Chapter I: HAVE endeavored, in the preceding part of my work, to explain and (3)

[footnote] *If, according to Burckhardt's determination, the Moon's radius be 0.2725 and its volume 1/49.00th, its density will be 0.5596, or nearly five ninths. Compare, also, Wilh. Beer and H. Madler, 'der Mond', 2, 10, and Madler, 'Ast.', 157. The material contents of the Moon are, according to Hansen, nearly 1/34th (and ädler 1/40.6th) that of the Earth, and its mass equal to 1/87.73d that of the Earth. In the largest of Jupiter's moons, the third, the relations of volume to the central body are 1/15370th, and of mass 1/11300th. On the polar flattening of Uranus, see Schum, 'Astron. Nachr.', 1844, No. 493.

While the density of the Moon is five ninths less than that of the Earth, it would appear, if we may sufficiently depend upon the determinations of their magnitudes and masses, that the second of Jupiter's moons is actually denser than that great planet itself. Among the fourteen satellites that have been investigated with any degree of certainty, the system of the seven satellites of Saturn presents an instance of the greatest possible contrast, both in absolute magnitude and in distance from the central body. The sixth of these satellites is probably not much smaller than Mars, while our moon has a diameter which does not amount to more than half that of the latter planet. With respect to volume, the two outer, the sixth and seventh of Saturn's satellites, approach the nearest to the third and brightest of Jupiter's moons. The two innermost of these satellites belong perhaps, together with the remote moons of Uranus to the smallest cosmical bodies of our solar system, being only made visible under favorable circumstances by the most powerful instruments. They were first discovered by the forty-foot telescope of William Herschel in 1789, and were seen again by John Herschel at the Cape of Good Hope, by Vico at Rome, and by Lamont at Munich. Determinations of the 'true' diameter of satellites, made by the measurement of the apparent size of their small disks, are subjected to many optical difficulties; but numerical astronomy, whose task it is to predetermine by calculation the motions of the heavenly bodies as they will appear when viewed from the Earth, is directed almost p 97 exclusively to motion and mass, and but little to volume. The absolute distance of a satellite from its central body is greatest in the case of the outermost or seventh satellite of Saturn, its distance from the body round which it revolves amounting to more than two millions of miles, or ten times as great a distance as that of our moon from the Earth. In the case of Jupiter we find that the outermost or fourth attendant moon is only 1,040,000 miles from that planet, while the distance between Uranus and its sixth satellite (if the latter really exist) amounts to as much as 1,360,000 miles. If we compare, in each of these subordinate systems, the volume of the satellite, we discover the existence of entirely new numerical relations. The distances of the outermost satellites of Uranus, Saturn, and Jupiter are when expressed in semi-diameters of the main planets, as 91, 64, and 27. The outermost satellite of Saturn appears, therefore, to be removed only about one fifteenth further from the center of that planet than our moon is from the Earth. The first or innermost of Saturn's satellites is nearer to its central body than any other of the secondary planets, and presents, moreover, the only instance of a period of revolution of less than twenty-four hours. Its distance from the center of Saturn may, according to Mädler and Wilhelm Beer, be expressed as 2.47 semi-diameters of that planet, or as 80,088 miles. Its distance from the surface of the main planet is therefore 47,480 miles, and from the outer-most edge of the ring only 4916 miles. The traveler may form to himself an estimate of the smallness of this amount by remembering the statement of an enterprising navigator, Captain Beechey, that he had in three years passed over 72,800 miles. If, instead of absolute distances, we take the semi-diameters of the principal planets, we shall find that even the first or nearest of the moons of Jupiter (which is 26,000 miles further removed from the center of that planet than our moon is from that of the Earth) is only six semi-diameters of Jupiter from its center, while our moon is removed from us fully 60 1/3d semi-diameters of the Earth.

In the subordinate systems of satellites, we find that the same laws of gravitation which regulate the revolutions of the principal planets round the Sun likewise govern the mutual relations existing between these planets among one another and with reference to their attendant satellites. The twelve moons of Saturn, Jupiter, and the Earth all most like the primary planets from west to east, and in elliptic orbits, deviating p 98 but little from circles. It is only in the case of one moon, and perhaps in that of the first and innermost of the satellites of Saturn (0.068), that we discover an eccentricity greater than that of Jupiter; according to the very exact observations of Bessel, the eccentricity of the sixth of Saturn's satellites (0.029) exceeds that of the Earth. On the extremest limits of the planetary system, where, at a distance nineteen times greater than that of our Earth, the centripetal force of the Sun is greatly diminished, the satellites of Uranus (which most striking contrasts from the facts observed with regard to other secondary planets. Instead, as in all other satellites, of having their orbits but slightly inclined toward the ecliptic and (not excepting even Saturn's ring, which may be regarded as a fusion of agglomerated satellites) moving from west to east, the satellites of Uranus are almost perpendicular to the ecliptic, and move retrogressively from east to west, as Sir John Herschel has proved by observations continued during many years. If the primary and secondary planets have been formed by the condensation of rotating rings of solar and planetary atmospheric vapor, there must have existed singular causes of retardation or impediment in the vaporous rings revolving round Uranus, by which, under the relations with which we are unacquainted, the revolution of the second and fourth of its satellites was made to assume a direction opposite to that of the rotation of the central planet.

It seems highly probable that the period of rotation of 'all' secondary planets is equal to that of their revolution round the main planet, and therefore that they always present to the latter the same side. Inequalities, occasioned by sight variations in the revolution, give rise to fluctuations of from 6 degrees to 8 degrees, or to an apparent libration in longitude as well as in latitude. Thus, in the case of our moon, we sometimes observe more than the half of its surface, the eastern and northern edges being more visible at one time, and the western or southern at another. By means of this libration* we are enabled to see the annular mountain Malapert (which occasionally conceals the Moon's south pole), the arctic landscape round the crater of Gioja, and the large gray plane near Endymion which exceeds in superficial extent the 'Mare Vaporum'.

[footnote] *Beer and Madler, op. cit., 185, s.208, and § 347, s. 332; and ix their 'Phys. Kenntniss der himml. Korper', s. 4 und 69, Tab. 1 (Physical History of the Heavenly Bodies).

Three sevenths of the Moon's surface are entirely p 99 concealed from our observation, and must always remain so, unless new and unexpected disturbing causes come into play. These cosmical relations involuntarily remind us of nearly similar conditions in the intellectual world, where, in the domain of deep research into the mysteries and the primeval creative forces of nature, there are regions similarly turned away from us, and apparently unattainable, of which only a narrow margin has revealed itself, for thousands of years, to the human mind, appearing, from time to time, either glimmering in true or delusive light. We have hitherto considered the primary planets, their satellites, and the concentric rings which belong to one, at least, of the outermost planets, as products of tangential force, and as closely connected together by mutual attraction; it therefore now only remains for us to speak of the unnumbered host of 'comets' which constitute a portion of the cosmical bodies revolving in independent orbits round the Sun. If we assume an equable distribution of their orbits, and the limits of their perihelia, or greatest proximities to the Sun, and the possibility of their remaining invisible to the inhabitants of the Earth, and base our estimates on the rules of the calculus of probabilities, we shall obtain as the result an amount of myriads perfectly astonishing. Kepler, with his usual animation of expression, said that there were more comets in the regions of space than fishes in the depths of the ocean. As yet, however, there are scarcely one hundred and fifty whose paths have been calculated, if we may assume at six or seven hundred the number of comets whose appearance and passage through known constellations have been ascertained by more or less precise observations. While the so-called classical nations of the West, the Greeks and Romans, although they may occasionally have indicated the position in which a comet first appeared, never afford any information regarding its apparent path, the copious literature of the Chinese (who observed nature carefully, and recorded with accuracy what they saw) contains circumstantial notices of the constellations through which each comet was observed to pass. These notices go back to more than five hundred years before the Christian era, and many of them are still found to be of value in astronomical observations.*

[footnote] *The first comets of whose orbits we have any knowledge, and which were calculated from Chinese observations, are those of 240 (under Gordian II.), 539 (under Justinian), 565, 568, 574, 837, 1337, and 1385. See John Russell Hind, in Schum., 'Astron. Nachr.', 1843, No. 498. While the comet of 837 (which, according to Du Sejour, continued during twenty-four hours within a distance of 2,000,000 miles from the Earth) terrified Louis I. of France to that degree that he busied himself in building churches and founding monastic establishments, in the hope of appeasing the evils threatened by its appearance, the Chinese astronomers made observations on the path of this cosmical body, whose tail extended over a space of 60 degrees, appearing sometimes single and sometimes multiple. The first comet that has been calculated solely from European observations was that of 1456, known as Halley's comet, from the belief long, but erroneously, entertained that the period when it was first observed by that astronomer was its first and only well-attested appearance. See Arago, in the 'Annuaire', 1836, p. 204, and Langier, 'Comptes Rendus des Seances de l'Acad.', 1843, t. xvi., 1006.

p 100 Although comets have a smaller mass than any other cosmical bodies -- being, according to our present knowledge, probably not equal to 1/5000th part of the Earth's mass -- yet they occupy the largest space, as their tails in several instances extend over many millions of miles. The cone of luminous vapor which radiates from them has been found, in some cases (as in 1680 and 1811), to equal the length of the Earth's distance from the Sun, forming a line that intersects both the orbits of Venus and Mercury. It is even probable that the vapor of the tails of comets mingled with our atmosphere in the years 1819 and 1823.

Comets exhibit such diversities of form, which appear rather to appertain to the individual than the class, that a description of one of these "wandering light-clouds," as they were already called by Xenophanes and Theon of Alexandria, contemporaries of Pappus, can only be applied with caution to another. The faintest telescopic comets are generally devoid of visible tails, and resemble Herschel's nebulous stars. They appear like circular nebulae of faintly-glimmering vapor, with the light concentrted toward the middle. This is the most simple type; but it can not, however, be regarded as rudimentary, since it might equally be the type of an older cosmical body, exhausted by exhalation. In the larger comets we may distinguish both the so-called "head" or "nucleus," and the single or multiple tail, which is characteristically denominated by the Chinese astronomers "the brush" ('sui'). The nucleus generally presents no definite outline, although, in a few rare cases, it appears like a star of the first or second magnitude, and has even been seen in bright sunshine;* as, p 101 for instance, in the large comets of 1402, 1532, 1577, 1744, and 1843.

[footnote] *Arago, 'Annuaire', 1832, p. 209, 211. The phenomenon of the tail of a comet being visible in bright sunshine, which is recorded of the comet of 1402, occurred again in the case of the large comet of 1843, whose nucleus and tail were seen in North America on the 28th of February (according to the testimony of J. G. Clarke, of Portland, state of Maine), between 1 and 3 o'clock in the afternoon.(a) The distance of the very dense nucleus from the sun's light admitted of being measured with much exactness. The nucleus and tail appeared like a very pure white cloud, a darker space intervening between the tail and the nucleus. ('Amer. Journ. of Science', vol. xiv., No. 1, p. 229.)

[footnote] (a) [The translator was at New Bedford, Massachusetts, U.S., on the 28th February, 1843, and distinctly saw the comet, between 1 and 2 in the afternoon. The sky at the time was intensely blue, and the sun shining with a dazzling brightness unknown in European climates.] -- Tr

This latter circumstance indicates, in particular individuals, a denser mass, capable of reflecting light with greater intensity. Even in Herschel's large telescope, only two comets, that discovered in Sicily in 1807, and the splendid one of 1811, exhibited well-defined disks;* the one at an angle of 1 second, and the other at 0.77 seconds, whence the true diameters are assumed to be 536 and 428 miles.

[footnote] *'Phil. Trans.' for 1808, Part ii., p. 155, and for 1812, Part i., p. 118. The diameters found by Herschel for the nuclei were 538 and 428 English miles. For the magnitudes of the comets of 1798 and 1805, see Arago, 'Annuaire', 1832, p. 203.

The diameters of the less well-defined nuclei of the comets of 1798 and 1805 did not appear to exceed 24 or 28 miles.

In several comets that have been investigated with great care, especially in the above-named one of 1811, which continued visible for so long a period, the nucleus and its nebulous envelope were entirely separated from the tail by a darker space. The intensity of light in the nucleus of comets does not augment toward the center in any uniform degree, brightly shining zones being in many cases separated by concentric nebulous envelopes. The tails sometimes appear single, sometimes, although more rarely, double; and in the comets of 1807 and 1843 the branches were of different lengths; in one instance (1744) the tail had six branches, the whole forming an angle of 60 degrees. The tails have been sometimes straight, sometimes curved, either toward both sides, or toward the side appearing to us as the exterior (as in 1811), or convex toward the direction in which the comet is moving (as in that of 1618); and sometimes the tail has even appeared like a flame in motion. The tails are always turned away from the sun, so that their line of prolongation passes through its center; a fact which, according to Edward Biot, was noticed by the Chinese astronomers as early as 837, but was first generally made known in Europe by Fracastoro and Peter Apian in the sixteenth century. These emanations may be regarded as conoidal envelopes of greater of less thickness, p 102 and, considered in this manner, they furnish a simple explanation of many of the remarkable optical phenomena already spoken of.

Comets are not only characteristically different in form, some being entirely without a visible tail, while others have a tail of immense length (as in the instance of the comet of 1618, whose tail measured 104 degrees), but we also see the same comets undergoing successive and rapidly-changing processes of configuration. These variations of form have been most accurately and admirably described in the comet of 1744, by Hensius, at St. Petersburg, and in Halley's comet, on its last reappearance in 1835, by Bessel, at Konigsberg. A more or less well-defined tuft of rays emanated from that part of the nucleus which was turned toward the Sun; and the rays being bent backward, formed a part of the tail. The nucleus of Halley's comet; with its emanations, presented the appearance of a burning rocket, the end of which was turned sideways by the force of the wind. The rays issuing from the head were seen by Arago and myself, at the Observatory at Paris, to assume very different forms on successive nights.*

[footnote] *Arago, 'Des Changements physiques de la Comete de Halley du 15-23 Oct., 1835. 'Annuaire', 1836, p. 218, 221. The ordinary direction of the emanations was noticed even in Nero's time. "Comae radios solis effugiunt." -- Seneca, 'Nat. Quaest.', vii., 20.

The great Konigsberg astronomer concluded from many measurements, and from theoretical considerations, "that the cone of light issuing from the comet deviated considerably both to the right and the left of the true direction of the Sun, but that it always returned to that direction, and passed over to the opposite side, so that both the cone of light and the body of the comet from whence it emanated experienced a rotatory, or, rather, a vibratory motion in the plane of the orbit." He finds that "the attractive force exercised by the Sun on heavy bodies is inadequate to explain such vibrations, and is of opinion that they indicate a polar force, which turns one semi-diameter of the comet toward the Sun, and strives to turn the opposite side away from that luminary. The magnetic polarity possessed by the Earth may present some analogy to this, and, should the Sun have an opposite polarity, an influence might be manifested, resulting in the precession of the equinoxes." This is not the place to enter more fully upon the grounds on which explanations of this subject have been based; but observations so remarkable,* and views of so exalted p 103 a character, regarding the most wonderful class of the cosmical bodies belonging to our solar system, ought not to be entirely passed over in this sketch of a general picture of nature.

[footnote] *Bessel, in Schumacher, 'Astr. Nachr.', 1836, No. 300-302, s. 188, 192, 197, 200, 202, und 230. Also in Schumacher, 'Jahrb.', 1837, s. 149, 168. William Herschel, in his observations on the beautiful comet of 1811, believed that he had discovered evidences of the rotation of the nucleus and tail ('Phil. Trans.' for 1812, Part i., p. 140). Dunlop, at Paramatta thought the same with reference to the third comet of 1825.

Although, as a rule, the tails of comets increase in magnitude and brilliancy in the vicinity of the sun, and are directed away from that central body, yet the comet of 1823 offered the remarkable example of two tails, one of which was turned toward the sun, and the other away from it, forming with each other an angle of 160 degrees. Modifications of polarity and the unequal manner of its distribution, and of the direction in which it is conducted, may in this rare instance have occasioned a double, unchecked, continuous emanation of nebulous matter.*

[footnote] *Bessel, in 'Astr. Nachr.', 1836, No. 302, s. 231. Schum, 'Jahrb.', 1837 s. 175. See, also Lehmann, 'Ueber Cometenschweife' (On the Tails of Comets), in Bode, 'Astron. Jahrb. fur' 1826, s. 168.

Aristotle, in his 'Natural Philosophy', makes these emanations the means of bringing the phenomena of comets into a singular connection with the existence of the Milky Way. According to his views, the innumerable quantity of stars which compose this starry zone give out a self-luminous, incandescent matter. The nebulous belt which separates the different portions of the vault of heaven was therefore regarded by the Stagirite as a large comet, the substance of which was incessantly being renewed.*

[footnote] *Aristot., 'Meteor.', i., 8, 11-14, und 19-21 (ed. Ideler, t. i., p. 32-34). Biese, 'Phil. des Aristoteles', bd. ii., s. 86. Since Aristotle exercised so great an influence throughout the whole of 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 Pythagorean school, according to which these cosmical bodies are supposed to be planets having long periods of revolution. (Aristot., i., 6, 2.) This Pythagorean doctrine, which, according to the testimony of Apollonius Myndius, was still more ancient, having originated with the Chaldeans, passed over to the Romans, who in this instance, as was their usual practice, were merely the copiers of others. The Myndian philosopher describes the path of comets as directed toward the upper and remote regions of heaven. Hence Seneca says, in his 'Nat. Quaest.', vii., 17: "Cometes non est species falsa, sed proprium sidus sicut solis et lunae: altiora mundi secat et tunc demum apparet quum in imum cursum sui venit;" and again (at vii., 27), "Cometes aternos esse et sortis ejusdem, cujus caetera (sidera), etiamsi faciem illis non habent similem." Pliny (ii., 25) also refers to Apollonius Myndius, when he says, "Sunt qui et haec sidera perpetua esse credant suoque ambitu ire, sed non nisi relicta a sole cerni."

p 104 The occulation of the fixed stars by the nucleus of a comet, or by its innermost vaporous envelopes, might throw some light on the physical character of these wonderful bodies; but we are unfortunately deficient in observations by which we may be assured* that the occulation was perfectly central; for, as it has already been observed, the parts of the envelope contiguous to the nucleus are alternately composed of layers of dense or very attenuated vapor.

[footnote] *Olbers, in 'Astr. Nachr.', 1828, s. 157, 184. Arago, 'De la Constitution physique des Cometes; Annuaire de' 1832, p. 203, 208. The ancients were struck by the phenomenon that it was possible to see through comets as through a flame. The earliest evidence to be met with of stars having been seen through comets is that of Democritus (Aristot., 'Meteor.', i., 6, 11), and the statement leads Aristotle to make the not unimportant remark, that he himself had observed the occulation of one of the stars of Gemini by Jupiter. Seneca only speaks decidedly of the transparence of the tail of comets. "We may see," says he, "stars through a comet as through a cloud ('Nat. Quaest.', vii., 18); but we can ony see through the rays of the tail, and not through the body of the comet itself: 'non in ea parte qua sidus ipsum est spissi et solidi ignis, sed qua rarus splendor occurrit et in crines dispergitur. Per intervalla ignium, non er ipsos, vides" (vii., 26). The last remark is unnecessary, since, as Galileo observed in the 'Saggiatore (Lettera a Monsignor Cesarini', 1619), we can certainly see through a flame when it is not of too great a thickness'.

On the other hand the carefully conducted measurements of Bessel prove, beyond all doubt, that on the 29th of September, 1835, the light of a star of the tenth magnitude, which was then at a distance of 7".78 from the central point of the head of Halley's comet, passed through very dense nebulous matter, without experiencing any deflection during its passage.*

[footnote] *Bessel, in the 'Astron. Nachr.', 1836, No. 301, s. 204, 206. Struve, in 'Recueil des Mem. de l'Acad. de St. Peterab.', 1836, p. 140, 143, and 'Astr. Nachr.', 1836, No. 303, s. 238, writes as follows: "At Dorpat the star was in conjunction only 2".2 from the brightest point of the comet. The star remained continually visible, and its light was not perceptibly diminished, while the nucleus of the comet seemed to be almost extinguished before the radiance of the small star of the ninth or tenth magnitude."

If such an absence of refracting power must be ascribed to the nucleus of a comet, we can scarcely regard the matter composing comets as a gaseous fluid. The question here arises whether this absence of refracting power may not be owing to the extreme tenuity of the fluid; or does the comet consist of separated particles, constituting a cosmical stratum of clouds, which, like the clouds of our atmosphere, that exercise no influence on the p 105 zenith distance of the stars, does not affect the ray of light passing through it? In the passage of a comet over a star, a more or less considerable diminution of light has often been observed; but this has been justly ascribed to the brightness of the ground from which the star seems to stand forth during the passage of the comet.

The most important and decisive observations that we possess on the nature and the light of comets are due to Arago's polarization experiments. His polariscope instructs us regarding the physical constitution of the Sun and comets, indicating whether a ray that reaches us from a distance of many millions of miles transmits light directly or by reflection; and if the former, whther the source of light is a solid, a liquid, or a gaseous body. His apparatus was used at the Paris Observatory in examining the light of Capella and that of the great comet of 1819. The latter showed polarized, and therefore reflected light, while the fixed star, as was to be expected, appeared to be a self-luminous sun.*

[footnote] *On the 3d of July, 1819, Arago made the first attempt to analyze the light of comets by polarization, on the evening of the sudden appearance of the great comet. I was present at the Paris Observatory, and was fully convinced, as were also Matthieu and the late Bouvard of the dissimilarity in the intensity of the light seen in the polariscope, when the instrument received cometary light. When it received light from Capella, which was near the comet, and at an equal altitude, the images were of equal intensity. On the reappearance of Halley's comet in 1835, the instrument was altered so as to give, according to Arago's chromatic polarization, two images of complementary colors (green and red). ('Annales de Chimie', t. xiii., p. 108; 'Annuaire', 1832, p. 216.) "We must conclude from these observations," says Arago, "that the cometary light was not entirely composed of rays having the properties of direct light, there being light which was reflected specularly or polarized, that is, coming from the sun. It can not be stated with absolute certainty that comets shine only with borrowed light, for bodies, in becoming self-luminous, do not, on that account, lose the power of reflecting foreign light."

The existance of polarized cometary light announced itself not only by the inequality of the images, but was proved with greater certainty on the reappearance of Halley's comet, in the year 1835, by the more striking contrast of the complementary colors, deduced from the laws of chromatic polarization discovered by Arago in 1811. These beautiful experiments still leave it undecided whether, in addition to this reflected solar light, comets may not have light of their own. Even in the case of the planets, as, for instance, in Venus, an evolution of independent light seems very probable.

The variable intensity of light in comets can not always be p 106 explained by the position of their orbits and their distance from the Sun. It would seem to indicate, in some individuals, the existence of an inherent process of condensation, and an increased or diminished capacity of reflecting borrowed light. In the comet of 1618, and in that which has a period of three years, it was observed first by Hevelius that the nucleus of the comet diminished at its perihelion and enlarged at its aphelion, a fact which, after remaining long unheeded, was again noticed by the talented astronomer Valz at Nismes. The regularity of the change of volume, according to the different degrees of distance from the Sun, appears very striking. The physical explanation of the phenomenon can not, however, be sought in the condensed layers of cosmical vapor occurring in the vicinity of the Sun, since it is difficult to imagine the nebulous envelope of the nucleus of the comet to be vesicular and impervious to the other.*

[footnote] *Arago, in the 'Annuaire', 1832, p. 217-220. Sir John Herschel, 'Astron.', 488.

The dissimilar eccentricity of the orbits of comets has, in recent times (1819), in the most brilliant manner enriched our knowledge of the solar system. Encke has discovered the existence of a comet of so short a period of revolution that it remains entirely within the limits of our planetary system, attaining its aphelion between the orbits of the smaller planets and that of Jupiter. Its eccentricity must be assumed at 0.845, that of Juno (which has the greatest eccentricity of any of the planets) being 0.255. Encke's comet has several times, although with difficulty, been observed by the naked eye, as in Europe in 1819, and according to Rumker, in New Holland in 1822. Its period of revolution is about 3 1/3d years; but, from a careful comparison of the epochs of its return to its perihelion, the remarkable fact has been discovered that these periods have diminished in the most regular manner between the years 1786 and 1838, the diminution amounting, in the course of 52 years, to about 1 3/10th days. The attempt to bring into unison the results of observation and calculation in the investigation of all the planetary disturbances, with the view of explaining this phenomenon, has led to the adoption of the very probable hypothesis that there exists dispersed in space a vaporous substance capable of acting as a resisting medium. This matter diminished the tangential force, and with it the major axis of the comet's orbit. The value of the constant of the resistance appears to be somewhat different before and after the perihelion; and this may, perhaps, be ascribed p 107 to the altered form of the small nebulous star in the vicinity of the Sun, and to the action of the unequal density of the strata of cosmical ether.*

[footnote] *Encke, in the 'Astronomiche Nachrichten', 1843, No. 489, s. 130-132.

These facts, and the investigations to which they have led, belong to the most interesting results of modern astronomy. Encke's comet has been the means of leading astronomers to a more exact investigation of Jupiter's mass (a most important point with reference to the calculation of perturbations); and, more recently, the course of this comet has obtained for us the first determination, although only an approximative one, of a smaller mass for Mercury.

The discovery of Encke's comet, which had a period of only 3 1/3d years, was speedily followed, in 1826, by that of another, Biela's comet, whose period of revolution is 6 3/4th years, and which is likewise planetary, having its aphelion beyond the orbit of Jupiter, but within that of Saturn. It has a fainter light than Encke's comet, and, like the latter, its motion is direct, while Halley's comet moves in a course opposite to that pursued by the planets. Biela's comet presents the first certain example of the orbit of a comet intersecting that of the Earth. This position, with reference to our planet, may therefore be productive of danger, if we can associate an idea of danger with so extraordinary a natural phenomenon, whose history presents no parallel, and the results of which we are consequently unable correctly to estimate. Small masses endowed with enormous velocity may certainly exercise a considerable power; but Laplace has shown that the mass of the comet of 1770 is probably not equal to 1/5000th that of the Earth, or about 1/2000th that of the Moon.*

[footnote] *Laplace, 'Expos. du Syst. du Monde', p. 216, 237.

We must not confound the passage of Biela's comet through the Earth's orbit with its proximity to, or collision with our globe. When this passage took place, on the 29th of October, 1832, it required a full month before the Earth would reach the point of intersection of the two orbits. These two comets of short periods of revolution also intersect each other, and it has been justly observed,* that amid the many perturbations experienced by such small bodies from the largr planets, there is a 'possibility' -- supposing a meeting of these comets to occur in October -- that the inhabitants of the Earth may witness the extraordinary spectacle of an encounter between two cosmical bodies, and possibly of their reciprocal penetration and amalgamation, or of their destruction by means of exhausting emanations.

[footnote] *Littrow, 'Beschreibende Astron.', 1835, s. 274. On the inner comet recently discovered by M. Faye, at the Observatory of Paris, and whose eccentricity is 0.551, its distance at its perihelion 1.690, and its distance at its aphelion 5.832, see Schumacher, 'Astron. Nachr.', 1844, No. 495. Regarding the supposed identity of the comet of 1766 with the third comet of 1819, see 'Astr. Nachr.', 1833, No. 239; and on the identity of the comet of 1743 and the fourth comet of 1819, see No. 237 or the last mentioned work.

Events of this nature, resulting either from deflection occasioned by disturbing masses or primevally intersecting orbits, must have been of frequent occurrence in the course of millions of years in the immeasurable regions of ethereal space; but they must be regarded as isolated occurrences, exercising no more general or alternative effects on cosmical relations than the breaking forth or extinction of a volcano within the limited sphere of our Earth.

A third interior comet, having likewise a short period of revolution was discovered by Faye on the 22d of November, 1843, at the Observatory at Paris. Its elliptic path, which approaches much more nearly to a circle than that of any other known comet, is included within the orbits of Mars and Saturn. This comet, therefore, which, according to Goldschmidt, passes beyond the orbit of Jupiter, is one of the few whose perihelia are beyond Mars. Its period of revolution is 7 29/100 years, and it is not improbable that the form of its present orbit may be owing to its great approximation to Jupiter at the close of the year 1839.

If we consider the comets in their inclosed elliptic orbits as members of our solar system, and with respect to the length of their major axes, the amount of their eccentricity, and their periods of revolution, we shall probably find that the three planetary comets of Encke, Biela, and Faye are most nearly approached in these respects, first, by the comet discovered in 1766 by Messier, and which is regarded by Clausen as identical with the third comet of 1819; and next, by the fourth comet of the last-mentioned year, discovered by Blaupain, but considered by Clausen as identical with that of the year 1743, and whose orbit appears, like that of Lexell's comet, to have suffered great variations from the proximity and attraction of Jupiter. The two last-named comets would likewise seem to have a period of revolution not exceeding five or six years, and their aphelia are in the vicinity of Jupiter's orbit. Among the comets that have a period of revolution of from seventy to p 109 seventy-six years, the first in point of importance with respect to theoretical and physical astronomy is Halley's comet, whose last appearance, in 1835, was much less brilliant than was to be expected from preceding ones; next we would notice Olbers's comet, discovered on the 6th of March, 1815; and, lastly, the comet discovered by Pons in the year 1812, and whose elliptic orbit has been determined by Encke. The two latter comets were invisible to the naked eye. We now know with certainty of nine returns of Halley's large comet, it having recently been proved by Laugier's calculations*, that in the Chinese table of comets, first made known to us by Edward Biot, the comet of 1378 is identical with Halley's; its periods of revolution have varied in the interval between 1378 and 1835 from 74.91 to 77.58 years, the mean being 76.1.

[footnote] *Laugier, in the 'Comptes Rendus des Seances de l'Academie', 1843, t. xvi., p. 1006.

A host of other comets may be contrasted with the cosmical bodies of which we have spoken, requiring several thousand years to perform their orbits, which it is difficult to determine with any degree of certainty. The beautiful comet of 1811 requires, according to Argelander, a period of 3065 years for its revolution, and the colossal one of 1680 as much as 8800 years, according to Encke's calculation. These bodies respectively recede, therefore, 21 and 44 times further than Uranus from the Sun, that is to say, 33,600 and 70,400 millions of miles. At this enormous distance the attractive force of the Sun is still manifested; but while the velocity of the comet of 1680 at its perihelion is 212 miles in a second, that is, thirteen times greater than that of the Earth, it scarcely moves ten feet in the second when at its aphelion. This velocity is only three times greater than that of water in our most sluggish European rivers, and equal only to half that which I have observed in the Cassiquiare, a branch of the Orinoco. It is highly probable that, among the innumerable host of uncalculated or undiscovered comets, there are many whose major axes greatly exceed that of the comet of 1680. In order to form some idea by numbers, I do not say of the sphere of attraction, but of the distance in space of a fixed star, or other sun, from the aphelion of the comet of 1680 (the furthest receding cosmical body with which we are acquainted in our solar system), it must be remembered that, according to the most recent determinations of parallaxes, the nearest fixed star is full 250 times further removed from our sun than the comet in its aphelion. The comet's distance is only 44 p 110 times that of Uranus, while 'a' Centauri is 11,000 and 61 Cygni 31,000 times that of Uranus, according to Bessel's determinations.

Having considered the greatest distances of comets from the central body, it now remains for us to notice instances of the greatest proximity hitherto measured. Lexell and Burckhardt's comet of 1770, so celebrated on account of the disturbances it experienced from Jupiter, has approached the Earth within a smaller distance than any other comet. On the 28th of June, 1770, its distance from the Earth was ony six times than of the Moon. The same comet passed twice, viz., in 1769 and 1779, through the system of Jupiter's four satellites without producing the slightest notable change in the well-known orbits of these bodies. The great comet of 1680 approached at its perihelion eight or nine times nearer to the surface of the Sun than Lexell's comet did to that of our Earth, being on the 17th of December a sixth part of the Sun's diameter, or seven tenths of the distance of the Moon from that luminary. Perihelia occurring beyond the orbit of Mars can seldom be observed by the inhabitants of the Earth, owing to the faintness of the light of distant comets; and among those already calculated the comet of 1729 is the only one which has its perihelion between the orbits of Pallas and Jupiter; it was even observed beyond the latter.

Since scientific knowledge, although frequently blended with vague and superficial views, has been more extensively diffused through wider circles of social life, apprehensions of the possible evils threatened by comets have acquired more weight as their direction has become more definite. The certainty that there are within the known planetary orbits comets which revisit our regions of space at short intervals -- that great disturbances have been produced by Jupiter and Saturn in their orbits, by which such as were apparently harmless have been converted into dangerous bodies -- the intersection of the Earth's orbit by Biela's comet -- the cosmical vapor, which, acting as a resisting and impeding medium, tends to contract all orbits -- the individual difference of comets, which would seem to indicate considerable decreasing gradations in the quantity of the mass of the nucleus, are all considerations more than equivalent, both as to number and variety, to the vague fears entertained in early ages of the general conflagration of the world by 'flaming swords', and stars with 'fiery streaming hair'. As the consolatory considerations which may be derived from the calculus of probabilities address themselves to reason and to p 111 meditative understanding only, and not to the imagination or to a desponding condition of mind, modern science has been accused, and not entirely without reason, of not attempting to allay apprehensions which it has been the very means of exciting. It is an inherent attribute of the human mind to experience fear, and not hope or joy, at the aspect of that which is unexpected and extraordinary.*

[footnote] *Fries, 'Vorlesungen uber die Sternkunde', 1833, s. 262-267 (Lectures on the Science of Astronomy). An infelicitously chosen instance of the good omen of a comet may be found in Seneca, 'Nat. Quest.', vii., 17 and 21. The philosopher thus writes of the comet: "Quem nos Neronis principatu latissimo vidimus et qui cometis detraxit infamiam."

The strange form of a large comet, its faint nebulous light, and its sudden appearance in the vault of heaven, have in all regions been almost invariably regarded by the people at large as some new and formidable agent inimical to the existing state of things. The sudden occurrence and short duration of the phenomenon lead to the belief of some equally rapid reflection of its agency in terrestrial matters, whose varied nature renders it easy to find events that may be regarded as the fulfillment of the evil foretold by the appearance of these mysterious cosmical bodies. In our own day, however, the public mind has taken another and more cheerful, although singular, turn with regard to comets; and in the German vineyards in the beautiful valleys of the Rhine and Moselle, a belief has arisen, ascribing to these once ill-omened bodies a beneficial influence on the ripening of the vine. The evidence yielded by experience, of which there is no lack in these days, when comets may so frequently be observed, has not been able to shake the common belief in the meteorological myth of the existence of wandering stars capable of radiating heat.

This material taken from pages 111- 147

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 by Alexander von Humboldt

Translated by E C Otte

from the 1858 Harper & Brothers edition of Cosmos, volume 1 --------------------------------------------------

From comets I would pass to the consideration of a far more enigmatical class of agglomerated matter -- the smallest of all asteroids, to which we apply the name 'aërolites', or 'meteoric stones',* when they reach our atmosphere in a fragmentary condition.

[footnote] * (Much valuable information may be obtained regarding the origin and composition of aërolites or meteoric stones in Memoirs on the subject, by Baumbeer and other writers, in the numbers of Poggendorf's 'Annalen', from 1845 to the present time.) -- Tr.

If I should seem to dwell on the specific enumeration of these bodies, and of comets, longer than the general nature of this work might warrant, I have not done so undesignedly. The diversity existing in the individual characteristics of comets has already been noticed. The imperfect knowledge we possess of their physical character renders it p 112 diifficult in a work like the present, to give the proper degree of circumstantiality to the phenomena, which, although of frequent recurrence, have been observed with such various degrees of accuracy, or to separate the necessary from the accidental. It is only with respect to measurements and computations that the astronomy of comets has made any marked advancement, and, consequently, a scientific consideration of these bodies must be limited to a specification of the differences of physiognomy and conformation in the nucleus and tail, the instances of great approximation to other cosmical bodies, and of the extremes in the length of their orbits and in their periods of revolution. A faithful delineation of these phenomena, as well as of those which we proceed to consider, can only be given by sketching individual features with the animated circumstantiality of reality.

Shooting stars, fire-balls, and meteoric stones are, with great probability, regarded as small bodies moving with planetary velocity, and revolving in obedience to the laws of general gravity in conic sections round the Sun. When these masses meet the Earth in their course, and are attracted by it, they enter within the limits of our atmosphere in a luminous condition, and frequently let fall more or less strongly heated stony fragments, covered with a shining black crust. When we enter into a careful investigation of the facts observed at those epochs when showers of shooting stars fell periodically in Cumana in 1799, and in North America during the years 1833 and 1834, we shall find that 'fire-balls' can not be considered separately from shooting stars. Both these phenomena are frequently not only simultaneous and blended together, but they likewise are often found to merge into one another, the one phenomenon gradually assuming the character of the other alike with respect to the size of their disks, the emanation of sparks, and the velocities of their motion. Although exploding smoking luminous fire-balls are sometimes seen, even in the brightness of tropical daylight,* equaling in size the apparent p 113 diameter of the Moon, innumerable quantities of shooting stars have, on the other hand, been observed to fall in forms of such extremely small dimensions that they appear only as moving points or 'phosphorescent lines.'**

[footnote] *A friend of mine, much accustomed to exact trigonometrical measurements, was in the year 1788 at Popayan, a city which is 2 degrees 26' north latitude, lying at an elevation of 5583 feet above the level of the sea, and at noon, when the sun was shining brightly in a cloudless sky, saw his room lighted up by a fire-ball. He had his back to the window at the time, and on turning round, perceived that great part of the path traversed by the fire-ball was still illuminated by the brightest radiance. Different nations have had the most various terms to express these phenomena: The Germans use the word 'Sternschnuppe', literally 'star snuff' -- an expression well suited to the physical views of the vulgar in former times, according to which, the lights in the firmament were said to undergo a process of 'snuffing' or cleaning; and other nations generally adopt a term expressive of a 'shot' or 'fall' of stars, as the Swedish 'stjernifall', the Italian 'stella cadente', and the English 'star shoot.' In the woody district of the Orinoco, on the dreary banks of the Cassiquiare, I heard the natives in the Mission of Vasiva use terms still more inelegant than the German 'star snuff.' ('Relation Historique du Voy. aux Régions Equinox.', t. ii., p. 513.) These same tribes term the pearly drops of dew which cover the beautiful leaves of the heliconia 'star spit.' In the Lithuanian mythology, the imagination of the people has embodied its ideas of the nature and signification of falling stars under nobler and more graceful symbols. The Parcæ, 'Werpeja', weave in heaven for the new-born child its thread of fate, attaching each separate thread to a star. When death approaches the person, the thread is rent, and the star wanes and sinks to the earth. Jacob Grimm, 'Deutsche Mythologie', 1843, s. 685.

[footnote] ** According to the testimony of Professor Denison Olmsted, of Yale College, New Haven, Connecticut. (See Poggend., 'Annalen der Physik', bd. xxx., s. 194.) Kepler, who excluded fire-balls and shooting stars from the domain of astronomy, because they were, according to his views, "meteors arising from the exhalations of the earth, and blending with the higher ether," expresses himself, however, generally with much caution. He says: "Stellæ cadentes sunt materia viscida inflammata. Earum aliquæ inter cadendum absumuntur, aliquæ verè in terram cadunt, pondere suo tractæ. Nec est dissimile vero, quasdam conglobatas esse ex materia fæculentâ, in ipsam auram ætheream immixta: exque aëtheris regione, tractu rectilineo, per aërem trajicere, ceu minutos competas, occultâ causa motus utrorumque." -- Kepler, 'Epit. Astron. Copernicanæ', t. i., p. 80.

It still remains undertermined whether the many luminous bodies that shoot across the sky may not vary in their nature. On my return from the equinoctial zones, I was impressed with an idea that in the torrid regions of the tropics I had more frequently than in our colder latitudes seen shooting stars fall as if from a height of twelve or fifteen thousand feet; that they were of brighter colors, and left a more brilliant line of light in their track; but this impression was no doubt owing to the greater transparency of the tropical atmosphere*, which enables the eye to penetrate further into distance.

[footnote] *'Relation Historique', t. i., p. 80, 213, 527. If in falling stars, as in comets, we distinguish between the head or nucleus and the tail, we shall find that the greater transparency of the atmosphere in tropical climates is evinced in the greater length and brilliancy of the tail which may be observed in those latitudes. The phenomenon is therefore not necessarily more frequent there, because it is oftener seen and continues longer visible. The influence exercised on shooting stars by the character of the atmosphere is shown occasionally even in our temperate zone, and at very small distances apart. Wartmann relates that on the occasion of a November phenomenon at two places lying very near each other, Geneva and Aux Planchettes, the number of the meteors counted were as 1 to 7. (Wartmann, 'Mém. sur les Etoiles filantes', p. 17.) The tail of a shooting star (or its 'train'), on the subject of which Brandes has made so many exact and delicate observations, is in no way to be ascribed to the continuance of the impression produced by light on the retina. It sometimes continues visible a whole minute, and in some rare instances longer than the light of the nucleus of the shooting star; in which case the luminous track remains motionless. (Gilb., 'Ann.', bd. xiv., s. 251.) This circumstance further indicates the analogy between large shooting stars and fire-balls. Admiral Krusenstern saw, in his voyage round the world, the train of a fire-ball shine for an hour after the lluminous body itself had disappeared, and scarcely move throughout the whole time. ('Reise', th. i., s. 58.) Sir Alexander Burnes gives a charming description of the transparency of the clear atmosphere of Bokhara, which was once so favorable to the pursuit of astronomical observations. Bokhara is situated in 39 degrees 48' north latitude, and at an elevation of 1280 feet above the level of the sea. "There is a constant serenity in its atmosphere, and an admirable clearness in the sky. At night, the stars have uncommon luster, and the Milky Way shines gloriously in the firmament. There is also a never-ceasing display of the most brilliant meteors, which dart like rockets in the sky; ten or twelve of them are sometimes seen in an hour, assuming every color -- fiery red, blue, pale, and faint. It is a noble country for astronomical science, and great must have been the advantage enjoyed by the famed observatory of Samarkand." (Burnes, 'Travels into Bokhara', vol. ii. (1834), p. 158.) A mere traveler must not be reproached for calling ten or twelve shooting stars in an hour "many," since it is only recently that we have learned, from careful observations on this subject in Europe, that eight is the mean number which may be seen in an hour in the field of vision of one individual (Quetelet, 'Corresp. Mathém.', Novem., 1837, p. 447); this number is, however, limited to five or six by that diligent observer, Olbers. (Schum., 'Jahrb.', 1838, s. 325.)

p 114 Sir Alexander Burnes likewise extols as a consequence of the purity of the atmosphere in Bokhara the enchanting and constantly-recurring spectacle of variously-colored shooting stars.