
 fragments revolving at different distances, but within 
 the orbits of Mars and Jupiter. In this way thei\ 
 the common origin of the asteroid planets may be 
 considered as conclusively established. It has, ac- 
 cordingly, been conjectured that since the comets 
 of short period may be supposed to have a common 
 origin also, the latter may have formed from por- 
 tions of the original planet; and that thus both the 
 asteroids and comets of short period may have 
 formerly constituted but one mass, which was subr 
 sequently divided by some unusual and violent phy- 
 sical convulsion. When the cohesion of the particles 
 of matter which constituted the original mass, was 
 overcome by the action of the explosive force, the 
 fragments which would thus be thrown off, would 
 be projected with a velocity depending on their 
 magnitude and density. The largest fragments 
 would continue to move on in orbits differing but 
 little from that of the original planet, while the 
 lighter fragments would assume orbits more or less 
 eccentric, depending on the velocity of projection. 
 If now we suppose that the planet was, just before 
 its dissolution, composed in part of matter in a 
 gaseous form, we may conclude with some degree 
 of probability, perhaps, that the comets of short 
 period may have been formed in this manner. The 
 
COMETS AND ASTEROID PLANETS. 269 
 
 very difference of inertia of the different parts of 
 the planet, might seem to have caused the separa- 
 tion, under these circumstances, of the nebulous or 
 cornetary matter, from that of greater density ; and 
 while portions of this same nebulous material were 
 still retained by the larger portions of the original 
 mass, the smaller portions would retain scarcely any 
 traces of it. In support of this hypothesis, the fact 
 has been cited that some of the asteroid planets are 
 actually observed to be surrounded by a very dense 
 atmosphere, or nebulous envelope. 
 
 However plausible these hypotheses in reference 
 to the similarity and common origin of the asteroid 
 planets and the comets of short period may seem to 
 be, yet we cannot by any means regard them as 
 having anything more than the very slightest degree 
 of probability. There is a very beautiful theory of 
 the formation of the solar system, which we shall 
 explain hereafter, by which the origin of cometary 
 worlds is illustrated with great distinctness, and 
 which presents far greater claims to our attentive 
 consideration than any other which has ever been 
 devised. Reference is here made to the nebular 
 hypothesis of Laplace, who, in his theory of planet- 
 ary genesis from rings of vapor revolving round the 
 sun, in which matter aggregates into spheres around 
 a nucleus, asserts that, according to the hypothesis 
 of zones of vapor, and of a nucleus increasing by 
 
270 TREATISE ON COMETS. 
 
 the condensation of the atmosphere which surrounds 
 them, the comets are to be regarded as strangers to 
 the planetary system. 
 
 When we consider the subject of the comets in its 
 fullest extent, it may indeed be said that there is no 
 other branch of astronomy more replete with inter- 
 est, as furnishing the most sublime examples of the 
 achievements of the human intellect in the investi- 
 gation of the intricate problems here presented. It 
 is certainly wonderful to notice the extreme preci- 
 sion of the calculations of modern astronomers re- 
 garding the orbits of these chaotic worlds, by which 
 they are enabled to conclude definitely what has 
 been the past, and what will be the future history 
 of any particular body. Each newly discovered 
 comet is at once subjected to the ordeal of a most 
 rigorous inquiry; and its elements, roughly calcu- 
 lated within a few days after its first appearance, 
 are gradually corrected as observations accumulate, 
 until finally the exact form and position of its orbit 
 in space are determined. Its connection or identity 
 with any comet which may have previously appeared 
 is carefully examined, and when a decided similarity 
 has been detected, the disturbances due to the action 
 of the planets during the interval which has elapsed, 
 are investigated; and the past, thus brought into 
 unbroken connection with the present, is made to 
 afford substantial ground for prediction of the future. 
 
PROBABLE COLLISION WITH PLANETS. 271 
 
 The increasing attention which is being bestowed 
 upon this branch of knowledge, may ultimately in- 
 crease the number of periodic comets to an extent 
 almost beyond conjecture ; and when, even now, we 
 come to consider the manner in which the orbits 
 very nearly intersect those of the various planets 
 belonging to our system, it may indeed seem proba- 
 ble that the time will come when some one of these 
 bodies will come into collision with some member 
 of the planetary system. How far the welfare of 
 the inhabitants of the earth may be dependent upon 
 any such event, when the collision is not directly 
 with our planet, it will be unnecessary to attempt to 
 determine ; and let it suffice to know that calcula- 
 tions, based on the theory of probabilities, and of 
 the most positive character, show that there is but 
 one chance in 281,000,000, that the earth will come 
 in collision with one of these bodies. The probable 
 effect of such a catastrophe, admitting its possi- 
 bility, has already been discussed in a previous con- 
 nection. For some of the planets, the probability 
 of coming in contact is much less than in the case 
 of the earth ; and for others it is even greater. It 
 has indeed been asserted that Mars was struck by a 
 comet in 1315 or 1316, or that there was, at least, 
 an appulse, if not an actual collision. What may 
 have been the effect produced on the comet, in case 
 this event did actually take place, we have no means 
 
272 TREATISE ON COMETS. 
 
 of determining ; but in respect to Mars, there is no 
 indication in its motions, or in those of the neigh- 
 boring planets, that it has ever suffered disturbance 
 from any such source. 
 
 It might also be expected, considering the great 
 number of comets which are supposed to be included 
 within the solar system, that a collision would occa- 
 sionally take place between two of these bodies. 
 The orbits of Encke's and Biela's comets actually 
 intersect each other, and it is highly probable that 
 at some future time they may meet in this point. 
 Should this event take place in the month of Octo- 
 ber, it is possible that the inhabitants of the earth 
 may witness the extraordinary spectacle of an en- 
 counter between these two bodies ; and, it may be, 
 of their reciprocal penetration and amalgamation, 
 or of their destruction by means of exhausting ema- 
 nations. In view of this fact, it has most justly 
 been stated that events of this nature, resulting 
 either from deflection occasioned by disturbing 
 masses orprimevally intersecting orbits, must have 
 been of frequent occurrence, in the course of millions 
 of years, in the immeasurable regions of ethereal 
 space. Still, they must be regarded as isolated oc- 
 currences, exercising no more general or alterative 
 effects on the order and harmony of the celestial 
 motions, than the breaking forth or extinction of a 
 volcano, within the limited sphere of our earth. 
 
PHYSICAL CHARACTER OF COMETS. 273 
 
 CHAPTER IV. 
 
 PHYSICAL CONSTITUTION OF COMETS PERTURBATIONS OF THE COMETS 
 
 BY THE PLANETS THE MASSES OF COMETS EFFECT OF INTENSE 
 
 HEAT AND COLD ON THEIR FORM LIGHT OF COMETS TAILS OF 
 
 COMETS THEORY OF THE FORMATION OF THE TAIL INFLUENCE 
 
 OF THE TAIL ON THE MOTION OF THE NUCLEUS IN ITS ORBIT 
 
 ORIGIN OF COMETS FORMATION OF NEW WORLDS NEBULAR 
 
 HYPOTHESIS. 
 
 WE have already stated briefly the general charac- 
 teristics by which the comets are distinguished from 
 the other members of the solar system, and have 
 also explained the terms by which each part of a 
 comet is designated. It is proposed, therefore, in 
 this connection, to consider more fully the physical 
 constitution of these wandering bodies, the effect 
 of the various degrees of temperature to which such 
 objects may be subjected, and, finally, their probable 
 origin. The theory of their motions we have found 
 to be in every respect complete, but in the case of 
 their physical constitution it is far different. The 
 data here presented are by no means of a positive 
 character, and the conclusions which may be derived 
 must be received with due allowance. There may, 
 indeed, be comets in existence which bear some 
 slight resemblance to the planets in their physical 
 
271 TREATISE ON COMETS. 
 
 constitution, and which, were it not for the great 
 eccentricity of their orbits, might justly be regarded 
 as planetary worlds. But by far the greater number 
 of the comets, and more especially those which are 
 visible only through a telescope, are seen generally 
 as clouds or masses of vapor, with or without a train 
 of fainter light. 
 
 It has long since been noticed that comets might 
 pass directly between the eye of the observer and 
 stars of the sixth and seventh magnitude, without 
 diminishing their light ; and that, in every such case, 
 the effect produced on the light of the star would 
 not be greater than might be expected if it were 
 shining through a very slight fog. There are nu- 
 merous cases of this kind on record, all of which 
 tend to show that the comets are composed of matter 
 in a state of the very greatest attenuation. From 
 considerations such as this, and the fact that the 
 light of the stars are greatly enfeebled by that of 
 the full moon shining through our atmosphere, 
 attempts have been made to determine the probable 
 masses of some of the comets ; and the results ob- 
 tained have invariably indicated that they contain 
 but a very small quantity of matter. In some cases 
 it has been found that the absolute quantity of 
 matter in the entire comet was almost inconceivably 
 small; and that, consequently, the comet was so ex- 
 tremely attenuated that the molecules of which it is 
 
PHYSICAL CHAKACTER OF COMETS. 275 
 
 composed were completely isolated, and destitute of 
 mutual elastic reaction. These results, however, 
 must be received with considerable allowance for 
 the insufficiency of the data on which they are 
 based. It is by no means certain that a central pas- 
 sage of a comet over a star will not almost invari- 
 ably cause a complete occultation. In those cases 
 in which stars have been seen through comets, they 
 have usually been situated barely within the exterior 
 nebulosity or envelope, and, consequently, not under 
 circumstances favorable to indicate the precise 
 density of the cometary matter. In April, 1857, the 
 complete occultation of a star by Brorsen's comet 
 was observed at Florence ; and a few months later 
 the same phenomenon, produced by another comet, 
 was observed at Altona, in Denmark. In both cases 
 the comet was a telescopic object, and belonged to 
 the class of diffused nebulosities. Thus, although 
 there were no indications that the comet was really 
 solid at the centre, yet the density of the nucleus 
 was such as to prevent a passage of the light of the 
 star. There can be no doubt but that many of the 
 comets have actually a solid nucleus surrounded by 
 an immense nebulous atmosphere. The solid part, 
 however, may no, t exceed a few miles, or perhaps a 
 feAV hundred miles, in diameter, while the nebulous 
 envelope, or cometic atmosphere, may be of such 
 enormous dimensions as to appear as if constituting 
 
276 TREATISE ON COMETS. 
 
 the whole matter of the comet. In this case the 
 comet might pass almost centrally over a star and 
 yet not affect its brilliancy very materially, or cause 
 a complete occupation. It is evident, therefore, 
 that the combination of circumstances which would 
 alone furnish a sufficient test of the density of a 
 comet, by passing apparently over a star, is of rare 
 occurrence. It would be necessary that the centre 
 of the head of the comet, although very small, 
 should pass critically over a star, in order to ascer- 
 tain whether the latter is visible through it. 
 
 In September, 1832, Biela's comet was observed 
 to pass directly over a small cluster of telescopic 
 stars of the sixteenth or seventeenth magnitude ; 
 and the entire cluster remained distinctly visible, 
 thus affording a striking proof of the extreme trans- 
 lucency of the matter of which this comet consists. 
 The most trifling fog or haze in the earth's atmo- 
 sphere would have entirely effaced this group of 
 stars, yet they continued visible through a thickness 
 of cometic matter which, at its central part, must 
 have been nearly 50,000 miles. In this case, there- 
 fore, the comet could not have had a solid nucleus 
 of any considerable size, but it does not follow that 
 a similar phenomenon could have been observed as 
 a general result for all comets. It is certainly known 
 that many of the comets which have hitherto ap- 
 peared have a solid and compact nucleus; and we 
 
PHYSICAL CHARACTER OF COMETS. 277 
 
 may, therefore, conclude, that while there are many 
 comets without any nucleus, properly so called, there 
 are some with a nucleus which, perhaps, may be 
 transparent ; and others more brilliant than planets, 
 having a nucleus which is solid and opaque. In 
 those comets in which the appearance of a nucleus 
 is most distinctly marked, the nebulous envelope, 
 instead of a uniform or of a progressively increasing 
 brilliancy from the circumference toward the centre, 
 is often apparently composed of one or more rings 
 or strata, many thousand miles in thickness, alter- 
 nating with fainter portions thus presenting an ap- 
 pearance not unlike what would be presented to a 
 spectator viewing from a distance our earth with its 
 different layers of clouds, one above another, sepa- 
 rated by portions of transparent atmosphere. 
 
 It appears from such considerations that, as a 
 general result, the absolute quantity of matter con- 
 tained in any of the comets which have hitherto 
 appeared, must be extremely small when compared 
 with that of the planets belonging to our system, 
 excepting, perhaps, some of the group of asteroid 
 planets between Mars and Jupiter. That they are 
 thus constituted is still further evident from the fact, 
 that for ages they have travelled among the planets 
 and their satellites without producing any disturb- 
 ances which have yet been detected. An example 
 of this kind has already been given in the case of 
 24 
 
278 TREATISE ON COMETS. 
 
 Lexell's comet of 1770, which passed directly among 
 the satellites of Jupiter, without in the least dis- 
 arranging that system. The comet, however, was 
 subjected to the most excessive perturbations by 
 Jupiter, its orbit being completely changed. We 
 have also numerous instances 011 record, in which 
 comets have approached very near some of the 
 smaller planets of the system without producing 
 the slightest perceptible disturbance, but in which, 
 without exception, the comet has been drawn very 
 considerably from its previous orbit. According to 
 the law of universal gravitation first discovered by 
 Newton, and subsequently demonstrated to the very 
 fullest extent, as the great law or bond which unites 
 the various bodies composing the material universe, 
 by the researches of Lagrange, Laplace, and others, 
 every portion of matter attracts and is attracted 
 directly as the quantity of matter, and inversely as 
 the square of the distance from the attracting body. 
 This is the law of universal gravitation ; and it may 
 be perceived at once, from the mere statement of the 
 law, that the comets must be greatly attracted from 
 their regular course by the various bodies of the 
 systems through which or near which they pass. 
 The disturbances to which they are thus subjected, 
 and to which we have briefly alluded in the pre- 
 ceding chapter, are called the perturbations. 
 
 To attempt an explanation of the manner in which 
 
PERTURBATIONS OF COMETS. 279 
 
 the perturbations of the comets maybe determined, 
 would require a higher degree of mathematical 
 knowledge than can be expected of those for whom 
 these explanations are intended; and we must there- 
 fore be contented with a mere statement of the 
 general features of the problem thus presented. 
 If the system were composed of simply the sun and 
 one planet, the latter would describe an exact ellipse 
 around the former, or rather both around their com- 
 mon centre of gravity, and continue to perform its 
 revolutions in an unchanged orbit through all time. 
 But if now we add to the system a third body, the 
 attraction of this third body will draw both of the 
 former bodies out of their mutual orbits, and by 
 acting on them unequally, will disturb their relation 
 to each other, and will completely efface all traces 
 of exact elliptic motion either around one another, 
 or about a fixed point in space. The disturbance 
 thus produced is not due to the whole attraction of 
 the third body, but rather to the difference of its 
 attractions on the two which we have supposed to 
 have originally constituted the system. 
 
 In the solar system, when compared with the sun, 
 the masses or weights of the different comets and 
 planets composing the system, are extremely minute. 
 Their attractions on each other, therefore, are all 
 very feeble when brought into comparison with the 
 presiding central power of the sun, and the effects 
 
280 TREATISE ON COMETS. 
 
 of their disturbing forces are proportionally small. 
 This fact enables us, in determining the perturba- 
 tions of any particular body, to estimate each of the 
 effects of the disturbing bodies separately, as if the 
 others did not take place, without causing any error 
 in the final results beyond what may be considered 
 necessarily incident to a first approximation. It 
 follows directly from the primary relations between 
 forces and the motions which they produce, that 
 when a number of very minute forces act at once on 
 a system, the combined effect is the sum or aggre- 
 gate of their separate effects, at least within such 
 limits, that the original relation of the parts of the 
 system shall not have been materially changed by 
 their joint action. This, in the case of the mutual 
 attractions of the planets and comets, may be sup- 
 posed to be true for a limited portion of time, but 
 when the orbits may have become slightly changed, 
 it becomes necessary to take account of these changes 
 in determining the subsequent effects. It is evident, 
 however, that in investigating the disturbing influ- 
 ence of the several planets on any particular comet, 
 we need only consider the case of a single disturb- 
 ing planet at a time, and having found their indi- 
 vidual effects, the sum of these will be the total 
 effect. The problem thus reduces to that of three 
 bodies; a predominant central body, corresponding 
 to the sun, a disturbing body, corresponding to a 
 
PERTUBATIONS OF COMETS. 281 
 
 planet, and a disturbed body, or the comet. It is 
 here supposed that the mass of the comet is small 
 when compared with either that of the sun or planet, 
 which has hitherto invariably been the case ; and it 
 may, perhaps, be stated as a general fact, that no 
 comet will probably ever appear which will render it 
 necessary to take into account its action on any of 
 the planets. 
 
 Since the orbits of the comets are inclined to the 
 orbit of the earth at all possible angles, while the 
 orbits of the other planets are all inclined to this 
 within very narrow limits, excepting, of course, some 
 of the asteroid planets, it follows that for different 
 comets the effect produced by the several planets on 
 the different elements will be greatly dissimilar. 
 For example, if a comet happened to pass near a 
 planet when in the vicinity of the node of the former, 
 the attraction of the latter might produce a great 
 change in the inclination of its orbit. If the comet, 
 on the contrary, were far distant from either node 
 of its orbit, the effect would be to change the place 
 of the node without affecting very considerably the 
 inclination. In the case of a comet whose orbit is 
 greatly inclined to the orbit of the earth, since its 
 distance from the planets in the remoter parts of its 
 orbit will be much greater than if it were less in- 
 clined, the perturbations will be proportionally 
 smaller. It is evident, therefore, that those comets 
 24* 
 
282 TREATISE ON COMETS. 
 
 whose orbits have the greatest inclination to those 
 of the planets, are subject to the least disturbance ; 
 while those which move very nearly in the plane of 
 these orbits, or else in that which would result as a 
 mean between the orbits of the principal planets, 
 would, on account of the great length of their orbits 
 which would bring them into the vicinity of several 
 different planets be subject to much greater dis- 
 turbance. 
 
 In computing the perturbations of a comet by a 
 planet, the attractive force of the planet on the 
 comet, depending, of course, on their mutual dis- 
 tance, is resolved into three parts called components, 
 each acting in the direction of one of the co-ordi- 
 nate axes, already explained in the preceding chap- 
 ter.* The amount of the disturbance of the motion 
 of the comet in the direction of the three rectangular 
 axes is thus obtained ; and it is only necessary to 
 add them to the rectangular co-ordinates of the 
 comet computed under the supposition of an undis- 
 turbed orbit, or subtract them from these, as the 
 case may be, in order to find the actual co-ordinates 
 of the comet at any given instant. In this way the 
 perturbations are to be computed from day to day, 
 or, when they are not very large, at intervals of 
 twenty or forty days, or even longer, correcting the 
 elements of the orbit, from time to time, by means 
 
 * See page 170. 
 
PERTURBATIONS OF COMETS. 
 
 of the variations of the co-ordinates already ob- 
 tained. Assuming then the elements of the orbit 
 to be accurately determined from observations for 
 any given epoch, the perturbations or variations of 
 the co-ordinates must be computed at regular inter- 
 vals, as above stated, for the whole period during 
 which it is required to trace the motions of the 
 comet. The same process must be performed for 
 each disturbing planet, and the sum of the separate 
 variations for each planet will give the total amount 
 of the perturbations. Having obtained these values, 
 and having corrected the assumed elements accord- 
 ingly, we obtain finally the elements of the orbit in 
 which the comet is moving at the instant for which 
 its true orbit is required. The values of the dis- 
 turbances thus formed are called the special pertur- 
 bations of the comet. In the case of the planets it 
 is possible to compute the variations of their elements 
 due to their mutual attractions for an indefinite num- 
 ber of revolutions. The formulae, however, which give 
 these variations, are so affected by the eccentricity 
 of the orbit of the planet, that the solution is possi- 
 ble only when the eccentricity is small. The varia- 
 tions of the elements thus obtained are called the 
 general perturbations, and can be computed only in 
 the case of the planets the eccentricity of their 
 orbits being small. In the case of the comets the 
 eccentricity of the orbit is always so great that it is 
 
284 TREATISE ON COMETS. 
 
 impossible to compute their general perturbations, 
 or for an indefinite number of revolutions ; and, 
 consequently, astronomers are enabled to determine 
 only the variations of the elements of their orbits 
 assuming the elements for a given epoch to be 
 known for a series of dates in regular progression. 
 When a comet passes to such an enormous distance 
 that the attraction of even the remote planets of 
 the system becomes insensible, it is no longer neces- 
 sary to compute the variations of its elements as 
 before, since it may then be supposed to revolve in 
 a pure ellipse around the common centre of gravity 
 of the system, which, however, coincides very nearly 
 with the centre of the sun. It is supposed to revolve 
 thus in an orbit which may be determined from the 
 assumed elements in connection with their variations 
 or perturbations as already determined, until finally 
 it again comes .within the sphere of the sensible 
 attraction of the planets, when the disturbance of 
 its motion is again computed for a regular series of 
 dates, precisely as before. In this way, therefore, 
 the computation of the perturbations of a comet 
 may be performed for a long series of years, or even 
 for a number of revolutions ; but the process, as may 
 be readily perceived, is very laborious. In the case 
 of the periodic comets the perturbations are com- 
 puted only to the date of their next appearance, 
 since it would be useless to continue them further 
 
THE MASSES OF COMETS. 285 
 
 \rithout correcting the elements by means of a series 
 of observations which might then be taken. In the 
 case of a comet moving in an ellipse of very great 
 eccentricity, or in which the period is more than 
 1000 years, and also in the case of a comet moving 
 in a parabola or hyperbola, it is necessary only to 
 compute the perturbations during the short period 
 in which it is visible from the earth, since in these 
 cases all that is required is to find the exact elements 
 of the orbit in which the comet was. moving while 
 visible. 
 
 Such is a general statement of the method by 
 which astronomers are enabled to follow the comets 
 in their long journeys through space, while invisible 
 to the inhabitants of our earth, and to predict, in 
 the case of the periodic comets, the exact period of 
 their return. To be able to solve this problem 
 requires the very highest degree of mathematical 
 knowledge ; and let it suffice here to say that it is 
 one of the most difficult problems of astronomy, 
 while the amount of labor required in making the 
 numerical computations, vastly exceeds what may 
 be readily conceived. An illustration of this fact 
 may be witnessed in the case of Halley's comet 
 already given. 
 
 We have already stated ijiat in all computations 
 respecting the motions of the comets, their masses 
 have been neglected as being inconsiderable in com- 
 
286 TREATISE ON COMETS. 
 
 parison with those of the sun and planets. Their 
 effect in the planetary system, even if there were 
 not other facts tending to the same result, would 
 alone be sufficient to indicate that this supposition is 
 entirely admissible. These bodies are continually 
 passing through our system, and often in the imme- 
 diate vicinity of or directly among the asteroid 
 planets, and yet do not cause the slightest disturb- 
 ance which can be determined by accurate observa- 
 tion. The physical appearance of the comet affords 
 also conclusive proof that their masses are at least 
 small. Various attempts have been made to deter- 
 mine the masses of these bodies; and although the 
 results obtained exhibit great discrepancies, yet they 
 all indicate conclusively that in the case of a very 
 great majority of the comets the mass is extremely 
 small, or, in reality, almost inconceivably small. 
 It may, perhaps, seem really absurd to assert that 
 the tails of some of the largest comets have not 
 contained a hundred pounds of matter, yet it has 
 been asserted that even the largest and longest did 
 not contain more than a few ounces. "We are, of 
 course, unable to deny such an assertion, since we 
 have no means of proving the contrary. It must be 
 admitted, however, that this is an extremely small 
 estimate, and that, although the comets are in reality 
 of extreme tenuity, and consequently have no con- 
 sideiable mass, yet that they contain vastly more 
 
INTENSE HEAT AND COLD. 287 
 
 matter than what would result from any such esti- 
 mate. The only correct and reliable method of 
 arriving at the exact value of the mass of a comet, 
 is by means of the disturbance which it may produce 
 in the motions of a planet near which it approaches ; 
 and until such a phenomenon is observed, no definite 
 knowledge of this element can be obtained. 
 
 When we come to consider the ever- varying tem- 
 perature to which the comets are subjected in their 
 motions through space, the question may very 
 naturally arise as to the probable effect which would 
 thus be produced. It might be supposed that, as a 
 necessary consequence, when a comet approaches 
 the sun, the continually increasing temperature 
 which may be supposed to affect it, would cause it 
 to expand gradually until it arrived at its perihelion, 
 when its absolute dimensions would be greatest. 
 For a similar reason it might be expected that in 
 receding from the sun, the temperature being sup- 
 posed to decrease continually, it would be gradually 
 contracted, and would finally appear precisely as 
 before its perihelion passage. It may, on this ac- 
 count, seem strange that the dimensions of a comet 
 are observed almost invariably to be enlarged as it 
 recedes from the source of heat, and more especially 
 in the case of those telescopic comets which have no 
 tail. This singular and somewhat unaccountable 
 phenomenon has been explained in various ways. 
 
288 TREATISE ON COMETS. 
 
 Valz ascribed it to the pressure of the solar atmo- 
 sphere, supposed to extend to a great distance, acting 
 upon the comet. He supposes that the atmosphere, 
 being more dense than the sun, compressed the 
 comet and diminished its dimensions ; while, at a 
 greater distance, being relieved from this coercion, 
 the body again expanded to its natural bulk. In 
 order to test the validity of this hypothesis, probable 
 values were assumed for the density of the solar 
 atmosphere and the elasticity of the comet; and the 
 variations of the bulk of a comet, deduced in accord- 
 ance with this assumption, exhibited a remarkably 
 close agreement with the observed change in its 
 dimensions. It is necessary, however, to suppose 
 also that the comet is composed of an elastic gas or 
 vapor ; and, further, that it is impervious to the solar 
 atmosphere through which it moves, both of which 
 assumptions are inadmissible, at least in the gene- 
 rality of cases. Another theory which has been 
 advanced in order to explain the fact that the 
 dimensions of a comet are enlarged as it recedes 
 from the source of heat, is, that as the particles or 
 molecules of the nebulous matter of the cornet are 
 distant, and held together by so feeble a power, they 
 may revolve to a certain limited extent independ- 
 ently of each other, each having its own perihelion ; 
 and that thus they would be brought nearer to each 
 other as the} 7 approach the sun, and separate again 
 
INTENSE HEAT AND COLD. 289 
 
 further and further as they depart from him. This 
 theory assumes that the mutual cohesion, or mutual 
 gravitation of these particles is a quantity evanescent 
 in comparison with their separate gravitation toward 
 the sun. 
 
 Sir John Herschel supposes that the nebulous 
 portion of the comet, or that portion which reflects 
 the sun's rays, is of the nature of a fog, or of a col- 
 lection of discrete particles of a volatile fluid floating 
 in a transparent medium. During the comet's ap- 
 proach to the sun, these molecules would absorb its 
 rays and become heated, and consequently a portion 
 of them would be constantly passing from the liquid 
 to the extremely gaseous or invisible state, just as a 
 fog or common vapor disappears before the rising 
 sun. As this change must commence from without, 
 and must be propagated toward the centre, the ulti- 
 mate effect would be a diminution of the visible 
 bulk of the comet. As the comet receded from the 
 sun, it would lose by radiation the heat thus acquired ; 
 which, in accordance with the general analogy of 
 radiant heat, might be expected to escape chiefly 
 from the unevaporated or nebulous mass within. 
 The dimensions of the latter would, therefore, begin 
 and continue to increase by the precipitation above 
 it of fresh nebula, just as fogs, on cold still nights, 
 are seen to form at first on the surface of the earth, 
 and as the heat near the surface becomes dissipated, 
 
290 TREATISE ON COMETS. 
 
 gradually extending upward. A comet might thus 
 appear to enlarge rapidly in its visible dimensions, 
 while its real volume was slowly contracting by the 
 general abstraction of its heat. Herschel supposes 
 that this process might go on in the absence of any 
 solid or fluid nucleus ; and in those cases in which 
 such a nucleus exists, the increase of temperature 
 in the vicinity of the sun, by causing an evaporation 
 from its surface, would afford a constant and copious 
 supply of vapor, which, rising into its atmosphere, 
 and condensing at its exterior parts, would tend 
 still more to dilate the visible limits of its nebulosity. 
 In this manner we may account for the appearances 
 which have been noticed in the head of certain 
 comets, where a stratum void of nebula has been 
 observed, interposed, as it were, between the denser 
 portion of the nucleus, and the coma or envelope. 
 Another theory which has been advanced by Her- 
 schel, in explanation of the phenomenon under con- 
 sideration, attributes it to the ethereal medium sur- 
 rounding the sun. It is supposed to be by no means 
 improbable that the region in which the earth re- 
 volves has a temperature of its own greatly superior 
 to what may be presumed to be the absolute zero, 
 and even to what may be produced by artificial 
 means. This temperature Herschel supposes to be 
 due not simply to the radiation of the stars, but 
 rather to the contact of an ether, possessing itself 
 
LIGHT OF COMETS. 291 
 
 a determinate temperature, and tending, like all 
 known fluids, to communicate this temperature to 
 bodies immersed in it. ]^ow, if the temperature of 
 the ether increases in approaching the sun, which 
 seems to be a necessary consequence regarding it 
 as endued with the ordinary relations of fluids to 
 heat an obvious explanation is furnished of the 
 phenomenon under consideration. A body of such 
 extreme tenuity as a comet, may be presumed to 
 assume very readily the temperature of the ether 
 by which it is enveloped; and the vicissitude of 
 warmth and cold thus experienced may ultimately 
 convert it into transparent vapor, and again pre- 
 cipitate the nebulous substance, just as fogs are 
 dissipated by an increase of atmospheric tempera- 
 ture, not by abstracting or annihilating its aqueous 
 particles, but by causing them to assume the elastic 
 and transparent state, which, when the temperature 
 falls, they again lose, and appear in the formation 
 of another fog. 
 
 Another important consideration immediately con- 
 nected with this branch of our subject, is whether 
 the comets shine partially by their own proper light, 
 or, in other words, whether they are to be regarded 
 as to a limited extent self-luminous. If a comet 
 should have a solid and opaque nucleus of any con- 
 siderable size, it would follow as a necessary con- 
 sequence, in case it did not have a light of its own, 
 
292 TKEATISE ON COMETS. 
 
 that it would exhibit the phases of the moon when 
 near the sun. The fact, however, that these have 
 rarely, if ever, been certainly observed, and also 
 that the comets are known to be composed of 
 nebulous matter in an extremely attenuated con- 
 dition, has rendered it uncertain whether they shine 
 by reflected light, like the planets, or by virtue of 
 their being self-luminous, like the sun and stars. 
 A mere mass of nebulous matter, not itself lumi- 
 nous, but rendered visible by reflected light, would 
 not exhibit phases similar to those of the moon and 
 interior planets. Its imperfect opacity would permit 
 the solar light to affect its constituent parts through- 
 out its entire depth so that, like a thin, fleecy 
 cloud, it would not appear as if superficially illumi- 
 nated, but as receiving and reflecting light through 
 all its dimensions, both externally and internally. 
 If we suppose the comets to be self-luminous, it 
 might be expected that they would be visible in the 
 remote parts of their orbits until their distance was 
 absolutely so great as to render their apparent 
 diameter inappreciable. It is an established pro- 
 perty of self-luminous bodies, that when viewed 
 from any distance whatever, they will appear of the 
 same intrinsic splendor, or, in other words, equal 
 portions of the apparent surface at different distances 
 will be equally brilliant. Thus the sun, when seen 
 from Neptune, the exterior planet of our system, 
 
LIGHT OF COMETS. 293 
 
 must appear as bright as when seen from the earth. 
 The absolute quantity of light is much smaller, since 
 the great increase of distance from the sun in pass- 
 ing from the earth to that planet will cause the 
 apparent angular diameter of the sun to be propor- 
 tionally diminished. It is evident, therefore, that 
 although equal portions of his disc, as seen from 
 these planets, may be equally bright, yet the total 
 amount of light received at each will be vastly 
 different. It follows from this, that, since the visi- 
 bility of a self-luminous object does not depend 
 upon the angle which it subtends as long as it is of 
 sensible magnitude, if a comet shines by its own 
 light, it should retain its brilliancy as long as its 
 diameter is of a sensible magnitude ; and that even 
 after it has receded to such a distance as to render 
 its apparent diameter inappreciable, it ought to be 
 visible like the fixed stars, and should only vanish 
 in consequence of extreme remoteness. The phe- 
 nomena, however, which are actually observed, do 
 not in the least accord with any such hypothesis. 
 The comets are observed to become gradually fainter 
 and fainter as their distance from the earth and sun 
 increases, and finally become invisible, even through 
 the most powerful telescopes, while their apparent 
 angular diameter is very considerable. This would 
 seem to indicate conclusively that they shine by 
 reflected light. It might be remarked also, in this 
 25* 
 
294 TREATISE ON COMETS. 
 
 connection, that the brilliancy of a comet computed 
 from day to day, during the period of its visibility, 
 under the supposition that it shines by reflected 
 light, is found to coincide very closely with the 
 actual appearance of the comet. The most brilliant 
 comets have ceased to be visible long before they 
 had receded to a distance of 500,000,000 miles ; and 
 a very great majority of these bodies have their 
 perihelia within the orbit of Mars, while none have 
 been visible from the earth whose perihelia were 
 exterior to the orbit of Jupiter. 
 
 The facts thus established are entirely incom- 
 patible with the hypothesis that the comets are self- 
 luminous bodies, unless it is supposed that, from 
 some physical cause, they gradually lose their lumi- 
 nosity. The phenomenon, already noticed, of the 
 gradual expansion of a comet in receding from the 
 sun, might perhaps serve to account partially for a 
 diminution of their luminosity. The luminous 
 matter thus existing in a less condensed state might 
 be supposed to shine with a proportionally enfeebled 
 splendor, until, at length, by the dilatation of the 
 body, the light would become so reduced as to be 
 incapable of affecting the retina of the eye to an 
 extent sufficient to produce a sensation. The ex- 
 pansion of the comet, however, would have precisely 
 the same effect in the case of reflected light ; and 
 it is evident, therefore, that nothing definite can be 
 
LIGHT OF COMETS. 295 
 
 arrived at iii this manner, except by experiment. 
 Arago has submitted to examination the rate in 
 which comets increase their dimensions in receding 
 from the sun, and has found the corresponding 
 diminution of intrinsic splendor which would arise 
 from such a cause. It remains then to determine 
 whether the circumstances are such as to render the 
 brightest comets visible beyond the orbit of Jupiter. 
 This Arago proposed to determine by actual experi- 
 ment. Let a telescope be selected having a large 
 aperture and low magnifying power, by the aid of 
 which the comet may be observed in every part of 
 its visible course. Let the comet be observed at 
 some determinate distance from the sun, and by 
 varying the magnifying power of the telescope, it 
 may be made to assume apparently different degrees 
 of brilliancy. The magnifying power of the tele- 
 scope may be so regulated as to exhibit the comet 
 with precisely that degree of brilliancy with which 
 it would appear at any given increased distance 
 from the sun, if observed with the lowest magnify- 
 ing power of the instrument, under the supposition 
 of its being a self-luminous body, and losing bright- 
 ness by reason of the enlargement of its dimensions. 
 In this manner the actual brilliancy of the comet, 
 if self-luminous, at any given distance from the 
 sun, might be predicted; and, in case the subse- 
 quent observations were found to agree completely 
 
296 TREATISE ON COMETS. 
 
 with the prediction, it might be presumed that the 
 comet was visible by means of its own light. But 
 if, as the observations actually indicate, the brilliancy 
 of the comet at different distances from the earth is 
 greatly less than in accordance with the prediction, 
 while it becomes invisible at distances at which its 
 apparent diameter is considerable, then it may be 
 considered as conclusively established, that the body 
 is not self-lurninous, but that it derives its light from 
 the sun, and that its final disappearance from our 
 view arises from the extreme faintness of the light 
 reflected by its attenuated matter. 
 
 It was the opinion of several astronomers that the 
 great comet of 1811 shone by inherent light, and it 
 was asserted that the rapid variations which were 
 observed to take place in the brilliancy of the 
 nucleus, together with the flashes of light which 
 characterized the appearance of the tail, cannot be 
 explained by means of any other hypothesis. In 
 opposition to this opinion, it may be remarked that 
 comets have been seen as dark spots crossing the 
 sun's disc, and also that their light exhibits traces 
 of polarization. On the 18th of November, 1826, 
 the transit of the dark body of a comet across the 
 sun's disc was observed at Marseilles and Yiviers, 
 in France ; and a similar phenomenon was observed 
 at various places on the 6th of June, 1818. There 
 are also numerous cases on record in which dark 
 
LIGHT or COMETS. 297 
 
 bodies have been observed in their transit over the 
 sun's disc, which were undoubtedly comets having 
 a solid and opaque nucleus. In these cases the 
 comet could not have been self-luminous, since it 
 would then have been invisible ; or, at least, would 
 have appeared as a bright instead of a dark spot. 
 Again, it is found that all direct light constantly 
 divides itself into two points of the same intensity 
 when it traverses a crystal possessing the power of 
 double refraction ; while reflected light, in certain 
 positions of the crystal through which it is made to 
 pass, gives two images of unequal intensity, in case 
 the angle of reflection is not ninety degrees ; or, in 
 other words, it is polarized in the act of reflection. 
 In general, when a ray of light is reflected from any 
 surface, it may be reflected a second time from 
 another surface, and may also be made to pass freely 
 through transparent bodies. If, however, a ray of 
 light be reflected from any surface at a given angle, 
 to be determined by experiment, it will be found 
 to have been rendered totally incapable of reflection 
 at another surface in certain definite positions; 
 while, in other positions, it will be completely re- 
 flected by the second surface. It will be found also 
 to have lost the property of penetrating transparent 
 bodies in particular positions, but, in others, to be 
 freely transmitted by them. Light thus modified 
 so as to be incapable of reflection and transmission 
 
298 TREATISE ON COMETS. 
 
 in certain directions, is said to be polarized. On 
 this principle Arago proposed a photometrical 
 method of determining whether the comets are 
 self-luminous, or shine by means of the light of the 
 sun reflected from their surface, and, perhaps, even 
 their internal parts. This method was applied at 
 the last return of Halley's comet, and it was found 
 that the apparatus gave two images presenting the 
 complementary colors, one of them being red and 
 the other green. By turning the instrument half 
 round the red image became green, and the reverse. 
 It was therefore concluded that the light of the 
 comet, or at least the whole of it, was not composed 
 of rays possessing the property of direct light. 
 The same experiment's were repeated with this 
 comet by different observers, and always with the 
 same result. Similar experiments have subsequently 
 been made on some of the brightest comets, and it 
 is found that their light exhibits precisely the same 
 phenomena. This shows that the light of comets 
 must be partly composed of that received from the 
 sun, even if they have a light of their own. All 
 that can be proved by the polarization of their light, 
 is that they may possibly shine wholly by reflection, 
 while the considerations previously adduced are 
 sufficient to establish the possibility thus admitted 
 as a physical fact, In view of all these facts it 
 seems reasonable to conclude, without hesitation, 
 
TAILS OF COMETS. 299 
 
 that the comets shine by means of the light which 
 they receive from the sun, reflected from all their 
 parts. 
 
 The next important consideration connected with 
 the theory of the physical constitution of comets, is 
 that which relates to the formation and develop- 
 ment of their tails. The idea of a tail or train of 
 vast extent is, in the popular mind, inseparably con- 
 nected with the physical appearance of these won- 
 derful bodies ; but it should be remarked, that by 
 far the great majority of comets are not thus 
 attended. Those which have been so brilliant as 
 to be visible to the naked eye, have usually exhibited 
 the phenomenon of a train of fainter light, extend- 
 ing in a direction nearly opposite the sun. The 
 direction of the tail is, however, by no means inva- 
 riable. Sometimes it has happened that it formed 
 a considerable angle with a line drawn to the sun, 
 and cases have occurred in which it was actually at 
 right angles to it. It has also been observed that 
 the tails of comets, as a general result, incline con- 
 stantly toward the region from which the comet is 
 moving, as if, in its progress through space, it were 
 subject to the retarding influence of some resisting 
 medium, its nebulous atmosphere being, on account 
 of the resistance thus imposed, compelled to remain 
 behind the solid nucleus in the form of a tail. 
 
 We have already, in the descriptions of some of 
 
300 TREATISE ON COMETS. 
 
 the most remarkable comets given in the preceding 
 pages, had occasion to allude to the curved form 
 and absolute dimensions of the tails of comets, and 
 have also noticed the various other characteristics 
 by which these somewhat remarkable appendages 
 are distinguished. It is proposed, therefore, in this 
 connection, to give simply some of the various the- 
 ories which have hitherto been advanced in expla- 
 nation of this phenomenon. It will, of course, be 
 useless to attempt to give all the theories which 
 human ingenuity has devised ; and we shall, there- 
 fore, explain only those which have been received 
 with the greatest favor, before proceeding to illus- 
 trate that which is now satisfactorily ascertained to 
 be the true theory of the development of these 
 anomalous appendages. 
 
 The more ancient theory of the fprmation of the 
 tails of comets, in which they are supposed to be 
 formed by the lighter parts being thrown off by the 
 resistance of the ether through which the comet 
 passed, has just been alluded to. In more modern 
 times the prevailing opinion has been, that these 
 particles are driven off by the impulsive force of the 
 sun's rays. The fallacy of the former opinion is 
 evident from the fact that the tails are generally 
 directed from the sun ; and that, whatever the rela- 
 tive direction may be, it remains very nearly the 
 same both before and after the perihelion passage. 
 
TAILS OF COMETS. 301 
 
 The more modern theory may be supported with 
 considerable force of reasoning; yet the latest in- 
 vestigations have indicated beyond a doubt the true 
 theory of the formation of the tail. Another theory 
 which has been advanced supposes the tail to be 
 formed by the sun's rays slightly refracted by the 
 nucleus in traversing the envelope of the comet, 
 and uniting in an infinite number of points beyond 
 it, throwing a stronger than ordinary light on the 
 ethereal medium, near to or more remote from the 
 comet, as the ray is more or less refracted from its 
 relative position and direction. 
 
 The phenomena presented by Halley's comet at 
 its last appearance, when considered in detail, led 
 Sir John Herschel to conclude: 1st. "That the 
 matter of the nucleus of a comet is powerfully ex- 
 cited and dilated into a vaporous state by the action 
 of the sun's rays, escaping in streams and jets at 
 those points of its surface which oppose the least 
 resistance, and in all probability throwing that sur- 
 face, or the nucleus itself, into irregular motions 
 by its reaction in the act of so escaping, and thus 
 altering its direction. 2d. That this process chiefly 
 takes place in that portion of the nucleus which is 
 turned toward the sun the vapor escaping chieflv 
 in that direction. 3d. That when so emitted, it is 
 prevented from proceeding in the direction origin- 
 ally impressed upon it, by some force directed from 
 26 
 
TREATISE ON COMETS. 
 
 the sun, drifting it back, and carrying it out to vast 
 distances behind the nucleus, forming the tail, or so 
 much of the tail as can be considered as consisting 
 of material substance. 4th. That this force, what- 
 ever its nature, acts unequally on the materials of 
 the comet, the greater portion remaining unva- 
 porized, and a considerable part of the vapor actu- 
 ally produced remaining in its neighborhood, form- 
 ing the head and coma. 5th. That the force thus 
 acting on the materials of the tail cannot possibly 
 be identical with the ordinary gravitation of matter, 
 being centrifugal or repulsive, as respects the sun, 
 and of an energy very far exceeding the gravitating 
 force toward that luminary. This will be evident 
 if we consider the enormous velocity with which 
 the matter of the tail is carried backward, in oppo- 
 sition both to the motion which it had as part of the 
 nucleus, and to that which it acquired in the act of 
 its emission, both which motions have to be de- 
 stroyed in the first instance, before any movement 
 in the contrary direction can be impressed. 6th. 
 That unless the matter of the tail thus repelled from 
 the sun be retained by a peculiar and highly ener- 
 getic attraction to the nucleus, differing from and 
 exceptional to the ordinary power of gravitation, it 
 must leave the nucleus altogether ; being in effect 
 carried far beyond the coercive power of so feeble 
 a gravitating force as would correspond to the 
 
TAILS OF COMETS. 303 
 
 minute mass of the nucleus; and it is therefore 
 very conceivable that a comet may lose, at every 
 approach to the sun, a portion of that peculiar 
 matter, whatever it be, on which the production of 
 its tail depends, the remainder being of course less 
 excitable by the solar action, and more impassive 
 to his rays, and therefore more nearly approximating 
 to the nature of the planetary bodies." 
 
 The repulsion which is thus assumed to be exer- 
 cised by the sun, was exhibited by Donati's comet 
 in the most palpable manner. The question may 
 now arise whether this force is real or apparent; 
 whether, if real, it is a polar force, like magnetism 
 and electricity, or a simple force like that of gravity; 
 and whether, if apparent, it results simply from the 
 difference of action of the solar attraction on the 
 various parts of the comet, or from its different 
 action on the molecules of a gravitating ether, and 
 upon those of the comet. There are thus exhibited 
 four cases to be considered, two of which, however, 
 may hardly with propriety be regarded as strictly 
 connected with a repulsive force, such as will be 
 proved to exist. The idea of an apparent repulsion 
 is to assimilate the phenomena of the comets to 
 that of the seas, only on a vastly greater scale. The 
 sun is supposed to act in two ways ; first, by its 
 attraction, or by its gravitation, and then by its 
 heat. The attraction of the sun on a solid body 
 
 I 
 
304 TREATISE ON COMETS. 
 
 surrounded by a nebulous or fluid atmosphere would 
 necessarily change its figure from the spherical form 
 to an oblong or ellipsoidal form, in case the exterior 
 or disturbing force is very small in comparison with 
 the central gravity of the body, and its diameter 
 very small in comparison with the distance of the 
 source of the disturbance. In this way the attraction 
 of the sun and moon produces the ebbing and flow- 
 ing of the tides of the sea, and of the atmosphere. 
 In the case of a comet the mass being small, and 
 the volume extended it is evident that the solar 
 attraction can, at a certain distance, be much greater 
 than that which exists between the different parts 
 of the comet ; and also, that this attraction will be 
 very different in various parts of its volume. In 
 order, therefore, to determine the figure of a comet 
 at a given distance from the sun, it is only neces- 
 sary to find the form which a fluid mass will assume, 
 when the attraction of the sun is comparable with 
 or even greater than the gravitation between its 
 parts, its volume being considered as very great in 
 comparison with its distance from him. Under these 
 conditions the law of equilibrium of a moving mass 
 has been determined, whose physical constitution 
 is supposed to be a nucleus of solid matter sur- 
 rounded by concentric layers of an elastic atmos- 
 phere, with a density continually decreasing from 
 the centre outward. It should be remarked, how- 
 
TAILS OF COMETS. 305 
 
 ever, that no direct solution of the question thus 
 presented has been satisfactorily accomplished ; and 
 it is Simply contended that, as a necessary conse- 
 quence, the densest portion of the comet, supposed 
 to be a fluid mass, would not remain at the centre, 
 but would, on the contrary, approach the sun ; and 
 that, in the case of a comet constituted chiefly of an 
 extremely attenuated vapor, its form would be 
 analogous to that of a column of expansible atmos- 
 phere, having its base supported by the attracting 
 body, the denser layers being always nearest the 
 base. In this way it is supposed that the tail of a 
 comet may be formed, resting in space, isolated and 
 extended, and always sensibly in a direction oppo- 
 site to the sun. The case of a solid body surrounded 
 by a nebulous atmosphere is considered as the limit 
 of a minimum change of form, and that of an 
 atmospheric column as the maximum limit. Be- 
 tween these an infinity of varied forms may be 
 imagined ; and hence this theory has been urged as 
 wholly sufficient to explain all the phenomena which 
 have been observed in the formation of the tails of 
 comets. The sudden and anomalous changes in 
 the vicinity of the nucleus as the comet approaches 
 the sun, are attributed to the influence of the solar 
 heat, which must necessarily produce an enormous 
 dilatation of its mass. It is further contended that 
 the expansion of the comet by heat, and its subse- 
 26* u 
 
306 TREATISE ON COMETS. 
 
 quent condensation or contraction by cold, in con- 
 nection with the operation of the solar attraction, 
 as just explained, are sufficient to account for every 
 phenomenon connected with the structure of comets 
 which has hitherto been observed; and meteoro- 
 logical phenomena on our earth are adduced as 
 beautiful examples of similar effects. This theory 
 has been urged with great force of reasoning, and 
 has, indeed, been received with considerable favor; 
 yet it must be admitted that a detailed examination 
 of the recorded phenomena presented by some of 
 the largest comets, shows conclusively that it is 
 inadequate to their complete explanation. 
 
 A favorite theory of Newton was that the molecules 
 of the circumambient ether, overheated by means 
 of the comet, wilich absorbs the solar rays, and 
 transmits the heat to the ether, would become lighter 
 than its exterior layers, and would, therefore, flow 
 backward in a direction opposite to the sun, carry- 
 ing with them a part of the molecules of the comet. 
 In this way it is imagined that the phenomena of 
 the tails of comets may be explained without resort- 
 ing to a direct repulsive force, thus avoiding com- 
 pletely whatever may seem to complicate and con- 
 fuse the beautiful conception of universal gravita- 
 tion, by the introduction of an opposing force. This 
 theory of the action of the radiant heat of the comet 
 on the ether which surrounds it, and that, previously 
 
TAILS OF COMETS. 307 
 
 stated, which connects the formation of the tails of 
 comets with the combined action of the attraction. of 
 the sun, and the solar heat acting directly on the 
 molecules of the comet, are those which have been 
 classed as belonging to what may be termed appa- 
 rent repulsion. 
 
 Kepler and others have attributed to the light of 
 the sun a repulsive action, sufficient to drive off the 
 lighter portions of a comet in the form of a tail. 
 This is the case of a direct repulsive force, operating 
 like gravity, only in a contrary direction. With 
 this idea of repulsion as a basis, and omitting all 
 considerations in respect to the character of light 
 as not being essentially of any account, in the con- 
 clusions which may be arrived at Faye has 
 endeavored to develop a complete theory of the 
 figure and physical constitution of comets. Before 
 proceeding, however, to give an explanation of. this 
 theory, it may be well to state that Bessel and 
 Olbers have attributed the formation of the tails 
 and envelopes of comets to the operation of a repul- 
 sive force emanating chiefly from the sun, and being, 
 like magnetism and electricity, a polar force. Olbers 
 supposed the repulsive agent to be electricity, and 
 imagined that it acted so as to produce a repulsive 
 force in the sun, a similar force in the comet itself, 
 and a specific action of the sun such as may be 
 supposed to be exhibited in the simultaneous exist- 
 
308 TREATISE ON COMETS. 
 
 ence of several distinct tails. Bessel subjected the 
 theory of Olbers to a rigorous mathematical calcu- 
 lation, introducing, however, such modifications as 
 the exigencies of the case demanded. He supposed 
 the action of the sun on a comet to be exercised in 
 two different ways, namely, by a general action 
 equal for every part, and by a differential action, 
 strongest at those points nearest the sun, and most 
 feeble at those parts most remote from him. This 
 he conceived to be a necessary consequence, no 
 matter what the nature of the force exercised may 
 be. Thus, for example, in the case of the force of 
 gravitation, there is a general action which produces 
 the motion of the centre of gravity, and a differ- 
 ential action which causes a motion of rotation ; the 
 latter being always feebler than the former. He, 
 therefore, supposes that the comet experiences at 
 first the general action of the sun, and that, under 
 this action, it emits from every part of its nucleus 
 particles of matter whose polarity is negative with 
 reference to the sun. On approaching its perihelion, 
 the comet will experience, finally, the second mode 
 of action, and this he imagines to give to it the two 
 poles, the positive pole, with reference to the sun, 
 being on the side nearest that luminary. At this 
 point an emission of nebulous matter will com- 
 mence ; and, since the polarity is positive, this emission 
 will be directed toward the sun. In passing through 
 
TAILS OF COMETS. 309 
 
 the nebulous atmosphere which is aiready filled 
 with negative particles, and which is continually 
 forming anew, the nebulous matter emitted will 
 lose, by degrees, its primitive polarity, and even 
 assume, finally, the opposite polarity, before it 
 reaches the limit of this atmosphere. This emission 
 from the nucleus will now be repelled by the sun, 
 and driven back to form the tail. This is a general 
 statement of Bessel's theory as modified from that 
 of Olbers. 
 
 The phenomenon of the regular formation of 
 luminous sectors, of which the great comet of 1811 
 furnished so beautiful an example, led Olbers to 
 conclude, without hesitation, that the hypothesis 
 made by Kepler and Euler, which we have already 
 stated, was necessarily untenable, since the solar 
 repulsion could not produce an emission directed 
 from the nucleus itself toward the sun. It was for 
 this reason that he considered it necessary to endow 
 both the comet and the sun with repulsive forces, 
 which he attributed to a common cause, namely, to 
 the electricity which might be supposed to be de- 
 veloped by the approach of the comet to the sun. 
 The periodic, or nearly periodic fluctuations of the 
 luminous sectors exhibited in the case of Halley's 
 comet, at its last return, led Bessel to assume the 
 existence of a polar force developed in the comet 
 under the influence of the sun, and in this way to 
 
310 TREATISE ON COMETS. 
 
 attempt to reconcile all the varied phenomena 
 which have been observed. Faye, adopting the 
 fundamental idea of a repulsive force, the existence 
 of which he considers to be conclusively established, 
 attempts to explain not only the figure of the comets, 
 but also the acceleration of their motion, which we 
 have stated to be due to the resistance of an ethereal 
 fluid pervading all space. He attributes to the 
 force of repulsion a mode of propagation, not in- 
 stantaneous like that of gravitation, but in regular 
 succession, like the undulations of the ether in pro- 
 ducing light. He supposes the repulsive force to 
 be resolved into two component parts, one acting 
 in the direction of the line joining the centre of the 
 sun with that of the comet, and the other in the 
 direction of a line tangent to the orbit of the comet 
 at the point where it is situated. The first com- 
 ponent force may be supposed to determine the 
 figure of the comets, in chiding the formation of 
 the tail, while the second may act to produce the 
 acceleration of their motions. The repulsive force 
 being supposed to be propagated by a series of suc- 
 cessive undulations, it follows that it will not depend 
 directly on the mass of the sun, but on the extent 
 of his surface and its physical condition. The in- 
 tensity of its action on any body whatever will vary 
 inversely as the square of its distance. In this way, 
 Faye considers it possible to give n most simple and 
 
TAII<S OF COMETS. 311 
 
 natural explanation of the phenomena which led to 
 the hypotheses of Gibers and Bessel, thus avoiding 
 a metaphysical objection, which he contends may 
 be justly urged against the existence of a force such 
 as these astronomers have assumed. The objection 
 referred to is that these hypotheses attribute simul- 
 taneously to the same matter a repulsive force and 
 an attractive force of the same nature, the same 
 direction, and the same law, with the exception 
 only of the specific and limited operation of the 
 former, which distinguishes it from the universality 
 of the latter. 
 
 In further confirmation of his theory, Faye adduces 
 the fact that the theories of Olbers and Bessel seem 
 to present almost insuperable difficulties in the 
 explanation of the phenomena, which completely 
 vanish when we adopt the simple and unique theory 
 of Kepler, which he has adopted as the basis of his 
 own. He regards the modifications of the original 
 theory of Olbers introduced by Bessel as extremely 
 complicated, in which it is supposed that, under the 
 influence of the sun, the nucleus acquires polarity, 
 and emits, in the direction of the sun, particles which 
 are negatively electrified, while, at the same time, 
 the sun exercises, on this part of the comet, a 
 positive action. He regards it as an important 
 objection to Bessel's theory, that, in order to explain 
 the manner in which these particles or molecules 
 
312 TREATISE ON COMETS. 
 
 of nebulous matter, notwithstanding their negative 
 polarity, cease to be attracted by the sun, and finally 
 become subject to an energetic repulsion on the 
 part of the sun, which drives them back from him, 
 it becomes necessary to suppose that by virtue of a 
 general action of the sun while the comet is yet 
 far distant, and before the development of the 
 polarity the part of the comet from which the 
 emission takes place has been positively polarized 
 like the sun. He also cites the fact that Bessel 
 admitted that the molecules which were thus differ- 
 ently polarized will become neutralized, and that 
 the particles emitted by the nucleus finally lose their 
 negative polarity, and assume a positive one, even 
 before they are very distant from the nucleus, and 
 ultimately become subject to the repulsive influence 
 of the sun, being driven back to form the tail. The 
 intensity of this force of repulsion will, it is urged, 
 vary with the nature of these molecules. In the 
 case of Halley's comet, at its last return, its mole- 
 cules were repelled by the sun by a force nearly 
 twice as great as that of attraction ; and those of 
 Donati's comet (1858) by a force whose intensity 
 was a little more than one-third that of gravity in 
 the case of the principal tail, and by a force nearly 
 seventeen times greater in the case of the secondary 
 train. The difficulty which Faye thinks very im- 
 portant, is to explain why the sun should exercise 
 
TAILS OF COMETS. 313 
 
 on the different particles of the comet two separate 
 and distinct repulsive forces of such unequal in- 
 tensity. This, however, has been explained by 
 supposing that different parts of the comet had a 
 vastly different specific gravity, being actually lighter 
 than the surrounding ether, and that these particles 
 are thus repelled through the ether with very differ- 
 ent velocities which was essentially the idea of 
 Newton. 
 
 Soon after the appearance of the great comet of 
 1843, Professor Norton proposed a theory of the 
 formation of the tails of comets which in some 
 respects resembled the theories of Bessel and Olbers. 
 He was led to the final hypothesis which he formed 
 by such considerations as the following, having been 
 entirely ignorant of the fact that in some points of 
 his theory he had been anticipated by others. 1st. 
 The general direction and situation of the tail with 
 respect to the sun, show that this luminary is con- 
 cerned, directly or indirectly, in its formation, while 
 the changes which take place in the dimensions and 
 figure of the comet, both in approaching its peri- 
 helion, and in receding from it, lead to the same 
 inference. 2d. Since the tail lies in the direction 
 very nearly of a line drawn from the centre of the 
 sun through that of the comet, the particles of 
 matter of which it is composed must have been 
 driven off from the head by some force exerted in a 
 27 
 
314 TREATISE ON COMETS. 
 
 direction from the sun. 3d. This force cannot 
 emanate from the nucleus, for such a force would 
 expel the nebulous matter surrounding the nucleus 
 in all directions, instead of one direction only. This 
 objection, however, might be removed by supposing 
 that a repulsive action is exerted by the nucleus 
 only from that side which is most remote from the 
 sun ; yet it may be perceived that such a force as 
 this would not give to the tail the form and direction 
 which observations indicate, when it is considered 
 that the tail has its origin in the nebulosity at the 
 side of the nucleus. It is thus evident that no one 
 force, having its origin in the head of the comet, 
 can be conceived of which would be adequate to 
 the production of the tail. There is, however, a 
 possibility that a repulsive action of the nucleus on 
 the matter of the nebulosity may be combined w T ith 
 some other force foreign to the comet, as an auxiliary 
 cause in producing the phenomena of the tail ; 
 its effects on the side toward the sun being con- 
 tracted, at a definite distance from the nucleus, by 
 this latter force. 4th. It seems to follow from what 
 has just been stated, as a necessary consequence, 
 that the matter of the tail is driven off from the 
 head by some force foreign to the comet, and taking 
 effect from the sun outward. 5th. This force, what- 
 ever its nature may be, must be supposed to extend 
 far beyond the orbit of the earth ; and although 
 
TAILS OF COMETS. 315 
 
 nothing can be predicated with certainty respecting 
 the law of its variation, yet it is at least probable 
 that, like all known central forces, it varies inversely 
 as the square of the distance. 
 
 From such considerations as these Korton was 
 led to the conclusion that there existed a repulsive 
 force in the sun, and that although it might operate 
 so as to drive off the nebulous matter to greater 
 and greater distances, without destroying the con- 
 nection of the parts the head and tail thus revolving 
 as one connected mass or, so as continually to 
 detach portions of the nebulosity, and repel them 
 to an indefinite distance into free space ; yet that it 
 was most probable that the latter mode of action 
 was the one which actually exists. After a long 
 train of reasoning based on the phenomena of the 
 comets which had hitherto appeared but which it 
 will be unnecessary to give in this connection he 
 arrived finally at the conclusion that the tail of a 
 comet is made up of particles of matter continually 
 flowing away, at a very rapid rate, from the head 
 into free space, and that at any one instant we see 
 the collection of all the particles that have been 
 emitted during a certain previous interval. Accord- 
 ing to this hypothesis, at the end of any such 
 interval we are looking at an entirely new tail. The 
 particles may be supposed to be detached from the 
 outer portions of the nebulosity of the comet, by 
 
316 TREATISE ON COMETS. 
 
 the repulsive force of the sun ; in which case they 
 would fly off in the directions of the lines diverging 
 from the sun, along which the force acts, hut would 
 be made hy the attraction of the nucleus to pursue 
 paths slightly deviating from these lines and con- 
 cave toward the axis of the tail ; or, as Bessel and 
 Olbers have supposed, they may undergo some 
 modification hy the action of the sun, by reason of 
 which they are first repelled outward from the 
 nucleus, and then driven away from the sun, into 
 the depths of space, by virtue of his superior repul- 
 sion. According to the latter supposition, they 
 acquire an initial velocity in leaving the nucleus, 
 and subsequently, under the action of the sun's 
 repulsive force, they will move off in hyperbolas, 
 having the sun in their remote focus, and concave 
 toward the axis of the tail. 
 
 By means of this theory Norton was enabled to 
 explain the curved form of the tail, and the general 
 fact of its deviation from the position of direct 
 opposition to the sun ; and also to indicate the 
 causes in operation to produce the various degrees 
 of curvature, and various positions of the tail, 
 noticed in the case of different comets, and in 
 different positions of the same comet in its orbit. 
 The appearance of Donati's comet in 1858 induced 
 him to modify his theory in some particulars, and 
 to reduce it to a complete and systematic form. The 
 
TAILS OF COMETS. 317 
 
 phenomena of successive envelopes presented by 
 this comet to the most remarkable degree, led him 
 to conclude that the nebulous envelope of the head 
 of a comet was probably in the same dynamical con- 
 dition with the tail ; that the cometic matter was, in 
 the first instance, expelled from the nucleus on the 
 side toward the sun, and then driven back to form the 
 tail, by virtue of the repulsive force emanating from 
 the sun; and that this process went on continuously; 
 the nebulous matter rising perpetually, like a gush- 
 ing fountain of light, and the luminous jets bending 
 back, in graceful curves, before the repelling energy 
 of the sun. He, therefore, regarded the nebulous 
 envelope of the head, and the tail of a comet, as 
 simply different parts of the same flowing stream ; 
 and imagined that the cometary matter was, in the 
 first place, urged away from the nucleus by an 
 incessant force of repulsion exerted by its mass, and 
 that this force and the repulsive action of the" sun, 
 like all radiant actions, varied inversely as the square 
 of the distance. Other suppositions were also made 
 as that the expelling action was an instantaneous 
 repulsive force ; and again, that the nebulous matter 
 was first detached by a superficial force of projection, 
 and subsequently acted on by the repulsion of the 
 mass of the nucleus. By means of this theory, 
 Professor Norton was enabled to explain all the 
 phenomena presented by the great comet of 1858, 
 27* 
 
318 TREATISE ON COMETS. 
 
 and also to derive an approximate value of the density 
 of the matter composing the nucleus of the comet. 
 He found that the nucleus of this comet was of from 
 five to six times the density of water, or about the 
 same as the mean density of the earth. He per- 
 formed similar calculations in the case of four other 
 comets, and found that their density is from four to 
 fourteen times that of water. These results may 
 seem to be at variance with the generally received 
 opinion in regard to the physical constitution of 
 these bodies ; and it must be admitted that they are 
 at most but rude approximations; yet, he con- 
 sidered them as affording sure indications that the 
 telescopic nucleus of a bright comet is not entirely 
 gaseous, or nebulous, but that it is either liquid or 
 solid, or composed of both liquid and solid matter, 
 like the earth. 
 
 He accounted for the rise and gradual recess of a 
 succession of envelopes from the nucleus, by sup- 
 posing that the evolution of cometic matter first 
 begins at the polar regions of the nucleus, and 
 gradually extends toward the equatorial parts, the 
 comet being supposed to rotate about an axis per- 
 pendicular to the plane of its orbit. Under these 
 circumstances, if the comet is observed under a 
 large angle to the plane of its orbit, as was the case 
 with Donati's comet, after the perihelion passage, 
 the outline of the visible envelope would steadily 
 
TAILS OF COMETS. 319 
 
 enlarge, as the process of evolution was propagated 
 farther and further from the polar regions of the 
 nucleus. In this explanation, he supposes that the 
 envelopes are not spherical in form, but that each 
 approximates in its form to that of a flat semicir- 
 cular disc, of a definite thickness. The dark stripe 
 extending from the nucleus to a considerable dis- 
 tance in the tail, which we have already noticed in 
 the description of Donati's comet, is supposed to 
 show conclusively that the envelopes from which 
 the tail proceeded were much flattened in the 
 direction of the line of sight ; or approximately in 
 that direction. 
 
 Such are the principal theories which have been 
 devised, in order to explain all the anomalous 
 appearances presented by the comets ; and a careful 
 consideration of these different hypotheses in con- 
 nection with observed facts, will make it evident 
 that there exists in nature a general force of repul- 
 sion, exerted by all masses, and operating tinder 
 certain circumstances upon matter in an extremely 
 attenuated state. The idea of repulsion being thus 
 established on a firm basis, as a physical fact, it is 
 only necessary to determine which of the various 
 theories of the mode of operation of the repulsive 
 force will best explain all the varied phenomena of 
 cornets in general. The hypotheses of Bessel, Olbers, 
 and Norton, are essentially the same, and differ only 
 
320 TREATISE ON COMETS. 
 
 in certain supposed peculiarities of the force of 
 repulsion, and in its mode of operation. Bessel was 
 the first to apply a vigorous mathematical analysis 
 in the determination of the effects of such a force 
 as is found to exist. The modifications which he 
 found it necessary to introduce into Gibers' theory, 
 in order to explain the phenomena observed in the 
 case of Halley's comet, at its last return, have been 
 objected to as rendering the results contradictory 
 when an attempt is made to derive the intensity of 
 the forces in operation. The appearance of the 
 great comet of 1858 under circumstances peculiarly 
 favorable to the final determination of the theory 
 of the formation of the tail by means of a repulsive 
 force common to all bodies, required simply that 
 the various theories advanced should be subjected 
 to the test of exact mathematical calculation. This 
 was performed by Professor Pierce in the case of 
 Gibers' hypothesis, and the results obtained were in 
 such strict accordance with the observations, that 
 this theory is now regarded as being established on 
 the surest possible foundation. 
 
 There is an inference which may be drawn from 
 an examination of the theory of the operation of a 
 repulsive force in the formation of the tail of a 
 comet, which is, that some portions, at least, of the 
 nebulous matter will be detached entirely from the 
 comet, and driven off into space. Under this view 
 
TAILS OP COMETS. 321 
 
 it may be supposed that the comets are gradually 
 wasting away, and will finally be extinguished. 
 Observations, however, do not by any means render 
 it certain that these bodies are thus affected, and 
 the necessity of such a supposition may perhaps be 
 obviated by regarding the ethereal fluid which per- 
 vades all space as of sufficient density to afford a 
 sensible resistance to the flow of the nebulous matter 
 from the head of the comet, thus limiting the extent 
 of the tail. There are, indeed, questions here pre- 
 sented which we are unable to answer satisfactorily, 
 and which require a long series of observations of 
 the successive returns of a comet, for their elucida- 
 tion. It may be that the comets are thus wasting 
 away, but that in their wanderings through endless 
 space they casually assimilate new cometic matter 
 from the waste of other comets, and thus undergo a 
 series of perpetual changes. It may also happen 
 that two different comets may approach each other 
 near enough to cause their separate masses to 
 coalesce, thus furnishing abundant material for 
 future losses. These and similar questions will 
 undoubtedly be finally solved, though the observa- 
 tions hitherto recorded fail to give a satisfactory 
 answer. 
 
 Another consideration which may appear in this 
 connection, is, whether the tail of a comet, being 
 composed of attenuated nebulous matter, can offer 
 
322 TREATISE ON COMETS. 
 
 any sensible resistance to the regular motion of the 
 centre of gravity of the nucleus. In observing these 
 bodies, their places on the celestial vault are deter- 
 mined under the supposition that the point of 
 greatest brilliancy corresponds to the centre of 
 gravity of the comet. The elements of the orbit 
 are computed under this hypothesis, and it might 
 De expected that the predicted places would not be 
 found to correspond with the actual observations. 
 In the case of all the comets, this is to a greater or 
 less extent true, but the deviations have usually 
 been attributed to the necessary uncertainty of the 
 observations. In the case, however, of Donati's 
 comet, it is found that it is absolutely impossible to 
 represent the entire series of observations by a 
 purely elliptic orbit, and that the discrepancies 
 between computation and observation are such as 
 to indicate beyond a doubt that the motion of the 
 nucleus in its orbit is sensibly affected by the tail, 
 or by the joint action of this and the repulsive forces. 
 The question may also arise as to the nature or 
 cause of the repulsive force whose existence is thus 
 so clearly established. The near approach of a 
 comet to the sun, under an ever-varying tempera- 
 ture, would necessarily produce, when near the sun, 
 the most intense electrical excitement ; and we may 
 thus infer that the force partakes in some degree of 
 the nature of electrical action. The existence of 
 

 TAILS OF COMETS. 323 
 
 an ethereal fluid pervading space is beyond all 
 doubt, and it is certainly known that light is pro- 
 duced by certain vibrations or pulsations, through 
 this ether, an effect which is imparted, to a greater 
 or less extent, by all material bodies. The develop- 
 ment of electric currents, as well by magnetic as 
 by electrical action, the similarity in their mode of 
 action in a great variety of circumstances, and 
 especially the production of the spark from a magnet, 
 the ignition of metallic wires, and chemical decom- 
 position, all tend to show that magnetism and elec- 
 tricity are to be regarded as identical. The evolu- 
 tion of light and heat during the passage of the 
 electric fluid, the development of electricity by heat, 
 the influence of heat on magnetic bodies, and that 
 of light on the vibration of the compass, show 
 decided indications of some common and invisible 
 bond uniting these mysterious but powerful agents 
 of nature. The fact that these agencies operate 
 each with an intensity varying in accordance with 
 the same law, is a further proof of their common 
 origin. The solar spots, it may likewise be re- 
 marked, are found to be connected intimately with 
 terrestrial magnetism, and in view of all these facts, 
 it seems reasonable to conclude that light, heat, and 
 electricity, are all produced by successive undula- 
 tions in the ethereal fluid just as sound is caused 
 by vibrations communicated to our fluid atmosphere, 
 
324 TREATISE ON COMETS. 
 
 yet under different circumstances with respect to 
 the operation of the cause. 
 
 Having thus considered in detail the various 
 theories of the physical constitution of these wan- 
 dering bodies, it may not be improper to add a few 
 remarks in reference to their probable origin. La- 
 place, following the speculations of Sir William 
 Herschel, applied the nebular theory of that astro- 
 nomer to the formation of the solar system, com- 
 prehending the comets as well as the planets and 
 their satellites. He supposes that in the primeval 
 condition of the solar system, the sun revolved on 
 his axis, surrounded by an atmosphere which, on 
 account of an excessive heat, extended far beyond 
 the orbit of the remotest planets. The heat gradu- 
 ally diminished, and as the solar atmosphere con- 
 tracted by cooling, the rapidity of his rotation be- 
 came increased, and the exterior portions of the 
 nebula by which he was surrounded, would be 
 detached from the rest, the central attraction being 
 no longer able to overcome the increased centrifugal 
 force. This ring of nebulous matter would, from 
 various causes, be broken into fragments forming 
 a planet and satellites, or, which is more probable, 
 the fragments would coalesce into one nebular mass, 
 which would revolve around the sun in an orbit 
 nearly, if not quite circular, lying in a plane nearly 
 coincident with the plane of the equator of the 
 
ORIGIN OF COMETS. 325 
 
 central body, and revolving in its orbit in the same 
 direction in which the central globe rotates on its 
 axis. The same process would again take place 
 when the central mass had again parted with a 
 sufficient quantity of heat, and thus the successive 
 planets may be supposed to have been formed in a 
 state of vapor, the process continuing until the 
 cohesion of the particles of the central mass were 
 finally able to resist any further change. These 
 planets, in their turn, having each its motion of 
 rotation, would, as they became gradually cooled 
 and condensed, produce satellites in the same 
 manner, and by the operation of the same laws, by 
 virtue of which they were themselves formed from 
 the nebulous matter surrounding the sun. It may 
 be perceived also that the motions of the satellites 
 thus produced, and the motions of rotation of the 
 planets, must be in the same direction. Thus may 
 this strange and apparently fanciful theory be made 
 to account for the most remarkable circumstances 
 in the structure of the solar system. The motions 
 of the planets in the same relative direction, and 
 almost in the same plane ; the motions of the satel- 
 lites in the same direction as those of the planets; 
 the small eccentricity of the orbits of the planets, 
 upon which condition, together with others to be 
 noticed in a subsequent connection, the stability of 
 the system depends ; and the position of the source 
 28 
 
326 TREATISE ON COMETS. 
 
 of light and heat in the centre of the system, are 
 all satisfactory by this hypothesis. This is the 
 nebular hypothesis of Laplace, and although it was 
 originally advanced as simply a theory of the de- 
 velopment of the solar system from one primitive 
 mass of nebulous matter, yet the facility with which 
 it explains various phenomena connected with the 
 structure of the system has induced many to regard 
 it as strictly true. 
 
 Laplace supposed that in the very beginning of 
 the creation of our system, the sun consisted of a 
 diffused and extremely attenuated nebulous matter, 
 extending over a vast extent of space. This nebu- 
 lous matter may have been so exceedingly attenuated 
 that its existence was barely a fact. As a confirma- 
 tion of this view of the case, he sought an analogy 
 in those nebulae which are now seen here and there 
 throughout the entire heavens, so feeble and diffused 
 as to present the appearance simply of an ill-defined 
 mass of vapor when seen through the most powerful 
 telescopes. He imagined that in the vast assem- 
 blage of irresolvable nebulae by which is meant, 
 those which the most powerful telescopes fail to 
 reduce to a group of minute stellar points of light, 
 or individual stars there were examples of similar 
 processes of development now going on in various 
 parts of the universe. In the first place we see the 
 nebulous matter dispersed in patches in the different 
 
NEBULAR HYPOTHESIS. 327 
 
 parts of the sky, while in some of these it is sup- 
 posed that traces are exhibited of this matter feebly 
 condensed around one or more faint nuclei. In 
 other cases these nuclei are brighter in proportion 
 to the surrounding nebulosity, and further on, in 
 the apparent progress of development, there are 
 supposed examples of the condensation of the atmo- 
 sphere of each nucleus, thus exhibiting a collection 
 or aggregation of nebulous stars, formed by brilliant 
 nuclei very near each other, and each surrounded 
 by an atmosphere. Still further on in the process 
 of development the nebulous matter, by condensing 
 uniformly, is supposed to form nebulous systems 
 which are called planetary; and, finally, a much 
 greater degree of condensation transforms all these 
 nebulous systems into stars. If this hypothesis be 
 true, then will all those nebulse which are now 
 irresolvable, even in the most powerful telescopes, 
 be ultimately transformed into stars, while it may 
 be inferred, from a similar course of reasoning, that 
 the anterior or primeval condition of the stars which 
 now exist, was that of nebulous matter highly 
 attenuated and distributed throughout space. This 
 is the most general view of the nebular hypothesis, 
 and granting what has been stated, we may conclude 
 that at this very moment creation is going on in 
 distant parts of the universe, and will continue for 
 ever. It must not be objected that there is anything 
 
328 TREATISE ON COMETS. 
 
 in this view of the creation which militates against 
 revealed religion. To supply the primitive matter, 
 and endow it with such properties as must have 
 been required to effect the various transformations 
 from inanimated existence to animated existence of 
 the highest order, must indicate the power and 
 presence of the Almighty Creator. It may also be 
 objected that in all such speculations we enter the 
 confines of the unknown; yet the anxious search 
 for truth which characterizes the human mind must 
 be considered a sufficient excuse for thus wandering 
 into the realms of the invisible and uncertain. 
 
 According to the nebular hypothesis of the forma- 
 tion of new worlds from original and primitive 
 masses of matter, the comets have their origin in 
 portions of the nebulous matter occupying positions 
 intermediate between two or more great centres. 
 These masses of attenuated matter are supposed to 
 be held in a state of equilibrium, until, finally, the 
 attraction of some one centre predominates, and 
 the uncondensed filmy mass begins to descend 
 slowly toward its controlling centre. The fact that 
 the comets come into our system in all possible 
 directions, and in orbits which conduct them far 
 beyond the limits of our system, is in itself conclu- 
 sive evidence to show that they wander here and 
 there among the various systems constituting the 
 universe. This would naturally be supposed to be 
 
NEBULAR HYPOTHESIS. 329 
 
 the case if they were formed in accordance with the 
 nebular hypothesis, their unconden,sed condition 
 being due to the fact that the feeble gravitation of 
 their parts toward the centre of the mass is inade- 
 quate to contract them into a more solid form ; and 
 also that, in passing near the suns which they visit, 
 they are necessarily converted by the intense heat 
 into an extremely vaporous state. It may be that 
 in the course of ages the effects of the cold in the 
 remote parts of their orbits would solidify them to 
 such an extent that in future they might retain, to 
 a limited extent, a solid form ; the exterior portions, 
 however, of the compact nucleus, in their successive 
 revolutions, going through alternate changes of 
 solidification and evaporation. We may remark also 
 that the idea of attributing the origin of comets to 
 the nebular hypothesis, does not in* the least interfere 
 with or modify the theory of the formation of their 
 tails already given. 
 
 We have now given explanations of the principal 
 phenomena presented by the comets directly, which 
 the researches of astronomers have indicated, and 
 thus far our task is accomplished. There is, how- 
 ever, one other consideration, which the study of 
 the motions of these wonderful bodies has presented, 
 and to which we shall next advert; namely, the 
 existence of a medium of resistance pervading space, 
 and sensibly opposing the motions of the heavenly 
 28* 
 
330 TREATISE ON COMETS. 
 
 bodies, at least those which are known as cometary 
 worlds. We cannot, however, conclude the con- 
 sideration of this branch of our subject without re- 
 marking further, that the phenomena of the comets 
 are the most varied, the most beautiful, and, at the 
 same time, fantastical, which the heavens present to 
 our view. The peculiar forms of their orbits, the 
 suddenness of their appearance, and the develop- 
 ment of their physical changes as they approach the 
 sun and again recede into illimitable space, all tend 
 to afford a sublime conception of the grandeur of 
 God's creation. Even if we ever feel inclined to 
 doubt in regard to the endless extent of the universe, 
 the circumstances which are so undeniably proved 
 to be connected with their motions, are sufficient to 
 convince us of the reality of infinity. Following, 
 therefore, these mysterious bodies in our imagination 
 in their endless wanderings through space, we may 
 perhaps be prepared to realize, to some extent, the 
 beautiful yet true conception of that celebrated 
 dream of Eichter : 
 
 "God called up from dreams a man into the 
 vestibule of heaven, saying, i Come thou hither, and 
 see the glory of my house.' And to the servants 
 that stood around his throne he said, ' Take him, 
 and undress him from his robes of flesh : cleanse 
 his vision, and put a new breath into his nostrils ; 
 only touch not with any change his human heart 
 
NEBULAR HYPOTHESIS. 331 
 
 the heart that weeps and trembles.' It was done: 
 and, with a mighty angel for his guide, the man 
 stood ready for his infinite voyage ; and from the 
 terraces of heaven, without sound or farewell, at 
 once they wheeled away into endless space. Some- 
 times with solemn flight of angel wing they fled 
 through Zaarrahs of darkness, through wildernesses 
 of death, that divided the worlds of life ; sometimes 
 they swept over frontiers that were quickening 
 under prophetic motions from God. Then, from a 
 distance that is counted only in heaven, light dawned 
 for a time through a sleepy film; by unutterable 
 pace the light swept to them, they by unutterable 
 pace to the light. In a moment the rushing of 
 planets was upon them : in a moment the blazing 
 of suns was around them. 
 
 " Then came eternities of twilight, that revealed, 
 but were not revealed. On the right hand and on 
 the left towered mighty constellations, that by self- 
 repetition and answers from afar, that by counter- 
 positions, built up triumphal gates, whose archi- 
 traves, whose arch-ways horizontal, upright 
 rested, rose at altitude by spans that seemed 
 ghostly from infinitude. Without measure were 
 the architraves, past number were the arch-ways, 
 beyond memory the gates. Within were stairs that 
 scaled the eternities below ; above was below 
 below was above, to the man stripped of gravitating 
 
332 TREATISE ON COMETS. 
 
 body: depth was swallowed up in height insur- 
 mountable, height was swallowed up in depth un- 
 fathomable. Suddenly, as thus they rode from in- 
 finite to infinite, suddenly, as thus they tilted over 
 abysmal worlds, a mighty cry arose that systems 
 more mysterious, that worlds more billowy other 
 heights and other depths were coming; were near- 
 ing ; were at hand. 
 
 " Then the man sighed, and stopped, shuddered, 
 and wept. His overladened heart uttered itself in 
 tears ; and he said 'Angel, I will go no further. 
 For the spirit of man acheth with this infinity. 
 Insufferable is the glory of God. Let me lie down 
 in the grave, and hide me from the persecution of 
 the infinite; for end, I see, there is none.' And 
 from all the listening stars that shone around issued 
 a choral voice, ' The man speaks truly : end there is 
 none that ever yet we heard of.' 'End is there 
 none?' the angel solemnly demanded: 'Is there 
 indeed no end ? and is this the sorrow that kills 
 you ? ' But no voice answered that he might answer 
 himself. Then the angel threw up his glorious 
 hands toward the heaven of heavens, saying, 'End 
 is there none to the universe of God. Lo ! also 
 there is no beginning.' " 
 
RESISTING MEDIUM IN SPACE. 333 
 
 CHAPTER V". 
 
 ON THE EXISTENCE OF A RESISTING MEDIUM IN SPACE CHARACTER 
 
 OF THE RESISTING MEDIUM ITS EFFECT ON THE MOTIONS OF THE 
 
 HEAVENLY BODIES THE STABILITY OF THE SOLAR SYSTEM 
 INFLUENCE OF THE COMETS GENERAL REMARKS. 
 
 THE question of a plenum and a vacuum has en- 
 gaged the attention of philosophers from the earliest 
 ages of the world. In their speculations respecting 
 the constitution of the universe, it often became a 
 matter of doubt whether the celestial and terrestrial 
 spaces are absolutely full, or whether there are, be- 
 tween and among the material parts of the universe, 
 empty spaces free from all matter. The general 
 opinion, however, seems to have been that the 
 regions around us, beyond our atmosphere, and to 
 an indefinite extent, are supplied with a rare invisi- 
 ble medium, of unknown composition and character, 
 in which all bodies are compelled to move. In the 
 astronomy of the Brahmins of India, which dates 
 its origin long before the commencement of the 
 Christian era, the existence of such a medium is 
 assumed, though the stars were by them supposed 
 to move in it like fishes in water. They regarded 
 it as a celestial element, pure and impalpable, and 
 
334 TREATISE ON COMETS. 
 
 offering no resistance to the motions of the heavenly 
 bodies. Indeed, almost every ancient astronomer 
 held similar opinions. Kepler, notwithstanding his 
 profound knowledge of astronomy, in seeking the 
 origin of comets, supposed them to he native in- 
 habitants of this fluid medium, as the fishes are of 
 the waters of the earth ; and that they were thus 
 created to inhabit the immense spaces of the uni- 
 verse. The sombre and bloody appearance which 
 the sun sometimes exhibits, he attributed to a 
 coagulation of the ethereal fluid, and the cessation 
 of these appearances he supposed to be due to the 
 collection of the denser portions of the ether in the 
 form of comets. 
 
 The theory of an ethereal medium throughout all 
 space remained thus until the development of a 
 system of the world by Descartes. He supposed 
 that matter possessed only the properties of ex- 
 tension, impenetrability, and inertia; that it filled 
 all space ; and that it was, in its parts, both great 
 and small, endued with motion in an infinity of 
 directions. He imagined that the matter would 
 thus be formed into innumerable vortices, differing 
 not only in extent, but also in velocity and density ; 
 the more attenuated parts constituting the real 
 vortex, in which the denser bodies float, and by 
 which they are pressed, though not equally, on all 
 sides. He therefore regarded the universe as con- 
 
RESISTING MEDIUM IN SPACE. 835 
 
 sisting of innumerable vortices, which limit and 
 circumscribe one another, and supposed that the 
 earth and planets are bodies carried round in the 
 great vortex of the solar system. He supposed also, 
 that, by the pressure of the attenuated matter which 
 he had assumed to fill all space, and which circu- 
 lates with great rapidity and great centrifugal force, 
 the denser bodies, which have less rapidity and less 
 centrifugal force, are forced toward the sun, the 
 centre of the vortex. In like manner, each planet 
 was itself regarded as the centre of a small vortex, 
 by the operation of which all the phenomena of 
 gravity would be produced. This is the Cartesian 
 system of the world, a theory which was very gene- 
 rally received for nearly half a century from the 
 date of its promulgation ; and it is evident that the 
 existence of an ethereal fluid pervading all space is 
 here adopted as the basis of the theory. The glaring 
 defects, however, of the doctrine of Descartes, were 
 conclusively established by the subsequent disco- 
 veries of Newton and Kepler. It was shown by the 
 former, that the planetary and cometary bodies re- 
 volve around the sun by virtue alone of the force 
 of gravitation, and the original force of projection 
 imparted to them at the time of their formation. 
 That this theory was correct, the complete verifica- 
 tion of Kepler's laws exhibited the most incontesta- 
 ble evidence, while the subsequent development of 
 
336 TREATISE ON COMETS. 
 
 the great structure which modern astronomy forms, 
 has completely verified and generalized it in every 
 particular. 
 
 The Newtonian theory of gravitation was con- 
 ceived by some to require that the motions of the 
 heavenly bodies should be performed, according to 
 mechanical laws, in space which was absolutely a 
 vacuum ; and hence, that the doctrine of an ethereal 
 medium of any conceivable density was inadmissi- 
 ble. This, however, is by no means a necessary 
 consequence of the law of universal gravitation, and 
 was not so considered by Newton himself. He be- 
 lieved firmly in the existence of a subtile ethereal 
 fluid pervading all space, and even the internal parts 
 of all material bodies. He supposed that possibly 
 it was this medium which furnished the means for 
 the operation of the gravitating force. The ethe- 
 real fluid was conceived of as a sort of spirit, by the 
 force and action of which the particles of bodies 
 mutually attract each other at near distances, and 
 cohere if contiguous. He imagined that possibly 
 this medium is much rarer within the dense bodies 
 of the sun, stars, planets, and comets, than in the 
 empty spaces between them; and that in passing 
 from them to great distances, it became gradually 
 denser and denser, thus, it may be, causing the 
 gravity of these bodies toward each other, and of 
 their denser parts toward a centre ; every body tend- 
 
RESISTING MEDIUM IN SPACE. 337 
 
 ing* continually to move from the denser portions 
 of the medium toward the rarer. These hypotheses 
 in regard to the existence, nature, and effect of the 
 ethereal fluid, were not, however/advanced by Few- 
 ton as any part of his theory of universal gravita- 
 tion, but simply as a possible method of explaining 
 the operation of the gravitating force. This was 
 all that it was necessary for him to maintain, since 
 his theory of the system of the world was founded 
 exclusively on observations, and had nothing to do 
 with abstract possibilities and metaphysical neces- 
 sities. 
 
 The motions of the planets computed strictly in 
 accordance with the law of gravitation, and under 
 the supposition that they move in an absolute 
 vacuum, or, at least, in a space void of any resisting 
 fluid, were found to accord so closely with observa- 
 tion, that it was for some time doubted whether any 
 such medium as Newton and others had suspected, 
 really existed. The phenomena of the planetary 
 motions were so accurately predicted by the New- 
 tonian theory of gravitation, that it became evident 
 either that there was no such fluid, or that it was so 
 extremely attenuated or rarefied, that no phenome- 
 non which had hitherto been observed was capable 
 of exhibiting its effects. In order, however, to test 
 this question thoroughly, the Academy of Sciences, 
 of Paris, in 1762, offered a prize for the solution of 
 29 w 
 
338 TREATISE ON COMETS. 
 
 the following definite problem : "Do the planets re- 
 volve in a medium of which the resistance produces a 
 sensible effect upon their movements ?" The prize was 
 awarded to Bossut, who found, by calculation, that 
 the effect of any such resistance, offered to the 
 planets, would be to diminish the greater axis of 
 their orbits, and, consequently, to shorten their 
 periods of revolution. The moon was known to 
 revolve in periods which were becoming shorter and 
 shorter, and he concluded that the observed accelera- 
 tion was due to the resistance of the ether. He 
 applied the same reasoning in the case of the planets, 
 and although no such acceleration could be detected, 
 yet he conjectured that in the course of time similar 
 effects would be exhibited. It should be remarked, 
 however, that up to the present time no acceleration 
 of the planets, due to action of a resisting medium, 
 has been detected; and the acceleration of the 
 moon's mean motion is explained thoroughly and 
 satisfactorily by the changes which the perturba- 
 tions by the sun and planets are producing in the 
 eccentricity of the earth's orbit thus showing con- 
 clusively, that so far as the motions of the planets 
 and their satellites are concerned, we have no evi- 
 dence of the existence of a resisting ethereal fluid 
 in space. 
 
 We have already stated the fact that light is cer- 
 tainly known to be produced by successive undula- 
 
RESISTING MEDIUM IN SPACE. 339 
 
 tions or pulsations in an extremely attenuated and 
 elastic ethereal fluid, which pervades all space ; and 
 that heat and electricity may with great probability 
 be attributed to the operation of the same cause, 
 only under different circumstances. Such being the 
 case, it seems evident that this ethereal fluid will 
 oppose itself to the motions of the heavenly bodies 
 in their orbits, and that its effect on the motions of 
 the comets would be first exhibited, since the ex- 
 tremely vaporous matter of which they are com- 
 posed would be most readily operated upon by the 
 resisting medium. If, therefore, it should be found 
 that the comets which return regularly and at short 
 intervals, actually exhibit the accelerated motion 
 which would result from the operation of such a 
 medium of resistance, it may be regarded as con- 
 clusively established, that the undulatory theory of 
 light is a physical fact, and that there exists through- 
 out this illimitable universe a subtile ethereal fluid. 
 The only objection which can be urged against this 
 assumption is that the resisting medium must neces- 
 sarily be composed of ponderable matter, while the 
 ethereal fluid, which serves for the transmission of 
 light, is supposed to be strictly imponderable. This 
 objection has indeed been urged in opposition to 
 the theory of a resisting medium, but it should be 
 remembered that the idea of imponderability is 
 essentially relative. The comets themselves, to a 
 
340 TREATISE ON COMETS. 
 
 limited extent, are imponderable, and when it is 
 considered that the tails of these bodies are so ex- 
 tremely attenuated that they approach almost to 
 the nature of the invisible ether, the identity of the 
 resisting medium which opposes their motions, and 
 the ethereal fluid which transmits light, is by no 
 means doubtful. The phenomena of double refrac- 
 tion and polarization have demonstrated with un- 
 erring certainty that the undulatory theory of light, 
 which presupposes the existence of a subtile ethe- 
 real fluid throughout space, is the only one which 
 can be recognized in the present state of optical 
 science ; while other considerations, which have 
 already been stated, tend to prove that there exists 
 some common and yet inscrutable bond, which con- 
 nects these powerful agents of nature. There can 
 be no doubt, therefore, even if there were not 
 additional phenomena, and of an entirely different 
 character, tending to the same final consequence, 
 but that the ethereal fluid exists in every part of the 
 universe in which creation is already begun or 
 ended, and that this subtile fluid must necessarily 
 oppose itself to the free motion of all material bodies. 
 There is nothing in this view of the system of the 
 world which can be objected to on account of any 
 assumed insufficiency of the data from which such 
 conclusions have been derived, or, as being in con- 
 travention with the teachings of revelation. 
 
RESISTING MEDIUM IN SPACE. 341 
 
 The existence of a resisting medium in space may 
 thus be supposed to be established on a purely 
 theoretical basis, and it remains now to see whether 
 there are any phenomena connected with the motions 
 of any of the heavenly bodies, by means of which, 
 even if no such ideas had been previously advanced, 
 the same conclusion would have been derived. The 
 planets do not, during the period in which they have 
 been accurately observed, afford any indication of 
 an opposing force, such as would result from a 
 resisting medium. Their great weight, compared 
 with the extent of their volume, would necessarily 
 render them less liable to be sensibly affected by 
 such a force of resistance. The case of the comets, 
 however, is far different. These bodies are them- 
 selves composed, for the most part, of matter so 
 attenuated, while the extent of their volume is often 
 enormous, that the greatest resistance would be 
 opposed to their motions ; and, consequently, it is 
 to these bodies that we are to look for an exemplifi- 
 cation of the effects of a resisting medium. It has 
 already been noticed in the case of Halley's comet 
 that the effect of a resisting medium on its motions, 
 from 1682 to 1759, was computed by Clairaut, and 
 found to be so small as "to be almost, if not entirely 
 insensible. Such being the case, the effect of a 
 resistance from the ethereal fluid was not considered 
 in the computations which were made for its return 
 29* 
 
842 TREATISE ON COMETS. 
 
 in 1835. It has also been noticed in the case ot 
 Encke's comet, and of Biela's comet, that their suc- 
 cessive returns to their perihelia take place at in- 
 tervals which are regularly becoming shorter and 
 shorter. This diminution of the period of these 
 comets has been attributed to the influence of a 
 resisting medium, and the amount of the diminu- 
 tion of the period corresponds precisely with what 
 might be expected to result from such a cause. In 
 the case of Encke's comet the diminution of the 
 period has been determined with the very greatest 
 precision, the short period of its revolution afford- 
 ing great facilities for settling the question as to 
 the existence of a medium of resistance. Encke has 
 computed the planetary perturbations for the entire 
 period which has elapsed since the comet first be- 
 came known to astronomers, and finds that, after 
 making due allowance for these disturbances, its 
 period is shorter at each successive return. The 
 amount of the diminution of its periodic time is 
 very nearly eleven-hundredths of a day, or two 
 hours, thirty-eight minutes, and twenty-four seconds, 
 at each return. The same result has been obtained 
 in the case of Biela's comet, but since the perturba- 
 tions have not yet been computed for its successive 
 returns with extreme accuracy, and, moreover, on 
 account of the anomalous phenomenon of its having 
 separated into two separate and distinct comets, the 
 
RESISTING MEDIUM IN SPACE. 343 
 
 exact amount of the acceleration lias not been ascer- 
 tained. There can be no doubt, however, but that 
 its period is becoming shorter at each successive 
 return to its perihelion, and that the other comets of 
 short period will eventually, after a sufficient num- 
 ber of revolutions, be found to exhibit precisely the 
 same phenomenon. It may, therefore, be considered 
 as well established that the motions of the comets 
 indicate, beyond all doubt, the existence of a resist- 
 ing medium within the limits included by the solar 
 system, while the identity of this with th^ ethereal 
 fluid which transmits light, being admitted, shows 
 that the same medium of resistance is universal in 
 the material universe. 
 
 The effect of a resisting medium on the motions 
 of the bodies which revolve around the sun will be 
 to continually accelerate their mean motion, and, 
 consequently, shorten their period of revolution. 
 The eccentricity of their orbits will gradually 
 dimmish, as the greater axis decreases, and, finally, 
 they will one by one be plunged into the sun, after 
 having described an indefinite number of circum- 
 volutions of a kind of spiral curve, w T hich gradually 
 at first, but afterward rapidly brings them to the 
 central body. The comets being most affected by 
 the resistance will necessarily be the first to fall to 
 the sun, and then in rapid succession the various 
 planets belonging to our system ; until finally all 
 
344 TREATISE ON COMETS. 
 
 traces of the former existence of a beautiful and 
 yet complicated system of comets and planets will 
 be completely obliterated. This will be the neces- 
 sary result of the resistance of an ethereal fluid in 
 space, though it shall be accomplished only after 
 the lapse of almost an indefinite number of ages. 
 It may, therefore, 011 account of our preconceived 
 idea of infinite duration, seem very improbable, if 
 there be not other forces which can overcome the 
 eifect of a resisting medium, that it can really exist, 
 notwithstanding the facts already presented. The 
 conception of universal gravitation is so simple and 
 yet so beautiful, that when we come to consider its 
 legitimate results, in connection with the existence 
 of a great central sun, around which the systems 
 and compound systems of the universe revolve, the 
 mind hesitates to assume the existence of any force 
 either of repulsion or of resistance, which can limit 
 the period of these revolutions and counter-revolu- 
 tions. For example, supposing it possible to view 
 our own solar system from some point in space 
 beyond its limits, and that it is possible to witness 
 both the revolutions of the planets and comets 
 around the sun, and the revolution of the entire 
 system around its distant centre, let us contemplate 
 in imagination the scene which would be presented 
 to our view. In the centre of the system would be 
 seen the great central orb, or sun, and around him 
 
RESISTING MEDIUM IN SPACE. 345 
 
 the planets, some with and some without satellites, 
 each performing its appointed revolution. Here 
 and there among these would be seen chaotic worlds, 
 with streams of light flowing from them, some 
 moving rapidly toward the central body with a 
 constantly accelerated velocity, and others moving 
 from it with a velocity decreasing just as in the 
 other case it increased, while with all this gorgeous 
 equipage of glittering worlds, the great sun itself 
 and system would be seen to move slowly but uni- 
 formly around its centre. This also might be con- 
 ceived of as revolving around a second great centre, 
 and this again around a third, and so on until, 
 finally, the mind rests on the contemplation of a 
 great centre of centres of revolution, around which 
 the entire universe is brought to an equipoise. This 
 conception of the structure of the universe may 
 perhaps be regarded as fanciful, and doubtless, to a 
 greater or less extent, it is so ; yet there is in it that 
 which causes the reason at first to revolt against the 
 assumption of the existence of any forces in nature 
 which may ultimately destroy the present state of 
 things, as contrary to the evident design of 2n 
 intelligent and omnipotent Creator. To this may 
 be traced the great opposition which the theory of 
 the existence of a resisting medium in space has re- 
 ceived, and which has led to various other hypotheses 
 
346 TREATISE ON COMETS. 
 
 to explain the phenomena of the diminution of the 
 periodic times of the comets. 
 
 Without attempting to consider the various theo- 
 ries which have heen devised in order to avoid the 
 assumption, or rather the confirmation, of the ex- 
 istence of an ethereal fluid pervading all space, 
 which opposes itself to the motions of the heavenly 
 bodies, let it suffice simply to state that all these are 
 so incomplete and unsatisfactory, that, however 
 much we may feel inclined to do otherwise, we are 
 compelled finally to resort to the hypothesis of a 
 resisting medium in order to explain the phenonienq, 
 observed. This we are compelled to do in order to 
 satisfy the results of observation, while, for reasons 
 which have already been given, we must admit the 
 existence of an ethereal fluid, whether sufficiently 
 dense or not to oppose sensibly the motions of the 
 heavenly bodies. It may be perceived, therefore, 
 that the theory of a resisting medium presents the 
 very greatest claims to our consideration, as the 
 only one which can explain simultaneously several 
 of the most important facts connected with the con- 
 stitution of the material universe; and, consequently, 
 before proceeding to investigate more fully its ulti- 
 mate effect on our system, it may not be improper 
 to notice briefly the provisions which the operation 
 of the law of gravitation has afforded for securing 
 the permanence of the system. 
 
RESISTING MEDIUM IN SPACE. 347 
 
 From what has previously been stated in regard 
 to the mutual attractions of the heavenly bodies, it 
 may be understood that, since all the planets are 
 small compared with the sun, the derangement 
 which they may produce on any one of their number 
 will be very small in the course of a few revolutions. 
 But since the same forces are constantly in opera- 
 tion, and must continue so, it may be easily con- 
 ceived that in the course of ages the derangement 
 of the planetary motions may accumulate, the orbits 
 may change their form, and their mutual distances 
 may be greatly increased or diminished. These 
 changes might be supposed to continue without 
 limit, and end finally, though perhaps at a date 
 almost infinitely remote, in the total subversion and 
 ruin of the system. Moreover, the careful obser- 
 vation of the planets for a long period of time has 
 shown that changes are actually taking place in 
 their relative motions, of such a nature as just re- 
 ferred to, which have been going on progressively 
 since the very dawn of science. The eccentricity 
 of the orbit of the earth has been decreasing for 
 thousands of years, and, consequently, the mean 
 motion of the moon has become more and more 
 accelerated. The moon, therefore, perpetually ap- 
 proaches nearer and nearer the earth at each revo- 
 lution, and should this change Continue forever, it 
 would eventually fall to our globe, an event which 
 
348 TREATISE ON COMETS. 
 
 would indeed be a dreadful calamity. The question 
 may thus arise, whether these and similar changes 
 will continue indefinitely, or whether, in the nature 
 of the planetary orbits, there exist conditions which 
 fix the limit of all these changes, and thus preserve 
 the stability of the system. 
 
 The problem which is thus presented is certainly 
 worthy of the highest exertions of the human mind, 
 while its solution is a task of no ordinary difficulty. 
 The problem is indeed one of such proportions that 
 no attempt was made to demonstrate either the 
 stability or instability of the solar system, until 
 near the close of the eighteenth century. It was 
 then shown by Lagrange and Laplace, that the 
 arrangements of the system are stable, that in the 
 course of time counter-changes will take place, so 
 that in the end the planetary orbits and motions 
 remain unchanged. The variations of the elements 
 of their orbits, caused by their mutual attractions 
 in accordance with the law of universal gravitation, 
 are periodical, and not indefinitely progressive. The 
 periods, however, in which these changes and 
 counter-changes are effected, are indeed enormous ; 
 not less than thousands, and, in some instances, 
 millions of years ; and hence it is that some of these 
 apparent derangements have been progressing in 
 the same direction since the very beginning of the 
 history of the world. The disturbances which will 
 
STABILITY OF THE SOLAR SYSTEM. 349 
 
 ever exist will not be sufficient to modify or alter 
 the adaptations of the system. 
 
 It was demonstrated by Laplace, that whatever 
 be the masses of the planets, in consequence of the 
 fact that they all move in the same direction in 
 orbits of small eccentricity, and slightly inclined to 
 each other their secular inequalities or changes 
 are periodical, and included within narrow limits; 
 so that the planetary system will only oscillate about 
 a mean state, and will never deviate from it, except 
 by a small quantity. The orbits of the planets have 
 been, and will always be, nearly circular. The 
 ecliptic will never coincide with the equator, and 
 the entire extent of the variation in its inclination 
 cannot exceed three degrees. Thus will the planets, 
 so far as gravity alone is concerned, continue to 
 revolve forever in orbits, whose planes will rock 
 slowly up and down, gradually contracting and ex- 
 panding, but yet compelled to oscillate about a mean 
 orbit whose form and position is fixed. 
 
 Such is the result of computations which are of 
 the most positive character; and it is beyond all 
 doubt that if the force of gravity alone operates 
 on the bodies of the system, it will ever continue, 
 so far as the effect of the planets may be considered, 
 to preserve the stability of the system. By it these 
 changes are produced, and by it the limits of the 
 changes themselves are fixed. These limits can 
 30 
 
350 TREATISE ON COMETS. 
 
 never be passed, and after the lapse of perhaps 
 millions of ages, the entire system will have resumed 
 its primitive condition. But if now we introduce 
 into the system an opposing force, such as would 
 result from a resisting medium, the beautiful ar- 
 rangement of the configurations of the planets 
 which, by the operation of the gravitating force 
 preserves and perpetuates the conditions of stability, 
 will no longer exist. Unless, therefore, we can con- 
 ceive of other forces in nature which can compen- 
 sate for the effect of a resisting medium, we must 
 regard it as inevitably decreed that all must end. 
 This would be a necessary and unavoidable result, 
 and its final consummation will depend simply on the 
 character of the medium which exists. Thus, it 
 has been found that Encke's comet would lose one- 
 half of its present mean velocity in less than twenty- 
 three thousand years. In a similar manner, it has 
 been found, or rather conjectured, that if Jupiter 
 were to lose a millionth part of his velocity in a 
 million of }^ears which is regarded as far more 
 than may be considered in any way probable he 
 would require seventy millions of years to lose one- 
 thousandth of his present velocity, and a period 
 seven hundred times as long to reduce it to one-half. 
 These immense periods are almost sufficient to over- 
 whelm the imagination nevertheless it is certain 
 that even if such changes are to be ultimately 
 
STABILITY OF THE SOLAR SYSTEM. 351 
 
 effected, the time which must elapse will be much 
 greater. It may indeed be millions of millions of 
 years before the earth's retardation will sensibly 
 change the length of our year and the course of 
 our seasons, yet, if there are no forces to counter- 
 balance, even this exemplification of the smallness 
 of the resistance does not in the least indicate its 
 uncertainty or that of the fate which thus inevitably 
 awaits our planet and its inhabitants, should they 
 be permitted to exist for so long a time. The same 
 effect will be produced throughout the system, and 
 unless counter-changes shall be produced, it is indeed 
 an appalling fact that all must end. 
 
 Such considerations as these have induced Whe- 
 well to indulge in the following reflections : " The 
 vast periods which are brought under our conside- 
 ration in tracing the effects of the resisting medium, 
 harmonize with all that we learn of the constitution 
 of the universe from other sources. Millions and 
 millions of years are expressions that at first sight 
 appear fitted only to overwhelm and confound all 
 our powers of thought; and such numbers are no 
 doubt beyond the limits of anything which we dis- 
 tinctly conceive. But our powers of conception are 
 suited rather to the wants and uses of common life, 
 than to a complete survey of the universe. It is in 
 no way unlikely that the whole duration of the 
 solar system should be a period immeasurably great 
 
352 TREATISE ON COMETS. 
 
 in our eyes, though demon str ably finite. Such 
 enormous numbers have been brought under our 
 notice by all the advances we have made in our 
 knowledge of nature. The smallness of the objects 
 detected by the microscope and of their parts ; 
 the multitude of the stars which the best telescopes 
 of modern times have discovered in the sky ; the 
 duration assigned to the globe of the earth by 
 geological investigation ; all these results require 
 for their probable expression, numbers which, so 
 far as we see, are on the same gigantic scale as the 
 number of years in which the solar system will 
 become entirely deranged. Such calculations de- 
 pend in some degree on our relation to the vast 
 aggregate of the works of our Creator; and no 
 person who is accustomed to meditate on these 
 subjects will be surprised that the numbers which 
 such an occasion requires should oppress our com- 
 prehension. No one who has dwelt on the thought 
 of a universal Creator and Preserver, will be sur- 
 prised to find the conviction forced upon the mind 
 by every new train of speculation, that viewed in 
 reference to Him, our space is a point, our time a 
 moment, our millions a handful, our permanence a 
 quick decay. 
 
 " Our knowledge of the vast periods, both geo- 
 logical and astronomical, of which we have spoken, 
 is most slight. It is in fact little more than that 
 
STABILITY OF THE SOLAR SYSTEM. 353 
 
 such periods exist; that the surface of the earth 
 has, at wide intervals of time, undergone great 
 changes in the disposition of land and water, and 
 in the forms of animal life ; and that the motions 
 of the heavenly bodies round the sun are effected, 
 though with inconceivable slowness, by a force 
 which must end by deranging them altogether. It 
 would, therefore, be rash to endeavor to establish 
 any analogy between the periods thus disclosed; 
 but we may observe that they agree in this, that 
 they reduce all things to the general rule of finite 
 duration. As all the geological states of which we 
 find evidence in the present state of the earth, have 
 had their termination, so also the astronomical con- 
 ditions under which the revolutions of the earth 
 itself proceed, involve the necessity of a future 
 cessation of these revolutions. 
 
 " The contemplative person may well be struck 
 by this universal law of the creation. We are in 
 the habit sometimes of contrasting the transient 
 destiny of man with the permanence of the forests, 
 the mountains, the ocean with the unwearied cir- 
 cuit of the sun. But this contrast is a delusion of 
 our own imagination ; the difference is after all but 
 one of degree. The forest tree endures for its 
 centuries, and then decays ; the mountains crumble 
 and change, and perhaps subside in some convulsion 
 of nature ; the sea retires, and the shore ceases to 
 30* x 
 
354 TREATISE ON COMETS. 
 
 resound with the ' everlasting ' voice of the ocean ; 
 such reflections have already crowded upon the 
 mind of the geologist; and it now appears that the 
 courses of the heavens themselves are not exempt 
 from the universal law of decay ; that not only the 
 rocks and the mountains, but the sun and moon, 
 have the sentence, 'to end,' stamped upon their 
 foreheads. They enjoy no privilege beyond man 
 except a longer respite. The ephemeron perishes 
 in an hour ; man endures for his three-score years 
 and ten ; an empire, a nation, numbers its centuries, 
 it may be its thousands of years; the continents 
 and islands which its dominion includes have per- 
 haps their date, as those which preceded them have 
 had ; and the very revolutions of the sky, by which 
 centuries are numbered, will at last languish and 
 stand still." 
 
 The final obliteration of the system, at least so far 
 as the comets are concerned, was a favorite theory 
 advanced by Newton, and one which, to a certain 
 degree, he seems to have cherished to the latest 
 hours of his life. He thus regarded the comets as 
 "the aliment by which suns are sustained"; and 
 remarked, that although it was impossible for him 
 to conjecture when the comet of 1680 one to 
 which he had devoted special attention will fall 
 to the sun, yet, whenever that time shall arrive, the 
 heat of the sun will be raised by it to such a degree, 
 
STABILITY OF THE SOLAR SYSTEM. 355 
 
 that our earth will be so intensely heated, that all 
 animal and vegetable existence will be destroyed. 
 It has also been urged that the assumption of a 
 resisting medium as resulting in the final destruction 
 of our system, indicates a direct analogy between 
 the laws which govern all parts of the material 
 universe ; that perpetual change and perpetual pro- 
 gression, increase and diminution, are found thus 
 to prevail without exception. It must be remarked, 
 however, that this view of the system of the world, 
 although supported by close analogy, does not on 
 that account present unavoidable claims to be re- 
 garded as strictly true, or even as being anything 
 more than a mere speculation. It would indeed 
 seem strange and unaccountable that the Omnipo- 
 tent Creator of the universe should construct systems 
 of worlds revolving around a common centre, in 
 accordance with laws so regulated, if not subjected 
 to external causes, as to perpetuate each individual 
 and compound system. There may be a beginning 
 without an end, although the reverse is impossible, 
 at least so far as human reason can understand; 
 and to conceive, therefore, that because the present 
 state of things has had a beginning, it must neces- 
 sarily have an end, is by no means admissible. Nor 
 is it enough to say that the resisting medium does 
 not in the least counteract what is most important 
 in the provision for the perpetuity of our system, 
 
356 TREATISE ON COMETS. 
 
 and that the order of nature will remain unchanged 
 for a period, compared with which the age of the 
 earth in its present state is insignificant. The ulti- 
 mate effect of the resisting medium will be, as 
 already noticed, to dimmish not only the periods of 
 the revolutions of the different planets and comets, 
 but also that of the earth, and thus to derange those 
 adaptations which depend on the length of the year, 
 and fit it for the abode of animal life such as now 
 exists. It may, however, have been preordained 
 that our own system, at least, should exist for a 
 specific purpose, and that, having fulfilled its pur- 
 pose, it should, as such, have its end of duration, 
 though millions of millions of years were designated 
 by the Creator as the period in which all its suc- 
 cessive transformations should be effected. When 
 the present order of things had its beginning is 
 beyond the power of the human reason to declare ; 
 and when, if ever, it shall end, is equally beyond 
 our comprehension. In all such inquiries we are 
 soon lost and bewildered by the uncertainties which 
 surround us, and entering, as we do, the confines 
 of the unknown, it becomes as impossible to proceed 
 with the investigation as to attempt to conceive of 
 the origin of Omnipotence itself. The power of 
 the human reason is finite, and, therefore, unequal 
 to the comprehension of the infinite; and, in view 
 of this, we can only hope that the time will come, 
 
INFLUENCE OF THE COMETS. 357 
 
 and at no very distant day, when some of the 
 mysteries in which many of the phenomena of the 
 material world are involved, will be finally dissipated, 
 and when the field of speculation, which, to a 
 limited extent, must ever remain, will be finally 
 compressed within very narrow limits. 
 
 In regard to the theory, that there exist evident 
 indications that all must end, it might be added 
 that the limit of even millions of millions of ages 
 is by no means suificient to dispel any tendency in 
 the mind to revolt against any such assumption. 
 Granting, therefore, the existence of a resisting 
 medium in space, and in view of the ultimate effect 
 which would thus be produced, if not counteracted, 
 we are naturally inclined to look to the operation 
 of some other force, yet unknown, to obviate, or, 
 at least, modify the nature and degree of the effect, 
 and thus avert the dreadful calamity which must 
 eventually be produced. The perturbations of the 
 comets on each other are yet undetermined, and 
 also the ultimate effect of the attractions of these 
 bodies on the solar system. The phenomena of the 
 tails of comets have indicated, beyond all doubt, 
 the existence of a repulsive force in all material 
 bodies, while the fact that the force of gravity, and 
 the undulations of the ethereal fluid in producing 
 the intensity of light, vary according to the same 
 law, taken in connection with the supposed and, 
 
358 TREATISE ON COMETS. 
 
 we believe, clearly established identity of light, 
 heat, and electricity, affords cogent reasons for con- 
 jecturing that there may exist some unknown bond 
 connecting all these forces of nature in such a 
 manner that their combined effect will be productive 
 of the most harmonious results. In this way it may 
 be considered possible that future investigations 
 will establish such relations between the operation 
 of the various physical forces, as will exhibit in the 
 clearest and most instructive manner, the evident 
 design of perpetuity which the Creator may have 
 enstamped on every part of his universe of creation, 
 already ended. 
 
 There is also another point of view from which 
 the conditions of the perpetuity of the systems of 
 the universe may be considered, which must not be 
 passed by unnoticed, and which possibly may furnish 
 an explanation, by no means improbable, of the 
 object of the comets in the economy of nature. If 
 we suppose, that, in the course of an indefinite 
 number of ages, an almost infinite number of these 
 bodies shall pass through our system, it may be 
 concluded that, assuming their individual masses or 
 weights to be small, their disturbing influence in 
 the system will be sufficient to counterbalance any 
 minute, or, so to speak, residual inequalities or 
 irregularities which may exist. We may suppose 
 from analogy and there can, in this case, be no 
 
GENERAL KEMARKS. 359 
 
 forcible objection to such a course of reasoning 
 that, in the other systems of the universe, there 
 exist the same conditions and arrangements for 
 preserving their stability as in the case of our own 
 system. From what we know of the motions of 
 the comets, after they cease to be visible from the 
 earth and in the remote regions of space, it may be 
 inferred that they pass through, and present in other 
 systems the same phenomena, only to a greater or 
 less extent, which have characterized their appear- 
 ance in our own ; and that although some of them 
 will be drawn for a time, at least, into orbits which 
 would retain them as members of the system, yet 
 by far the greater number would move in orbits 
 which would conduct them beyond the confines of 
 the various systems which they may visit ; and so, 
 passing on in a meandering course, from sun to 
 sun, and from system to system, they may poise 
 even by their infinitesimal masses, as circumstances 
 may require, every minute irregularity in the sys- 
 tems which they visit, thus preserving, through 
 time and eternity, the stability and equilibrium of 
 the universe. 
 
 Such is the present state of cometary astronomy, 
 divested, so far as practicable, of its technicalities, 
 and brought within the comprehension of every 
 
360 TREATISE ON COMETS. 
 
 general reader. The phenomena which have been 
 presented are such as furnish the most sublime 
 objects for contemplation; and in what has been 
 stated, whether on the solid basis of established 
 truth, or in the form of a speculation, we have 
 endeavored to be careful to avoid everything which 
 might, should the train of thought be continued 
 further, lead to erroneous conclusions. We have 
 also endeavored to give all the most important facts 
 which the researches of astronomers have developed 
 in connection with the theory of these wonderful 
 bodies, and have shown conclusively, as must be 
 admitted, that these objects have no direct influence 
 in the local affairs of our earth. We have not 
 alluded to any supposed possibility of the comets 
 being regarded as the abode of animal or vegetable 
 life, except in the statement that Winston supposed 
 these bodies to be the residence of the damned, 
 who are punished by being alternately wheeled into 
 regions of intolerable heat and of the most intense 
 cold. It should not, however, be supposed that, 
 among the various speculations respecting the cha- 
 racter of these bodies, they have never been re- 
 garded as the abode of rational and intelligent 
 beings. Lambert, in contemplating the excursions 
 which these bodies make in the immensity of space, 
 concludes with the following reflections, which must 
 
GENERAL REMARKS. 361 
 
 necessarily be regarded as simply a fanciful con- 
 ception of a brilliant imagination : 
 
 "I love to figure to myself those travelling globes, 
 peopled with astronomers, who are stationed there 
 for the express purpose of contemplating nature on 
 a large, as we contemplate it on a small, scale. 
 Their movable observatory cruising from sun to 
 sun, carries them in succession through every dif- 
 ferent point of view, places them in a situation to 
 survey all, to determine the position and motion of 
 each star, to measure the orbits of the planets and 
 comets which revolve around them, to observe how 
 particular are resolved into general laws, in one 
 word, to get acquainted with the whole as well as 
 the detail. We may suppose that their year is 
 measured by the length of their route from one sun 
 to another. Winter falls in the middle of their 
 journey; each passage of a perihelion is the return 
 of summer; each introduction to a new world is 
 the revival of spring ; and the period of quitting it 
 is the beginning of their autumn. The place of 
 their abode is accommodated to all their distances 
 from the fixed stars, and the different degrees of 
 their heat make the fruits and vegetables designed 
 for their use blossom and ripen. Happy intelli- 
 gences, how excellent must be the frame of your 
 nature ! Myriads of ages pass away with you like 
 so many days with the inhabitants of the earth. 
 31 
 
362 TREATISE ON COMETS. 
 
 Our largest measurements are your infinitely small 
 quantities; our millions the elements of your arith- 
 metic ; we breathe but a moment ; our lot is error 
 and death, yours science and immortality. All this 
 is agreeable to the analogy of the works of creation. 
 The frame of the universe furnishes matter of con- 
 templation as a whole as well as in each of its parts. 
 There is not a point that does not merit our obser- 
 vation ; this magnificent fabric is portioned out in 
 detached parts to created beings ; but it is in the 
 unity of the whole that sovereign perfection shines ; 
 and can we suppose that this whole has no ob- 
 servers ? The imagination, indeed, after so sublime 
 a flight, may be astonished at its own temerity; 
 but, in short, here the cause is proportioned to the 
 effect, and there is nothing great or small in im- 
 mensity and eternity." 
 
 Whether the comets are in a state capable of 
 being the abode of rational intelligences, or whether, 
 as is more probable, they are only exhibitions of 
 matter in a state of transformation, are questions 
 which we shall not consider. They are by no 
 means necessary to a complete understanding of 
 our subject, and for this reason have been omitted. 
 We may, therefore, remark in conclusion, that 
 enough has been presented to almost overwhelm 
 the mind in the contemplation of the material uni- 
 verse. 
 
GENERAL REMARKS. 363 
 
 Well hath the great Creator of the world, 
 Fram'd it in that exact and perfect form, 
 That by itself immovable might stand, 
 Supported only by his providence. 
 Well hath his powerful wisdom ordered 
 The, in nature, disagreeing elements, 
 That all affecting their peculiar place, 
 Maintain the conservation of the whole. 
 Well hath he taught the swelling ocean 
 To know its bounds, lest in luxurious pride 
 He should insult upon the conquer' d land: 
 Well hath he plac'd those torches in the heavens, 
 To give light to our else darkened eyes: 
 The crystal windows through which our soul, 
 Looking upon the world's most beauteous face, 
 Is blest with sight and knowledge of his works." 
 
 THE END. 
 

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