PLATE XI.—THE GREAT COMET OF 1881.Observed on the night of June 25-26 at 1h. 30m. A.M.
PLATE XI.—THE GREAT COMET OF 1881.Observed on the night of June 25-26 at 1h. 30m. A.M.
PLATE XI.—THE GREAT COMET OF 1881.
Observed on the night of June 25-26 at 1h. 30m. A.M.
From my observations upon the comets which have appeared since the year 1873, it is apparent that the changes in the nucleus, coma and tail, are due to a solar action, which contracts or expands these objects in such a manner that the nuclei become either bright and star-like, or dim and diffused, in a very short time. I had excellent opportunity, especially in the two large comets of 1881, to observe some of these curious changes, a description of which will give an idea of their extent and rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881, III., which is represented on Plate XI., appeared sharply defined, bright and considerably flattened crosswise; but half an hour later it had considerably enlarged and had become so diffused that it could hardly be distinguished from the coma, with which it gradually blended. It is perhaps worth mention that, at the time this last observation was made, an aurora borealis was visible. This comet 1881, III., underwent other very important changes of its nucleus, coma and tail. On June 25th, the nucleus, which was bright and clearly defined, was ornamented with four bright diverging conical wings of light, as shown on Plate XI. On the 26th these luminous wings had gone, and the nucleus appeared one-third smaller. On the 28th it had enlarged, but on the 29th its shape was considerably altered, the nucleus extending in one direction to three or four times its diameter on previous nights, and being curved, so as to resemble a comma. On the 6th of July the nucleus of this comet showed the greatest disturbances. The nucleus, which had appeared perfectly round on the evening of the 5th, was found much elongated at 10 o'clock on the 6th, forming then a straight, acute, and well-defined wedge of light, inclined upwards to the left. The length of the nucleus, at this time, was three or four times its ordinary diameter. At the same time rapid changes occurred; the strangely shaped nucleus soon became unsteady, extending and contracting alternately, and varying greatly in brightness. At 10h. 45m., the elongated nucleus, then gently curved, took the shape of a succession of luminous knots, which at times became so brilliant and distinct that they seemed to be about to divide and form separate nuclei; but such a separation did not actually occur, at least while I was observing. While these important changes were going on in the comet, a bright auroral arch appeared in the north, which lasted only a short time. On July 7th, the sky being cloudy, no observations were made, but on the 8th I observed the comet again. The nucleus had then resumed its circular form, but it was yet very unsteady, being sometimes small, bright and sharp, while a few seconds later it appeared twice as large, but dim in outlines; and sometimes an ill-defined secondary nucleus appeared at its centre. On several occasions the nucleus appeared as if it were double, one nucleus being apparently projected partly upon the other.
The nuclei of comets are sometimes very small, and in other cases very large. Among those which have been measured, the nucleus of the comet of 1798, I., was only 28 miles in diameter, but that of Donati's comet, in 1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in diameter.
The coma of comets is found to be even more variable than the nucleus. The changes observed in the coma are generally in close connection with those of the nucleus and tail, the same perturbations affecting simultaneously the whole comet. While the coma of the comet of 1847 was only 18,000 miles in diameter, that of Halley's comet, in 1835, was 357,000 miles, and that of the comet of 1811 was 1,125,000 miles in diameter. In general, as already stated, the coma of a comet decreases in size in approaching the Sun. That of Encke's comet, which, on October 9th, 1838, had a diameter of 281,000 miles, gradually decreased at a daily mean rate of 4,088 miles in going towards the Sun; so that, on December 17th, when the distance of the comet from the Sun was more than four times less than it was on the first date, its diameter was reduced to 3,000 miles.
The form of the coma, in that part which is free from the tail, is in general a portion of a circle, but is sometimes irregular, with its border deformed. Thus, the border of the coma of Halley's comet was depressed at one point towards the Sun. I observed a similar phenomenon in Coggia's comet, with the great refractor of the Harvard College Observatory, on July 13th, 1874, when its border appeared deeply depressed on the side nearest to the Sun, as if repelled by this body. The coma of comet 1881, III., showed also very singular outlines on the nights of the 25th and 26th of June, when its border was so deeply depressed that the coma appeared as if it were double. Luminous rays and jets often radiate from the nucleus across the coma, and describe graceful lateral curves, falling backwards and gradually fading away into the tail, of which they then form a part. The rays and jets emitted by the nucleus seem at first to obey the solar attraction and travel towards the Sun; but they are soon repelled, and move backward towards the tail. It is a mystery, as yet unexplained, how these cometary jets, which at first seem to obey to the laws of attraction, are compelled to retreat apparently by superior opposing forces. Among the forces of nature, we know of no other than those of an electrical sort, which would act in a similar manner; but this explanation would require us to assume some direct electrical communication between the comet and the Sun. Considering the distance between the two bodies, and the probable absence or great tenuity of the gaseous material in interstellar space, such an assumption is a difficult one.
Under the action of the solar forces, the coma also very frequently forms itself into concentric luminous arcs, separated by comparatively dark intervals. These luminous semi-circles vary in number, but sometimes there are as many as four or five at a time. All great comets show these concentric curves more or less, but sometimes only a portion is visible, the rest of the coma having a different structure. When great comets approach near the Sun, their coma is generally composed of two distinct parts, an inner and an outer coma, the inner one being due to the luminous jets issuing from the nucleus, which, never extending very far, form a distinct, bright zone within the fainter exterior coma.
The tails of comets, which are in fact a prolongation of the coma, are likewise extremely variable in form. They are sometimes straight like a rod; again, are curved like a sabre, or even crooked like an S, as was that of the comet of 1769. They are also fan-shaped, pointed, or of the same width throughout. Many of these appendages appear longitudinally divided through their middle by a narrow, darkish rift, extending from the nucleus to the extremity. This peculiarity appears in the comet shown on Plate XI. Sometimes the dark rift does not commence near the nucleus, but at some distance from it, as I observed in the case of comet 1881, III., on June 26th. This dark rift is not a permanent feature of a comet's tail, but may be visible one day and not at all the next. Comet 1881, III., which had shown a dark rift towards the end of June, did not exhibit any such rift during July and August, when, on the contrary, its tail appeared brighter in the middle. Coggia's comet, which showed so prominent a dark rift in July, 1874, had none on June 10th. On the contrary, the tail was on that date very bright along its middle, as also along each of its edges.
The tail of a comet does not invariably point directly away from the Sun, as above mentioned, and sometimes the deviation is considerable; for instance, the tail of the comet of 1577 deviated 21° from the point opposite to the Sun.
In general, the tail inclines its extremity towards the regions of space which it has just left, always presenting its convex border to the regions towards which it is moving. It is also a remarkable fact that this convex border, moving first in space, always appears brighter and sharper than the opposite one, which is often diffused. From these peculiarities it would seem that in moving about the Sun the comets encounter some resistance to their motion, from the medium through which they pass, and that this resistance is sufficient to curve their tails away from the course in which they move, and to crowd their particles together on the forward side. It is especially when they approach their perihelion, and move more rapidly on a curve of a shorter radius, that the comets' tails show the greatest curvature, unless their position in regard to the observer prevents their being advantageously seen. The tail of Donati's comet presented a fair illustration of this peculiarity, its curvature having augmented with the velocity of the comet's motion about the Sun. But possibly this phenomenon has another cause, and may be found rather in the solar repulsion which acts on comets and is not instantaneously propagated throughout their mass.
Although, in general, comets have but one tail, it is not very rare to see them with multiple tails. The comets of 1807 and 1843 had each a double tail; Donati's comet, in 1858, showed several narrow, long rectilinear rays, issuing from its abruptly curved tail. The comet of 1825 had five branches, while that of 1744 exhibited no less than six distinct tails diverging from the coma at various angles. In general character the multiple and single tails are similar. When a comet has two tails, it is not rare for the second to extend in the general direction of the Sun, as was the case with the great comet of 1881, III., represented on Plate XI. From July 14th to the 21st it exhibited quite an extended conical tail, starting obliquely downwards from the right side of the coma, and directed towards the Sun. From the 24th of July to the 2d of August this secondary tail was exactly opposite in its direction from that of the primary tail, and gave to the head a very elongated appearance. Comet 1881, IV., also exhibited a secondary appendage, not directed towards the Sun, but making an angle of about 45° with the main tail.
These cometary appendages sometimes attain prodigious dimensions. The comets of 1680 and 1769 had tails so extended that, after their heads had set under the horizon, the extremities of these immense appendages were still seen as far up as the zenith. In a single day the tail of the comet of 1843 extended 100°, and it was thrust from the comet "as a dart of light" to the enormous distance of 48,500,000 miles, and yet of this immense appendage nothing was left on the following day. The tail of Donati's comet, in 1858, attained a real length of 42,000,000 miles, while that of the great comet of 1843 had the enormous length of 200,000,000 miles. If this last comet had occupied the position of the Sun, which it approached very nearly for a moment, the extremity of its tail would have extended 60,000,000 miles beyond the orbit of Mars.
In some cases the tails of comets have been seen undulating and vibrating in a manner similar to the undulations and coruscations of light characteristic of some auroras. Many observers report having seen such phenomena. The comet of 1769 was traversed by luminous waves and pulsations, comparable to those seen in the aurora borealis. I myself observed these curious undulations in Coggia's comet in 1874, while the head of this object was below the horizon. For an hour the undulations rapidly succeeded each other, and ran along the whole length of the tail.
Some of the brightest comets have shone with such splendor that they could be observed easily in full sunshine. Many comets, such as those of 1577 and 1744, have equaled Sirius and Venus in brilliancy. The great comet of 1843, which suddenly appeared in our sky, was so brilliant that it was seen by many observers at noon time, within a few degrees from the Sun. I remember that I myself saw this remarkable object in the day time, with a number of persons, who were gazing at the wonderful apparition. So brilliant was this comet, that besides its nucleus and head, a portion of its tail was also visible in the day time, provided the observer screened his eyes from the full sunlight by standing in the shadow of some building.
Of all the bodies revolving around the Sun, none have been known to approach so near its surface as did the comet of 1843. When it arrived at perihelion, the distance from the centre of its nucleus to the surface of the Sun's photosphere was only 96,000 miles, while the distance from surface to surface was less than 60,000 miles. This comet, then, went through the solar atmosphere, and in traversing it with its tremendous velocity of 366 miles per second, may very possibly have swept through some solar protuberances, many of which attain much higher elevations than that at which the comet passed. The comet of 1680 also approached quite near the surface of the Sun, and near enough to encounter some of the high solar protuberances, its distance at perihelion being about two-thirds of the Moon's distance from the Earth. The rapidity of motion of the comet of 1843 was such, when it approached the Sun, that it swept through all that part of its orbit which is situated north of the plane of the ecliptic in a little more than two hours, moving in this short time from one node to the other, or 1800.
But if some comets have a very short perihelion distance, that of others is considerable. Such a comet was that of 1729, whose perihelion distance was 383,000,000 miles, the perihelion point being situated between the orbits of Mars and Jupiter.
While some comets come near enough to the Sun at perihelion to be volatilized by its intense heat, others recede so far from it at aphelion that they may be said to be frozen. The shortest cometary aphelion distance known is that of Encke's comet, whose greatest distance from the sun is 388,000,000 miles. But that of the comet of 1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864 are so remote in space when they reach their aphelion points that light, with its velocity of 185,500 miles a second, would require 171 days in the first case, and 230 in the last, to pass from them to the Earth.
The period of revolution of different comets also varies immensely. While that of Encke's comet is only 3½ years, that of comet 1864, II., is 280,000 years.
Among the periodic comets of short period, some have exhibited highly interesting phenomena. Encke's comet, discovered in 1818, is remarkable for the fact that its period of revolution diminishes at each of its successive returns, and consequently this comet, with each revolution, approaches nearer and nearer to the Sun. The decrease of the period is about 2½ hours at each return. Although the decrease is small, if it go on in future as it does at present, the inevitable consequence will be that this comet will finally fall into the Sun. This curious phenomenon of retardation has been attributed by astronomers to the existence of a resisting medium filling space, but so rare and ethereal that it does not produce any sensible effect on the movements of the planets. But some other causes may retard this comet, as similar retardations have not been observed in the case of other periodic comets of short period. These, however, are not so near to the Sun, and perhaps our luminary may be surrounded by matter of extreme tenuity, which does not exist at a greater distance from it.
Another of the periodic comets which has exhibited a very remarkable phenomenon of transformation is Biela's comet, which divided into two distinct parts, moving together in the same direction. When this comet was first detected at its return in 1845, it presented nothing unusual, but in the early part of 1846 it was noticed by several astronomers to be divided into two parts of unequal brightness, forming thus a twin comet. At its next return in 1852, the two sister comets were still traveling in company, but their distance apart, which in 1846 was 157,000 miles, had increased to 1,500,000 miles. At the two next returns in 1859 and 1865, their position not being very favorably situated for observation, the comets were not seen. In 1872 the position should have been favorable for observation, and they were consequently searched for, but in vain; neither comet was found. An astronomer in the southern hemisphere, however, found a comet on the track of Biela's, but calculation has shown that the two objects are probably not identical, since this comet was two months behind the computed position for Biela's. It will be shown in the following chapter that our globe probably crossed the orbit of Biela's comet on November 27th, 1872, and the phenomena resulting from this passage will be there described.
It is seen from these observations that comets may be lost or dissipated in space by causes entirely unknown to us. Biela's comet is not the only one which has been thus disintegrated. Ancient historians speak of the separation of large comets into two or more parts. In 1661 Hevelius observed the apparent division of the comet of that year and its reduction to fragments. The return of this comet, calculated for 1790, was vainly waited for; the comet was not seen.
Other comets, whose periods of revolution were well known, have disappeared, probably never to return. Such is Lexell's comet, whose period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are now lost. It is supposed that Lexell's comet, which passed twice very near the giant planet Jupiter, had its orbit changed from an ellipse to a parabola, by the powerful disturbing influence of this planet, and was thus lost from our system. Several other comets, in traveling over their different orbits, have approached near enough to Saturn, Jupiter and the Earth to have their orbits decidedly altered by the powerful attraction of these bodies.
But since comets are liable to pass near the planets, and several have orbits which approach that of the Earth, it becomes important for us to know whether an encounter of such a body with our globe is possible, and what would then be the result for us. Although that knowledge would not enable us to modify the possibilities of an encounter, yet it is better to know the dangers of our navigation through space than to ignore them. This question of a collision of the Earth with a comet has been answered in different ways, according to the ideas entertained in regard to the mass of these bodies. While some have predicted calamities of all kinds, such as deluges, conflagrations, or the reduction of the Earth to incandescent gases, others have asserted that it would produce no more effect than does a fly on encountering a railroad train. In our days astronomers entertain very little fears from such an encounter, because the probabilities of danger from an occurrence of this sort are very slight, the mass of an ordinary comet being so small compared with that of our globe. We know with certainty that the Earth has never had an encounter with a cometby which it has been transformed into gases, at least within the several millions of years during which animal and vegetable life have left their marks upon the stony pages of its history, otherwise these marks would not now be seen. If, then, such an accident has not happened during this long period, the chances for its occurring must be very small, so small indeed that they might almost be left out of the question. It is true that our globe shows signs of great perturbations of its surface, but we have not the slightest proofs that they resulted from an encounter with a celestial body. It seems very probable that our globe passed through the tail of the comet of 1861, before it was first seen on June 29th; but nothing unusual was observed, except perhaps some phosphorescent light in the atmosphere, which was afterwards attributed to this cause.
The density and mass of comets must be comparatively very small. Their tails consist of matter of such extreme tenuity that it affects but very little the light of the small stars over which they pass. The coma and nucleus, however, are not quite so transparent, and may have greater masses. On several occasions I have seen the light of stars reduced by the interposition of cometary matter, comet 1881, III., presenting remarkable cases of this sort. On July 8th, at 10h. 50m., several small stars were involved in this comet, one of which passed quite near the nucleus through the bright inner coma. At that time the comet was greatly disturbed, its nucleus was contracting and enlarging rapidly, and becoming bright and again faint in an instant. Every time that the nucleus grew larger, the star became invisible, but reappeared the moment the nucleus was reduced in size. This phenomenon could not be attributed to an atmospheric effect, since, while the nucleus was enlarging, a very small inner nucleus was visible within the large diffused one, the matter of which had apparently spread over the part of the coma in which the star was involved, making it invisible.
That the mass of comets is small, is proved by the fact that they have sometimes passed near the planets without disturbing them in any sensible manner. Lexell's comet, which in 1770 remained four months very near Jupiter, did not affect in the least the orbits, or the motions of its satellites. The same comet also came within less than 1,500,000 miles from the Earth, and on this occasion it was calculated that its mass could not have been the ¹⁄₅₀₀₀ part of that of our globe, since otherwise the perturbations which it would have caused in the elements of the Earth's orbit would have been sensible. There was, however, no change. If this comet's mass had been equal to that of our globe, the length of our year would have been increased by 2h. 47m. The comet of 1837 remained four days within 3,500,000 miles of the Earth, with no sensible effect.
It seems quite difficult to admit that the denser part of a comet forming the nucleus is solid, as supposed by some physicists, since it is so rapidly contracted and dilated by the solar forces, while the comet is yet at a too great distance from the Sun to allow these effects to be attributed to solar heat alone. This part of a comet, as indeed the other parts, seems rather to be in the gaseous than in the solid state; the changes observed in the intensity of its light and in its structure may be conceived as due to some solar action partaking of the nature of electricity.
It has been a question whether comets are self-luminous, or whether they simply reflect the solar light. When their light is analyzed by the spectroscope, it is found that the nucleus of a comet generally gives a continuous spectrum, while the coma and tail give a spectrum consisting of several bright diffused bands. The spectrum given by the nucleus is rarely bright enough to allow the dark lines of the solar spectrum to be discerned upon it; but such lines were reported in the spectrum of comet 1881, III., a fact proving that this nucleus at least reflected some solar light. The nucleus of a comet may be partly self-luminous, and either solid, liquid, or composed of incandescent gases submitted to a great pressure. As to the coma and tail, they are evidently gaseous, and partly, if not entirely, self-luminous, as is proved by the band spectrum which they give. The position of these bands, moreover, indicates that the luminous gases of which they are composed contain carbon. The phenomena of polarization, however, seem to prove that these parts of comets also reflect some solar light.
No theory so far proposed, to explain comets and the strange phenomena they exhibit, seems to have been successful in its attempts, and the mystery in which these bodies have been involved from the beginning of their apparition, seems to be now nearly as great as ever. It has been supposed that their tails have no real existence, but are due to an optical illusion. Prof. Tyndall has endeavored to explain cometary phenomena by supposing these bodies to be composed of vapors subject to decomposition by the solar radiations, and thus made visible, the head and tail being an actinic cloud due to such decompositions. According to this view, the tails of comets would not consist of matter projected into spacer but simply of matter precipitated by the solar rays in traversing the cometary nebulosity. The endeavor has also been made to explain the various phenomena presented by comets by an electrical action of the Sun on the gases composing these objects. Theories taking this as a base seem to us to be more likely to lead to valuable results. M. Faye, who has devoted much time and learning to this subject, assumes a real repulsive force of the Sun, acting inversely to the square of the distance and proportionally to the surface, and not to the mass as attraction does. He supposes, however, that this repulsive force is generated by the solar heat, and not by electricity. Prof. Wm. Harkness says that many circumstances seem to indicate that the comets' tails are due, in a great measure, to electrical phenomena.
The fact that the tails of comets are better defined and brighter on the forward side, associated with the other fact that they curve the most when their motion is most rapid, sufficiently indicates that these appendages are material, and that they either encounter some resistance from the medium in which they move, or from a solar repulsion. The phenomena of condensation and extension, which I have observed in the comets of 1874 and 1881, added to the curious behavior exhibited by the jets issuing from the nucleus, seem to indicate the action of electrical forces rather than of heat. The main difficulty encountered in the framing of a theory of comets consists in explaining how so delicate and extended objects as their tails seem to be, can be transported and whirled around the Sun at their perihelion with such an enormous velocity, always keeping opposite to the Sun, and, as expressed by Sir John Herschel, "in defiance of the law of gravitation, nay, even of the received laws of motion."
To consider the direction of the comets' tails as an indirect effect of attraction, seems out of the question; the phenomenon of repulsion so plainly exhibited by these objects seems to point to a positive solar repulsion, as alone competent to produce these great changes. The repulsive action of the Sun on comets' tails might be conceived, for instance, as acting in a manner similar to that of a powerful current of wind starting from the Sun, and constantly changing in direction, but always keeping on a line with the comet. Such a current, acting on a comet's tail as if it were a pennant, would drive it behind the nucleus just as observed. If it could once be ascertained that the great disturbances on comets correspond with the magnetic disturbances on our globe and with the display of the auroral light, the electric nature of the forces acting so strangely on the comets would be substantially demonstrated. I have shown that some of the great disturbances observed in the comets of 1874 and 1881 have coincided with auroral displays, and it will be shown hereafter that similar displays have also coincided with the passage of meteoric showers through our atmosphere. Whether these simultaneous phenomena were simple coincidences having no connection, or whether they are the result of a common cause, can only be ascertained by long continued future observations.
While contemplating the heavens on a clear moonless night, we occasionally witness the sudden blazing forth of a star-like meteor, which glides swiftly and silently across some of the constellations, and as suddenly disappears, leaving sometimes along its track a phosphorescent trail, which remains visible for a while and gradually vanishes. These strange apparitions of the night are calledFallingorShooting-stars.
There is certainly no clear night throughout the year during which some of these meteors do not make their appearance, but their number is quite variable. In ordinary nights only four or five will be observed by a single person in the course of an hour; but on others they are so numerous that it becomes impossible to count them. When the falling stars are only a few in number, and appear scattered in the sky, they are calledSporadic Meteors, and when they appear in great numbers they constituteMeteoric ShowersorSwarms.
Probably there is no celestial phenomenon more impressive than are these wonderful pyrotechnic displays, during which the heavens seem to break open and give passage to fiery showers, whose luminous drops describe fantastic hieroglyphics in the sky. While observing them, one can fully realize the terror with which they have sometimes filled beholders, to whom it seemed that the stability of the universe had come to an end, and that all the stars of the firmament were pouring down upon the Earth in deluges of fire.
The ancients have left record of many great meteoric displays, and the manner in which they describe them sufficiently indicates the fear caused by these mysterious objects. Among the many meteoric showers recorded by ancient historians may be mentioned one observed in Constantinople, in the month of November, 472, when all the sky appeared as if on fire with meteors. In the year 599, meteors were seen on a certain night flying in all directions like fiery grasshoppers, and giving much alarm to the people. In March, 763, "the stars fell suddenly, and in such crowded number that people were much frightened, and believed the end of the world had come." On April 10th, 1095, the stars fell in such enormous quantity from midnight till morning that they were as crowded as are the hail stones during a severe storm.
In modern times the fall of the shooting-stars in great number has been frequently recorded. One of the most remarkable meteoric showers of the eighteenth century occurred on the night of November 13th, 1799, and was observed throughout North and South America and Europe. On this memorable night thousands of falling stars were seen traversing the sky between midnight and morning. Humboldt and Boupland, then traveling in South America, observed the phenomena at Cumana, between two and five o'clock in the morning. They saw an innumerable number of shooting-stars going from north to south, appearing like brilliant fire-works. Several of these meteors left long phosphorescent trails in the sky, and had nuclei whose apparent diameter, in some cases, surpassed that of the Moon.
The shower of November 13th, 1833, was still more remarkable for the great number of meteors which traversed the heavens, and was visible over the whole of North and South America. On that occasion the falling stars were far too numerous to be counted, and they fell so thickly that Prof. Olmsted, of New Haven, who observed them carefully, compared their number at the moment of their maximum fall to half that of the flakes of snow falling during a heavy storm. This observer estimated at 240,000 the number of meteors which must have traversed the heavens above the horizon during the seven hours while the display was visible.
PLATE XII.—THE NOVEMBER METEORS.As observed between midnight and 5 o'clock A.M. on the night of November 13-14 1868.
PLATE XII.—THE NOVEMBER METEORS.As observed between midnight and 5 o'clock A.M. on the night of November 13-14 1868.
PLATE XII.—THE NOVEMBER METEORS.
As observed between midnight and 5 o'clock A.M. on the night of November 13-14 1868.
In the years 1866, 1867 and 1868, there were also extraordinary meteoric displays on the night of November 13th. It was on the last mentioned date that I had the opportunity to observe the remarkable shower of shooting-stars of which I have attempted to represent all the characteristic points in Plate XII. My observations were begun a little after midnight, and continued without interruption till sunrise. Over three thousand meteors were observed during this interval of time in the part of the sky visible from a northern window of my house. The maximum fall occurred between four and five o'clock, when they appeared at a mean rate of 15 in a minute.
In general, the falling stars were quite large, many being superior to Jupiter in brightness and apparent size, while a few even surpassed Venus, and were so brilliant that opaque objects cast a strong shadow during their flight. A great many left behind them a luminous train, which remained visible for more or less time after the nucleus had vanished. In general, these meteors appeared to move either in straight or slightly curved orbits; but quite a number among them exhibited very extraordinary motions, and followed very complicated paths, some of which were quite incomprehensible.
While some moved either in wavy or zig-zag lines, strongly accentuated, others, after moving for a time in a straight line, gradually changed their course, curving upward or downward, thus moving in a new direction. Several among them, which were apparently moving in a straight line with great rapidity, suddenly altered their course, starting at an abrupt angle in another direction, with no apparent slackening in their motion. One of them, which was a very conspicuous object, was moving slowly in a straight course, when of a sudden it made a sharp turn and continued to travel in a straight line, at an acute angle with the first, retreating, and almost going back towards the regions from which it originally came. As nearly all the meteors which exhibited these extraordinary motions left the trace of their passage in the sky by a luminous trail, it was easily ascertained that these appearances were not deceptive. On one occasion I noticed that the change of direction in the orbit corresponded with the brightening up of the meteor thus disturbed in its progress.
Among these meteors, some traveled very slowly, and a few seemed to advance as if by jerks, but in general they moved very rapidly. One of the meteors thus appearing to move by jerks left a luminous trail, upon which the various jerks seemed to be left impressed by a succession of bright and faint spaces along the train. Some of the largest meteors appeared to rotate upon an axis as they advanced, and most of these revolving meteors, as also a great number of the others, seemed to explode just before they disappeared, sending bright fiery sparks of different colors in all directions, although no sound was at any time heard. The largest and most brilliant meteor observed on that night appeared at 5h. 30m., a little before sunrise. It was very bright, and appeared considerably larger than Venus, having quite a distinct disk. This meteor moved very slowly, leaving behind a large phosphorescent trail, which seemed to issue from the inside of the nucleus as it advanced. For a moment the train increased in size and brightness close to the nucleus, which then appeared as an empty transparent sphere, sprinkled all over with minute fiery sparks; the nucleus then suddenly burst out into luminous particles, which immediately vanished, only the luminous trail of considerable dimensions being left.
Many of the trails thus left by the meteors retained their luminosity for several minutes, and sometimes for over a quarter of an hour. These trails slowly changed their form and position; but it is perhaps remarkable that almost all those which I observed on that night assumed the same general form—that of an open, irregular ring, or horse-shoe, somewhat resembling the letter C. This ring form was subsequently transformed into an irregular, roundish cumulus-like cloud. The trail left by a very large meteor, which I observed on the evening of September 5th, 1880, also exhibited the same general character of transformation.
While I was observing a long brilliant trail left by a meteor on the night of November 13th, 1868, it was suddenly crossed by another bright shooting-star. The latter apparently went through the luminous substance forming the trail, which was suddenly altered in form, and considerably diminished in brightness simultaneously with this passage, although electrical action at some distance might perhaps as well explain the sudden change observed.
In the majority of cases the meteors appeared white; but many, especially the largest, exhibited a variety of brilliant colors, among which the red, blue, green, yellow and purple were the most common. In general the trails exhibited about the same color as the nucleus, but much fainter, and they were usually pervaded by a greenish tint. In some instances the trails were of quite a different color from the nucleus.
The luminous cloud observed at 5h. 30m. on the morning of November 14th, 1868, after having passed through the series of transformations above described, remained visible for a long while after sunrise, appearing then as a small cirrus cloud, exactly similar in appearance to the hundreds of small cirrus clouds then visible in the sky, which had probably the same meteoric origin. For over three hours after sunrise, these cirrus clouds remained visible in the sky, moving all together with the wind in the high regions of the atmosphere.
Although Plate XII. is intended to represent all the characteristics exhibited by the meteors observed on that night, every form represented having been obtained by direct observation, yet the number is much greater than it was at any single moment during the particular shower of 1868. As regards number, the intention was to give an idea of a great meteoric shower, such as that of 1833, for instance. Although many of the falling stars seem to be close to the Earth's surface, yet this is only an effect of perspective due to their great distance, very few of these meteors ever coming into the lower regions of our atmosphere at all.
The phenomena exhibited during other great meteoric showers have been similar to those presented by the shower just described, the only differences consisting in variations of size and brightness in the meteors, and also in the trails, which sometimes are not so numerous as they were in 1868.
While some shooting-stars move so rapidly that they can hardly be followed in their orbits, others move so slowly that the sight can easily follow them, and even remark the peculiarities of their movements, some remaining visible for half a minute. Some of the falling stars move at the rapid rate of 100 miles a second, but others only 10 miles a second, and even less. In general, they move about half as fast again as the Earth in its orbit. The arcs described by the meteors in the sky are variable. While some extend 8o° and even 100°, others are hardly half a degree in length. While some shooting-stars are so faint that they can hardly be seen through the largest telescopes, others are so large and brilliant that they can be seen in the day-time. In general, a shooting-star of average brightness resembles a star of the third or fourth magnitude.
Whatever may be the origin of the shooting-stars, they are, when we see them, not in the celestial spaces, like the planets, the comets, or the stars, but in our atmosphere, through which they travel as long as they remain visible. The height at which they appear and disappear is variable, but in general they are about 80 miles above the surface of our globe when they are first seen, and at about 55 miles when they disappear. In many cases, however, they have been observed at greater elevations, as also at smaller. A meteor simultaneously observed at two different stations first appeared at the height of 285 miles, and was last seen at 192 miles above the Earth's surface; but in rare cases the falling stars have been seen below a layer of clouds completely covering the sky. I myself saw one such shooting-star a few years since. The fact that the meteors are visible at so great elevations, proves that our atmosphere extends much farther than was formerly supposed, although at these great heights it must be extremely rarefied, and very different from what it is in its lower regions.
There is a remarkable difference between the sporadic meteors seen in the sky on every night, and the meteoric showers observed only at comparatively rare intervals. While the first appear from different points in the sky and travel in all directions, being perfectly independent, the meteors of a shower all come from the same point of the heavens, from which they apparently diverge in all directions. This point of divergence of the meteors is called theradiant pointof the shower. Although the meteors seem to diverge in all directions from the radiant point, yet they all move in approximately parallel lines, the divergence being an effect of perspective.
Whatever may be the position of the radiant point in the constellations, it remains as fixed in the sky as the stars themselves, and participates with them in the apparent motion which they undergo by the effect of the diurnal motion, and thus rises and sets with the constellation to which it belongs. This fact is sufficient to prove that the orbits of these meteors are independent of the Earth's motion, and that consequently they do not originate in our atmosphere. It has been shown by Encke that the radiant point of the meteoric shower of November 13th is precisely the point towards which our globe moves in space on November 13th; a tangent to the Earth's orbit would pass through this radiant point.
The meteoric showers are particularly remarkable, not merely because of the large number of meteors which are visible and the fact that they all follow a common orbit, but chiefly because they have a periodic return, either after an interval of a year, or after a lapse of several years. At the beginning of the present century only two meteoric showers were known, those of August 10th and of November 13th, and their periodicity had not yet been recognized, although it had begun to be suspected. It was only in 1836 that Quetelet and Olbers ventured to predict the reappearance of the November meteors in the year 1867. Having made further investigations, Prof. Newton, of Yale College, announced their return in the year 1866. In both of these years, as also in 1868, the meteors were very numerous, and were observed in Europe and in America on the night of November 13th. The predictions having thus been fulfilled, the periodicity of the meteors was established. Since then, other periodic showers have been recognized, although they are much less important in regard to number than those of August and November, except that of November 27th, which exhibited so brilliant a display in Europe in 1872. These successive appearances have established the main fact that meteoric showers are more or less visible every year when the Earth occupies certain positions in its orbit.
The meteoric shower of the 10th of August has its radiant point situated in the vicinity of the variable star Algol, in the constellation Perseus, from which its meteors have received the name of Perseids. Although varying in splendor, this meteoric swarm never fails to make its appearance every year. The Perseids move through our atmosphere at the rate of 37 miles per second. The shower usually lasts about six hours.
The meteoric shower of November 13th has its radiant point situated in the vicinity of the star Gamma, in the constellation Leo, from which its meteors have been called Leonids. But while the August meteors recur regularly every year, with slight variations, the shower of November does not occur with the same regularity. During several years it is hardly noticeable, and is even totally absent, while in other years it is very remarkable. Every 33 years an extraordinary meteoric shower occurs on the 13th of November, and the phenomenon is repeated on the two succeeding years at the same date, but with a diminution in its splendor at each successive return. The Leonids move in an opposite direction to that of the Earth, and travel in our atmosphere with an apparent velocity of 45 miles per second, this being about the maximum velocity observed in falling stars. But when the motion of our globe is taken into account, and a deduction is made of the 18 miles which it travels per second, it is found that these meteors move at an actual mean rate of 27 miles a second.
In a meteoric shower the stars do not fall uniformly throughout the night, there being a time when they appear in greater numbers. Usually it is towards morning, between 4 and 6 o'clock, that the maximum occurs. The probable cause of this phenomenon will be explained in its place hereafter.
The orbits of the meteoric showers are not all approximately in the same plane, like those of the planets, but rather resemble those of comets, and have all possible inclinations to the ecliptic. Like the comets, too, the different meteoric showers have either direct or retrograde motion.
The shooting-stars were formerly considered as atmospheric meteors, caused by the combustion of inflammable gases generated at the surface of the Earth, and transported to the high regions of our atmosphere by their low specific gravity. But the considerable height at which they usually appear, the great velocity of their motion, the common orbit followed by the meteors of the same shower, and the periodicity of their recurrence, do not permit us now to entertain these ideas, or to doubt their cosmical origin. But what is their nature?
It is now generally admitted that innumerable minute bodies, moving in various directions around the Sun, are scattered in the interplanetary spaces through which our globe travels. It has been supposed that congregations of such minute bodies form elliptical rings, within which they are all moving in close parallel orbits around the Sun. On the supposition that such rings intersect the orbit of the Earth at the proper places, it was practicable to account for the shooting-stars by the passage through our atmosphere of the numerous minute cosmical bodies composing the rings, and the Leonid and Perseid showers were so explained. But when the elements of the orbits of these two last swarms came to be better known, and were compared with those of other celestial bodies, it was found necessary to alter this theory.
It had for a long while been suspected that some kind of relation existed between the shooting-stars and the comets. This idea, vaguely formulated by Kepler more than two centuries ago, more clearly expressed by Chladni, and still more by Mr. Grey, before the British Association, at Liverpool, in 1855, has recently received a brilliant confirmation by the researches of Professor Schiaparelli, Director of the Observatory of Milan. A thorough investigation of the orbits of the August and November meteors led Schiaparelli to the discovery of a remarkable relation between meteoric and cometary orbits. By comparing the elements of these meteoric orbits with those of comets, he found a very close resemblance between the orbit of the August meteors and that of the comet 1862, III., and again between the orbit of the November meteors and that of Tempel's comet, 1866, I. These resemblances were too striking to be the result of mere chance, and demonstrated the identity of these cometary orbits with those of the Perseid and Leonid showers. In accordance with these new facts, it is now admitted that the meteoric showers result from the passage of our globe through swarms of meteoric particles following the orbits of comets, which intersect the orbit of the Earth.
Professor Schiaparelli has attempted to show how these meteoric swarms were originally scattered along the orbits of comets, by supposing these bodies to originate from nebulous masses, which, in entering the sphere of attraction of the Sun, are gradually scattered along their orbits, and finally form comets followed by long trails of meteoric particles.
It has been shown that in approaching the Sun the comets become considerably elongated, their particles being disseminated over immense distances by the solar repulsion. It seems probable that, owing to its feeble attractive power, the nucleus is incompetent to recall the scattered cometary particles and retain them in its grasp when they are relieved from the solar repulsion, so that they remain free from the nucleus, although they continue to move along its orbit. It is supposable that these cometary particles will scatter more and more in course of time. Forming at first an elongated meteoric cloud, they will finally spread along the whole orbit, and thus form a ring of meteoric particles. Since our globe constantly moves in its orbit and daily occupies a different position, it follows that at any point where such a cometary orbit happens to cross that of the Earth, our globe will necessarily encounter the cometary particles as a shower of meteors. This encounter will take place at a certain time of the year, either yearly, if they form a continuous ring, or after a succession of years, if they simply form an elongated cloud. Such meteoric clouds or rings would not be visible in ordinary circumstances, even through the largest telescopes, except on penetrating the upper regions of our atmosphere, when they would appear as showers of falling stars. It is supposed that in penetrating our atmosphere, even in its most rarefied regions, these meteors are heated by the resistance offered by the air to their motion, first becoming luminous and then being finally vaporized and burnt before they can reach the surface of the Earth.
The orbit of the comet of 1862, III., which so closely corresponds with that of the Perseid meteors, is much more extended than that of Tempel's comet corresponding with that of the Leonids. While the first extends far beyond the orbit of Neptune, the latter only goes a little beyond that of Uranus. The former orbit makes a considerable angle with the plane of the Earth's orbit, but the latter is much nearer to parallelism with it. The period of revolution of the first is 108 years, and that of the last about 33¼ years.
From the fact that the Perseid shower occurs yearly on the 10th of August, when the Earth crosses the orbit of the comet of 1862, III., it is supposed that the cometary particles producing this shower are disseminated along the whole orbit, and form a ring encircling the Sun and Earth. To explain the yearly variations in the number of the shooting-stars observed, these particles are supposed to be unequally distributed over the orbit, being more crowded at one place than they are at another. In order to explain the meteoric shower of Leonids, which appears in all its splendor every 33 years, and then with diminished intensity for two successive years, after which it is without importance, it is supposed that the cometary particles of the comet of 1866, I., have not as yet spread all along the orbit, a sufficient time not having been allowed, but form an elongated meteoric cloud, more dense in its front than in its rear part. From these considerations it has been supposed also that the comet of 1866, I., is of a more recent date than that of 1862, III. While Tempel's comet makes its revolution around the Sun in about 33 years, this meteoric cloud, which has the same period and returns to the same point of its orbit every 33 years, encounters our globe for three successive years. The first year we are passing through its densest parts, and the two following years in less and less crowded parts, from which result the observed phenomena. An idea of the extent of this meteoric cloud may be formed from the fact that, with its cometary velocity of motion, it takes this cloud three years at least to cross the Earth's orbit. From recent researches it would appear that the Leonid cloud is not single, but that at least two others of smaller importance exist, and have periods of 33¼ years.
Biela's comet, which was divided into two parts in 1846, is another of the few comets whose orbit approaches that of the Earth. Possessing this knowledge, and knowing then the close connection existing between meteors and comets, astronomers supposed that there were sufficient reasons to expect a meteoric shower when this comet was passing near the Earth. They consequently expected a meteoric display in 1872, when our globe was to cross its orbit. Their anticipation was plainly fulfilled, and on the night of November 27th, 1872, a splendid meteoric display, having its radiant point in the constellation Andromeda, was observed in Europe, and also in America, but the meteors seen here were not so numerous as in Europe. Other meteoric showers of less importance, such as that of April 20th, for instance, have also been identified with cometary orbits, so that now no doubt seems to remain as to the identity of cometary particles and shooting-stars.
The fact that the maximum number of meteors is always observed in the morning hours, supports the hypothesis of the cosmic origin of the shooting-stars, since the regions of the Earth where it is morning are precisely those fronting the regions towards which our globe is moving in space, and accordingly encounter more directly the meteors moving in their orbit. The greater abundance of falling stars at that time may thus be accounted for.
The number of meteors penetrating our atmosphere must be very great; there is not an hour and probably not a minute during which none fall. From various considerations, some astronomers have estimated at from 65,000,000,000 to 146,000,000,000 the total number of shooting-stars yearly penetrating in our atmosphere. The actual number is undoubtedly great, yet the fact that the meteors are rarely seen through the telescope while employed in observing various celestial objects, does not indicate that they are so numerous as these figures imply. It is only occasionally that one is seen traversing the field of the instrument. Even when the sky is observed with a low power eye-piece for several hours in succession, many nights may pass without disclosing one, although an observer, sweeping the sky more freely with the naked eye, may often perceive four or five during an ordinary night.
About the true nature of these bodies nothing is known with certainty. From spectrum analysis it seems to be established that most of them contain sodium and magnesium, while a few indicate the presence of strontium and iron, and in some rare cases there are traces of coal-gas. Some of the nuclei give a continuous spectrum, and others a spectrum of lines. The trail always gives a spectrum of bright lines which indicates its gaseous state. The traces of coal-gas rarely seen in meteors are, however, of great importance, as it identifies them more closely with the comets, which generally show a similar spectrum. The continuous spectra exhibited by some nuclei would indicate that they are incandescent and either solid or liquid; but it is difficult to conclude from their spectra what is their true nature, since we do not know exactly what part the terrestrial atmosphere may play in producing the results.
The mass of the shooting-stars is not known with certainty, but the fact that during great meteoric showers, none are seen to reach the surface of the Earth, all being consumed in a few seconds, sufficiently indicates that it must be very small. It has been calculated that those equal to Venus in apparent size and brilliancy may weigh several pounds, while the faint ones would weigh only a few grains.
If the shooting-stars have even such a mass as that here attributed to some of them, the extraordinary motions which I have described above seem to be unaccountable. The change of direction of a heavy mass moving swiftly cannot be sudden. The semi-circular, the wavy and the angular orbits observed could not be described, it would seem, by such a mass animated with a great velocity. Although the meteors are said to be ignited by the transformation of part of their progressive motion into molecular motion, yet it is not observed that the velocity of the falling stars diminishes when they are about to disappear. The luminous trails they leave in the atmosphere do not appear to be endowed with any motion, but remain for a time in their original positions. These facts are apparently opposed to the hypothesis that such meteors have any appreciable mass. The extraordinary motions exhibited by some meteors seem to indicate that some unsuspected force resides in these bodies, and causes them to deviate from the laws of ordinary motion.
Although it is very probable that the ordinary shooting-stars have no appreciable mass, yet it is known that very heavy meteoric masses sometimes fall at the surface of the Earth. Such falls are generally preceded by the sudden apparition in the sky of a large, and usually very brilliant fire-ball, which traverses the air at a great speed, sometimes leaving behind it a luminous trail, after which it explodes with a loud sound, and heavy fiery meteoric fragments, diverging in all directions, fall at the surface of the Earth. The name ofAerolitesorMeteorolitesis given to these ponderous fragments. As these meteors, before they explode and fall to the ground, have many points of resemblance with the shooting-stars, they are generally supposed to be connected with them, and to have a similar cometary origin. The fact that the aerolites differ widely from each other in constitution, and are all composed of substances found on the Earth, associated with other facts given below, would rather seem to indicate a terrestrial than a celestial origin.
If the aerolites belong to the same class of bodies as the falling stars, differing from them only in size and mass, it is difficult to see why so very few should fall upon the Earth during the great meteoric showers, when thousands of shooting-stars traverse our atmosphere. In Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the falls of aerolites and fire-balls which have been observed at the time of the periodic meteoric showers of the 10th of August and the 13th of November, during a period of 221 years for the Perseids, or August showers, and of 318 years for the Leonids, or November showers. During 221 years, 10 falls of aerolites have been witnessed simultaneously with the fall of the Perseids; while during 318 years, only 4 such falls have been recorded as having occurred at the time of the Leonid shower. If there is any close connection between the shooting-stars and the aerolites, we should expect to find a maximum in their fall at the time of the great meteoric displays. So far, no maxima or minima have yet been discovered in the fall of aerolites; they do not seem, like meteoric showers, to be governed by a law of periodicity.
A very remarkable peculiarity of the aerolites is that they seem to have a tendency to fall in certain regions. Such are the southern part of France, the north of Italy, Hindostan, the central states of North America, and Mexico and Brazil. There is a curious contrast existing between the quick cometary motion of the aerolites before their explosion, and the comparatively slow motion of their fragments as they reach the Earth; motion which seems to be no greater than that corresponding to their natural fall impeded by the resistance of the air. In general, their penetration into the soil upon which they fall does not at all correspond to the great velocity with which they move in the atmosphere. The fragmentary structures of the aerolites, their identity of substance with that of our globe, their great resemblance to the volcanic minerals of the Earth, and the fractures and faults which some of them exhibit, do not correspond at all with the idea that they are cometary particles fallen on the Earth. As far as their structure and appearance is concerned, they seem rather to be a volcanic product of the interior of the Earth than parts of disintegrated comets. It must be admitted that their identity with the shooting-stars is far from established, and that they are still involved in mystery.
The so-called meteoric dust gathered at sea and on high mountains may have various origins, and may be partly furnished by volcanic dust carried to great distances in the atmosphere.
Since millions of shooting-stars penetrate our atmosphere every year and remain in it, becoming definitively a part of the Earth, it follows that, no matter how small may be the quantity of matter of which they are composed, they must gradually increase the volume and mass of our globe, although the increase may be exceedingly slow. Supposing every one of the shooting-stars penetrating our atmosphere to contain one cubic millimeter of matter, it has been calculated that it would take nearly 35,000 years to make a deposit one centimeter in thickness all over the surface of our globe. Insignificant as this may appear, it is probable that the quantity of matter of meteoric origin which is added to our globe is much less than has just been supposed.
During clear nights, when the Moon is below the horizon, the starry vault is greatly adorned by an immense belt of soft white light, spanning the heavens from one point of the horizon to the opposite point, and girdling the celestial sphere in its delicate folds. Every one is familiar with this remarkable celestial object, called theMilky-wayorGalaxy.
Seen with the naked eye, the Galaxy appears as an irregular, narrow, nebulous belt, apparently composed of cloud-like luminous masses of different forms and sizes, separated by comparatively dark intervals. These cloud-like masses vary much in luminous intensity, and while some among them are very bright and conspicuous, others are so faint that they are hard to recognize. In general, the brightest parts of the Milky-way are situated along the middle of its belt, while its borders, which are usually very faint, gradually vanish in the sky. Some parts of the Galaxy, however, show very little of the cloudy structure so characteristic of other parts, being almost uniform throughout, except towards the borders, which are always fainter. These parts showing greater uniformity are also the faintest.
Such is the general appearance of the Milky-way on ordinary nights, but on rare occasions, when the atmosphere is particularly pure, it presents one of the grandest sights that can be imagined. At such favorable moments I have seen the Galaxy gleaming with light, and appearing as if composed of star-dust or of precious stones. The strange belt then appeared all mottled over and fleecy, its large cloud-like masses being subdivided into numerous small, irregular cloudlets of great brilliancy, which appeared projected upon a soft luminous background.
The width of the Galaxy is far from being uniform; while in some places it is only 4° or 5°, in others it is 15° and even more. In some places it appears wavy in outline, at others quite straight; then it contracts, to expand a few degrees distant; while at other places it sends off branches and loops, varying in form, size and direction, some of which are quite prominent, while others are very faint.
Although very irregular in form, the general appearance of the galactic belt is that of a regular curve occupying one of the great circles of the celestial sphere. The Milky-way completely encircles the heavens, but, of course, only one-half is visible at any one moment, since our globe prevents the other half from being seen. If, for a moment, we imagine ourselves left in space, our globe having vanished from under our feet, we should then see the whole Galaxy forming a continuous belt in the heavens, at the centre of which we should apparently be situated.
While only one-half of the galactic belt can be seen at once from any point on the Earth, yet, according to the position of the observer, a larger or smaller portion of the whole can be seen at different times. In high northern or southern latitudes but little more than half can be seen even by continuous observations; but as we approach the equatorial regions, more and more of it becomes visible, until the whole may be seen at different hours and seasons. In the latitudes of the northern states, about two-thirds of the Galaxy is visible, the rest remaining hidden below the horizon; but from the southern states very nearly the whole can be seen. The half of the Milky-way visible at any one time from any latitude on the Earth never entirely sets below the horizon, although in some places it may be so near the horizon as to be rendered invisible by vapors. In the latitude of Cambridge, when in its lowest position, the summit of its arc is still about 12° or 15° above the northern horizon. The great circle of the celestial sphere, occupied by the galactic belt, is inclined at an angle of about 63° to the celestial equator, and intersects this great circle on one side in the constellation Monoceros in 6h. 47m., and on the opposite side in the constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so that its northern pole is situated in the constellation Coma Berenices in R. A. 12h. 47m., declination N. 27°, and the southern in the constellation Cetus in R. A. 0h. 47m., declination S. 27°.
According to the seasons and to the hours of the night at which it is observed, the galactic arch presents different inclinations in the sky. Owing to its inclination to the equator of the celestial sphere, its opposite parts exhibit opposite inclinations when they pass the meridian of a place. That part of the Galaxy which is represented on Plate XIII., and which intersects the celestial equator in the constellation Aquila, is inclined to the left or towards the east, when it is on the meridian; while the opposite part, situated in Monoceros, is inclined to the right, or towards the west, when it reaches the meridian. The former passes the meridian in the evening in the summer and autumn months; the latter, in the winter and spring months.