THE
TROUVELOT
ASTRONOMICAL DRAWINGS
MANUAL

BY

E. L. TROUVELOT,

FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
GOVERNMENT EXPEDITION TO OBSERVE THE
TOTAL SOLAR ECLIPSE OF 1878.

NEW YORK

CHARLES SCRIBNER'S SONS

1882

INTRODUCTION

During a study of the heavens, which has now been continued for more than fifteen years, I have made a large number of observations pertaining to physical astronomy, together with many original drawings representing the most interesting celestial objects and phenomena.

With a view to making these observations more generally useful, I was led, some years ago, to prepare, from this collection of drawings, a series of astronomical pictures, which were intended to represent the celestial phenomena as they appear to a trained eye and to an experienced draughtsman through the great modern telescopes, provided with the most delicate instrumental appliances. Over two years were spent in the preparation of this series, which consisted of a number of large drawings executed in pastel. In 1876, these drawings were displayed at the United States Centennial Exhibition at Philadelphia, forming a part of the Massachusetts exhibit, in the Department of Education and Science.

The drawings forming the present series comprise only a part of those exhibited at Philadelphia; but, although fewer in number, they are quite sufficient to illustrate the principal classes of celestial objects and phenomena.

While my aim in this work has been to combine scrupulous fidelity and accuracy in the details, I have also endeavored to preserve the natural elegance and the delicate outlines peculiar to the objects depicted; but in this, only a little more than a suggestion is possible, since no human skill can reproduce upon paper the majestic beauty and radiance of the celestial objects.

The plates were prepared under my supervision, from the original pastel drawings, and great care has been taken to make the reproduction exact.

The instruments employed in the observations, and in the delineation of the heavenly bodies represented in the series, have varied in aperture from 6 to 26 inches, according to circumstances, and to the nature of the object to be studied. The great Washington refractor, kindly placed at my disposal by the late Admiral C. H. Davis, has contributed to this work, as has also the 26 inch telescope of the University of Virginia, while in the hands of its celebrated constructors, Alvan Clark & Sons. The spectroscope used was made by Alvan Clark & Sons. Attached to it is an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose kindness I am indebted for it.

Those unacquainted with the use of optical instruments generally suppose that all astronomical drawings are obtained by the photographic process, and are, therefore, comparatively easy to procure; but this is not true. Although photography renders valuable assistance to the astronomer in the case of the Sun and Moon, as proved by the fine photographs of these objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects, its products are in general so blurred and indistinct that no details of any great value can be secured. A well-trained eye alone is capable of seizing the delicate details of structure and of configuration of the heavenly bodies, which are liable to be affected, and even rendered invisible, by the slightest changes in our atmosphere.

The method employed to secure correctness in the proportions of the original drawings is simple, but well adapted to the purpose in view. It consists in placing a fine reticule, cut on glass, at the common focus of the objective and the eye-piece, so that in viewing an object, its telescopic image, appearing projected on the reticule, can be drawn very accurately on a sheet of paper ruled with corresponding squares. For a series of such reticules I am indebted to the kindness of Professor William A. Rogers, of the Harvard College Observatory.

The drawings representing telescopic views are inverted, as they appear in a refracting telescope—the South being upward, the North downward, the East on the right, and the West on the left. The Comet, the Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal Light and the Meteors are represented as seen directly in the sky with the naked eye. The Comet was, however, drawn with the aid of the telescope, without which the delicate structure shown in the drawing would not have been visible.

The plate representing the November Meteors, or so-called "Leonids," may be called an ideal view, since the shooting stars delineated, were not observed at the same moment of time, but during the same night. Over three thousand Meteors were observed between midnight and five o'clock in the morning of the day on which this shower occurred; a dozen being sometimes in sight at the same instant. The paths of the Meteors, whether curved, wavy, or crooked, and also their delicate colors, are in all cases depicted as they were actually observed.

In the Manual, I have endeavored to present a general outline of what is known, or supposed, on the different subjects and phenomena illustrated in the series. The statements made are derived either from the best authorities on physical astronomy, or from my original observations, which are, for the most part, yet unpublished.

The figures in the Manual relating to distance, size, volume, mass, etc., are not intended to be strictly exact, being only round numbers, which can, therefore, be more easily remembered.

It gives me pleasure to acknowledge that the experience acquired in making the astronomical drawings published in Volume VIII. of the Annals of the Harvard College Observatory, while I was connected with that institution, has been of considerable assistance to me in preparing this work; although no drawings made while I was so connected have been used for this series.

E. L. TROUVELOT.

Cambridge, March, 1882.

CONTENTS

INTRODUCTION
LIST OF PLATES

THE SUN

GENERAL REMARKS ON THE SUN
SUN-SPOTS AND VEILED SPOTS
SOLAR PROTUBERANCES
TOTAL ECLIPSE OF THE SUN

THE AURORAL AND ZODIACAL LIGHTS

THE AURORA BOREALIS
THE ZODIACAL LIGHT

THE MOON

THE MOON
ECLIPSES OF THE MOON

THE PLANETS

THE PLANET MARS
THE PLANET JUPITER
THE PLANET SATURN

COMETS AND METEORS

COMETS
SHOOTING-STARS AND METEORS

THE STELLAR SYSTEMS

THE MILKY-WAY OR GALAXY
THE STAR-CLUSTERS
THE NEBULÆ
APPENDIX

LIST OF PLATES[1]

PLATE

I. GROUP OF SUN-SPOTS AND VEILED SPOTS.
Observed June 17, 1875, at 7 h. 30m. A. M.
II. SOLAR PROTUBERANCES.
Observed May 5, 1873, at 9h. 40m. A. M.
III. TOTAL ECLIPSE OF THE SUN.
Observed July 29, 1878, at Creston, Wyoming Territory.
IV. AURORA BOREALIS.
As observed March 1, 1872, at 9h. 25m. P. M.
V. THE ZODIACAL LIGHT.
Observed February 20, 1876.
VI. MARE HUMORUM.
From a study made in 1875.
VII. PARTIAL ECLIPSE OF THE MOON.
Observed October 24, 1874.
VIII. THE PLANET MARS.
Observed September 3, 1877, at 11h. 55m. P. M.
IX. THE PLANET JUPITER.
Observed November 1, 1880, at 9h. 30m. P. M.
X. THE PLANET SATURN.
Observed November 30, 1874, at 5th. 50m. P. M.
XI. THE GREAT COMET OF 1881.
Observed on the night of June 25-26, at 1h. 30m. A. M.
XII. THE NOVEMBER METEORS.
As observed between midnight and 3 o'clock A. M., on the night
of November 13-14, 1868.
XIII. PART OF THE MILKY-WAY.
From a study made during the years 1874, 1875 and 1876.
XIV. STAR-CLUSTER IN HERCULES.
From a study made in June, 1877.
XV. THE GREAT NEBULA IN ORION.
From a study made in the years 1873-76.

Reproduced from the Original Drawings, by Armstrong & Company,
Riverside Press, Cambridge, Mass.

GENERAL REMARKS ON THE SUN

The Sun, the centre of the system which bears its name, is a self-luminous sphere, constantly radiating heat and light.

Its apparent diameter, as seen at its mean from the Earth, subtends an angle of 32', or a little over half a degree. A dime, placed about six feet from the eye, would appear of the same proportions, and cover the Sun's disk, if projected upon it.

That the diameter of the Sun does not appear larger, is due to the great distance which separates us from that body. Its distance from the Earth is no less than 92,000,000 miles. To bridge this immense gap, would require 11,623 globes like the Earth, placed side by side, like beads on a string.

The Sun is an enormous sphere whose diameter is over 108 times the diameter of our globe, or very nearly 860,000 miles. Its radius is nearly double the distance from the Earth to the Moon. If we suppose, for a moment, the Sun to be hollow, and our globe to be placed at the centre of this immense spherical shell, not only could our satellite revolve around us at its mean distance of 238,800 miles, as now, but another satellite, placed 190,000 miles farther than the Moon, could freely revolve likewise, without ever coming in contact with the solar envelope.

The circumference of this immense sphere measures 2,800,000 miles. While a steamer, going at the rate of 300 miles a day, would circumnavigate the Earth in 83 days, it would take, at the same rate, nearly 25 years to travel around the Sun.

The surface of the Sun is nearly 12,000 times the surface of the Earth, and its volume is equal to 1,300,000 globes like our own. If all the known planets and satellites were united in a single mass, 600 such compound masses would be needed to equal the volume of our luminary.

Although the density of the Sun is only one-quarter that of the Earth, yet the bulk of this body is so enormous that, to counterpoise it, no less than 314,760 globes like our Earth would be required.

The Sun uniformly revolves around its axis in about 25½ days. Its equator is inclined 7° 15' to the plane of the ecliptic, the axis of rotation forming, therefore, an angle of 82° 45' with the same plane. As the Earth revolves about the Sun in the same direction as that of the Sun's rotation, the apparent time of this rotation, as seen by a terrestrial spectator, is prolonged from 25½ days to about 27 days and 7 hours.

The rotation of the Sun on its axis, like that of the Earth and the other planets, is direct, or accomplished from West to East. To an observer on the Earth, looking directly at the Sun, the rotation of this body is from left to right, or from East to West.

The general appearance of the Sun is that of an intensely luminous disk, whose limb, or border, is sharply defined on the heavens. When its telescopic image is projected on a screen, or fixed on paper by photography, it is noticed that its disk is not uniformly bright throughout, but is notably more luminous in its central parts. This phenomenon is not accidental, but permanent, and is due in reality to a very rare but extensive atmosphere which surrounds the Sun, and absorbs the light which that body radiates, proportionally to its thickness, which, of course, increases towards the limb, to an observer on the Earth.

THE ENVELOPING LAYERS OF THE SUN

The luminous surface of the Sun, or that part visible at all times, and which forms its disk, is called the Photosphere, from the property it is supposed to possess of generating light. The photosphere does not extend to a great depth below the luminous surface, but forms a comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct from the interior parts, above which it seems to be kept in suspense by internal forces. From the observations of some astronomers it would appear that the diameter of the photosphere is subject to slight variations, and, therefore, that the solar diameter is not a constant quantity. From the nature of this envelope, such a result does not seem at all impossible, but rather probable.

Immediately above the photosphere lies a comparatively thin stratum, less than a thousand miles in thickness, called the Reversing Layer. This stratum is composed of metallic vapors, which, by absorbing the light of particular refrangibilities emanating from the photosphere below, produces the dark Fraunhofer lines of the solar spectrum.

Above the reversing layer, and resting immediately upon it, is a shallow, semi-transparent gaseous layer, which has been called the Chromosphere, from the fine tints which it exhibits during total eclipses of the Sun, in contrast with the colorless white light radiated by the photosphere below. Although visible to a certain extent on the disk, the chromosphere is totally invisible on the limb, except with the spectroscope, and during eclipses, on account of the nature of its light, which is mainly monochromatic, and too feeble, compared with that emitted by the photosphere, to be seen.

The chromospheric layer, which has a thickness of from 3,000 to 4,000 miles, is uneven, and is usually upheaved in certain regions, its matter being transported to considerable elevations above its general surface, apparently by some internal forces. The portions of the chromosphere thus lifted up, form curious and complicated figures, which are known under the names of Solar Protuberances, or Solar Flames.

Above the chromosphere, and rising to an immense but unknown height, is the solar atmosphere proper, which is only visible during total eclipses of the Sun, and which then surrounds the dark body of the Moon with the beautiful rays and glorious nimbus, called the Corona.

These four envelopes: the photosphere, the reversing layer, the chromosphere, and the corona, constitute the outer portions of our luminary.

Below the photosphere little can be seen, although it is known, as will appear below, that at certain depths cloud-like forms exist, and freely float in an interior atmosphere of invisible gases. Beyond this all is mystery, and belongs to the domain of hypothesis.

STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE

The apparent uniformity of the solar surface disappears when it is examined with a telescope of sufficient aperture and magnifying powers. Seen under good atmospheric conditions, the greater part of the solar surface appears mottled with an infinite number of small, bright granules, irregularly distributed, and separated from each other by a gray-tinted background.

These objects are known under different names. The terms granules and granulations answer very well for the purpose, as they do not imply anything positive as to their form and true nature. They have also been called Luculœ, Rice Grains, Willow Leaves, etc., by different observers.

Although having different shapes, the granulations partake more or less of the circular or slightly elongated form. Their diameter, which varies considerably, has been estimated at from 0".5 to 3", or from 224 to 1,344 miles. The granulations which attain the largest size appear, under good atmospheric conditions, to be composed of several granules, closely united and forming an irregular mass, from which short appendages protrude in various directions.

The number of granulations on the surface of the Sun varies considerably under the action of unknown causes. Sometimes they are small and very numerous, while at other times they are larger, less numerous, and more widely separated. Other things being equal, the granulations are better seen in the central regions of the Sun than they are near the limb.

Usually the granulations are very unstable; their relative position, form, and size undergoing continual changes. Sometimes they are seen to congregate or to disperse in an instant, as if acting under the influence of attractive and repulsive forces; assembling in groups or files, and oftentimes forming capricious figures which are very remarkable, but usually of short duration. In an area of great solar disturbances, the granulations are often stretched to great distances, and form into parallel lines, either straight, wavy, or curved, and they have then some resemblance to the flowing of viscous liquids.

The granulations are usually terminated either by rounded or sharply pointed summits, but they do not all rise to the same height, as can be ascertained with the spectroscope when they are seen sidewise on the limb. In the regions where they are most abundant, they usually attain greater elevations, and when observed on the limb with the spectroscope, they appear as slender acute flames.

The granulations terminated by sharply-pointed crests, although observed in all latitudes, seem to be characteristic of certain regions. A daily study of the chromosphere, extending over a period of ten years, has shown me that the polar regions are rarely ever free from these objects, which are less frequent in other parts of the Sun. In the polar regions they are sometimes so abundant that they completely form the solar limb. These forms of granulation are comparatively rare in the equatorial zones, and when seen there, they never have the permanency which they exhibit in the polar regions. When observed in the equatorial regions, they usually appear in small groups, in the vicinity of sun spots, or they are at least enclosed in areas of disturbances where such spots are in process of formation. In these regions they often attain greater elevations than those seen in high latitudes.

As we are certain that in the equatorial zones these slender flames (i. e., granulations) are a sure sign of local disturbance, it may be reasonably supposed that the same kind of energy producing them nearly always prevails in the polar regions, although it is there much weaker, and never reaches beyond certain narrow limits.

Studied with the spectroscope, the granulations are found to be composed in the main of incandescent hydrogen gas, and of an unknown substance provisionally called "helium." Among the most brilliant of them are found traces of incandescent metallic vapors, belonging to various substances found on our globe.

The chromosphere is not fixed, but varies considerably in thickness in its different parts, from day to day. Its thickness is usually greater in the polar regions, where it sometimes exceeds 6,700 miles. In the equatorial regions the chromosphere very rarely attains this height, and when it does, the rising is local and occupies only a small area. In these regions it is sometimes so shallow that its depth is only a few seconds, and is then quite difficult to measure. These numbers give, of course, the extreme limits of the variations of the chromosphere; but, nearly always, it is more shallow in the equatorial regions; and, as far as my observations go, the difference in thickness between the polar and equatorial zones is greater in years of calm than it is in years of great solar activity. But ten years of observation are not sufficient to warrant any definite conclusions on this subject.

There is undoubtedly some relation between the greater thickness of the chromosphere in the polar regions, and the abundance and permanence of the sharply-pointed granulations observed in the same regions. This becomes more evident when we know that the appearance of similarly-pointed flames in the equatorial zones is always accompanied with a local thickening of the chromosphere. The thickening in the polar regions may be only apparent, and not due to a greater accumulation of chromospheric gases there; but may be caused by some kind of repulsive action or polarity, which lifts up and extends the summit of the granulations in a manner similar to the well-known mode of electric repulsion and polarity.

As it seems very probable that the heat and light emanating from the Sun are mainly generated at the base of the granulations, in the filamentary elements composing the chromosphere and photosphere, it would follow that, as the size and number of these objects constantly vary, the amount of heat and light emitted by the Sun should also vary in the same proportion.

The granulations of the solar surface are represented on Plate I., and form the general background to the group of Sun-spots forming the picture.

THE FACULÆ

Although the solar surface is mainly covered with the luminous granulations and the grayish background above described, it is very rare that its appearance is so simple and uniform as already represented. For the most part, on the contrary, it is diversified by larger, brighter, and more complicated forms, which are especially visible towards the border of the Sun. Owing to their extraordinary brilliancy, these objects have been called Faculœ (torches).

Although the faculæ are very seldom seen well beyond 50 heliocentric degrees from the limb, yet they exist, and are as numerous in the central parts of the disk as they are towards the border; since they form a part of the solar surface, and participate in its movement of rotation. Their appearance near the limb has been attributed to the effect of absorption produced by the solar atmosphere on the light from the photosphere; but this explanation seems inadequate, and does not solve the problem. The well-known fact that the solar protuberances—which are in a great measure identical with the faculæ—are much brighter at the base than they are at the summit, perhaps gives a clue to the explanation of the phenomenon; especially since we know that, in general, the summit of the protuberances is considerably broader than their base. When these objects are observed in the vicinity of the limb, they present their brightest parts to the observer, since, in this position, they are seen more or less sidewise; and, therefore, they appear bright and distinct. But as the faculæ recede from the limb, their sides, being seen under a constantly decreasing angle, appear more and more foreshortened; and, therefore, these objects grow less bright and less distinct, until they finally become invisible, when their bases are covered over by the broad, dusky summit generally terminating the protuberances.

The faculæ appear as bright and luminous masses or streaks on the granular surface of the Sun, but they differ considerably in form and size. Two types at least are distinguishable. In their simplest form they appear either as isolated white spots, or as groups of such spots covering large surfaces, and somewhat resembling large flakes of snow. In their most characteristic types they appear as intensely luminous, heavy masses, from which, in most cases, issue intricate ramifications, sometimes extending to great distances. Generally, the ramifications issuing from the masses of faculæ have their largest branches directed in the main towards the eastern limb of the Sun. Some of these branches have gigantic proportions. Occasionally they extend over 60° and even 80° of the solar surface, and, therefore, attain a length of from 450,000 to 600,000 miles.

Although the faculæ may be said to be seen everywhere on the surface of the Sun, there is a vast difference in different regions, with regard to their size, number, and brilliancy. They are largest, most abundant, and brightest on two intermediate zones parallel to the solar equator, and extending 35° or 40° to the north and to the south of this line. The breadth of these zones varies considerably with the activity of the solar forces. When they are most active, the faculæ spread on either side, but especially towards the equator, where they sometimes nearly meet those of the other zone. In years of little solar activity the belts formed by the faculæ are very narrow—the elements composing them being very few and small, although they never entirely disappear.

The faculæ are very unstable, and are constantly changing: those of the small types sometimes form and vanish in a few minutes. When an area of disturbance of the solar surface is observed for some time, all seems in confusion; the movements of the granulations become unusually violent; they congregate in all sorts of ways, and thus frequently form temporary faculæ. Action of this kind is, for the most part, peculiar to the polar regions of the Sun.

The larger faculæ have undoubtedly another origin, as they seem to be mainly formed by the ejection of incandescent gases and metallic vapors from the interior of the photosphere. In their process of development some of the heavy masses of faculæ are swollen up to great heights, being torn in all sorts of ways, showing large rents and fissures through which the sight can penetrate.

Very few faculæ are represented in Plate I.; several streaks are shown at the upper left-hand corner, some appearing as whitish ramifications among the granulations representing the general solar surface.

SUN-SPOTS AND VEILED SPOTS

PLATE I

Besides the brilliant faculæ already described, much more conspicuous markings, though of a totally different character, are very frequently observed on the Sun. On account of their darkish appearance, which is in strong contrast with the white envelope of our luminary, these markings were called Maculæ, or Sun-spots, by their earlier observers.

The Sun-spots are not equally distributed on the solar surface; but like the faculæ, to which they are closely related, they occupy two zones—one on each side of the equator. These zones are comprised between 10° and 35° of north latitude, and 10° and 35° of south latitude. Between these two zones is a belt 20° in width, where the Sun-spots are rarely seen.

Above the latitudes 35° north and south, the Sun-spots are rare, and it is only occasionally, and during years of great solar activity, that they appear in these regions; in only a few cases have spots of considerable size been seen there. A few observers, however, have seen spots as far as 40° and 50 from the equator; and La Hire even observed one in 70° of north latitude; but these cases are exceedingly rare. It is not uncommon, however, to see very small spots, or groups of such spots, within 8° or 10° from the poles.

The activity of the Sun is subject to considerable fluctuation, and accordingly the Sun-spots vary in size and number in different years. During some years they are large, complicated, and very numerous; while in others they are small and scarce, and are sometimes totally absent for weeks and months together. The fluctuations in the frequency of Sun-spots are supposed to be periodical in their character, although their periods do not always appear to recur at exactly regular intervals. Sometimes the period is found to be only nine years, while at other times it extends to twelve years. The period generally adopted now is 11⅒ years, nearly; but further investigations are needed to understand the true nature of the phenomenon.

The number of Sun-spots does not symmetrically augment and diminish, but the increase is more rapid than the diminution.

The period of increase is only about four years, while that of decrease is over seven years; each period of Sun-spot maximum being nearer the preceding period of Sun-spot minimum than it is to that next following.

The cause of these fluctuations in the solar energy is at present wholly unknown. Some astronomers, however, have attributed it to the influence of the planets Venus and Jupiter, the period of revolution of the latter planet being not much longer than the Sun-spot period; but this supposition lacks confirmation from direct observations, which, so far, do not seem to be in favor of the hypothesis. At the present time the solar activity is on the increase, and the Sun-spots will probably reach their maximum in 1883. The last minimum occurred in 1879, when only sixteen small groups of spots were observed during the whole year.

Sun-spots vary in size and appearance; but, unless they are very small, in which case they appear as simple black dots, they generally consist of two distinct and well-characterized parts, nearly always present. There is first, a central part, much darker than the other, and sharply divided from it, called the "Umbra;" second, a broad, irregular radiated fringe of lighter shade, completely surrounding the first, and called the "Penumbra."

Reduced to its simplest expression, a Sun-spot is a funnel-shaped opening through the chromosphere and the photosphere. The inner end of the funnel, or opening, gives the form to the umbra, while its sloping sides form the penumbra.

The umbra of Sun-spots, whose outlines approximately follow the irregularities of the penumbral fringe, has a diameter which generally exceeds the width of the penumbral ring. Sometimes it appears uniformly black throughout; but it is only so by contrast, as is proved when either Mercury or Venus passes near a spot during a transit over the Sun's disk. The umbra then appears grayish, when compared with the jet-black disk of the planet.

The umbra of spots is rarely so simple as just described; but it is frequently occupied, either partly or wholly, by grayish and rosy forms, somewhat resembling loosely-entangled muscular fibres. These forms have been called the Gray and Rosy Veils. Frequently these veils appear as if perforated by roundish black holes, improperly called Nuclei, which permit the sight to penetrate deeper into the interior. To all appearance the gray and rosy veils are of the same nature as the chromosphere and the faculæ, and are therefore mainly composed of hydrogen gas.

Whatever can be known about the interior of the Sun, must be learned from the observations of these openings, which are comparatively small. But whatever this interior may be, we certainly know that it is not homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very thin shell. Below this shell exists a large open space filled with invisible gases, in which, through the openings constituting the Sun-spots, the gray and rosy veils described above are occasionally seen floating.

The fringe forming the penumbra of spots is much more complicated than the umbra. In its simpler form, it is composed of a multitude of bright, independent filaments of different forms and sizes, partly projecting one above the other, on the sloping wall of the penumbra, from which they seem to proceed. Seen from the Earth, these filaments have somewhat the appearance of thatched straw, converging towards the centre of the umbra. It is very rare, however, that the convergence of the penumbral filaments is regular, and great confusion sometimes arises from the entanglement of these filaments. Some of these elements appear straight, others are curved or loop-shaped; while still others, much larger and brighter than the rest, give a final touch to this chaos of filaments, from which results the general thatched and radiating appearance of the penumbra.

The extremities of the penumbral filaments, especially of those forming the border of the umbra, are usually club-shaped and appear very brilliant, as if these elements had been superheated by some forces escaping through the opening of the spots.

Besides these characteristics, the Sun-spots have others, which, although not always present, properly belong to them. Comparatively few spots are so simple as the form just described. Very frequently a spot is accompanied by brilliant faculæ, covering part of its umbra and penumbra, and appearing to form a part of the spot itself.

When seen projected over Sun-spots, the faculæ appear intensely bright, and from these peculiarities they have been called Luminous Bridges. They are, in fact, bridges, but in most cases they are at considerable heights above the spots, kept there by invisible forces. When such spots with luminous bridges approach the Sun's limb, it is easy to see, by the rapid apparent displacement which they undergo, that they are above the general level.

When the spots are closing up, the inverse effect is sometimes observed. On several occasions, I have seen huge masses of faculæ advance slowly over the penumbra of a spot and fall into the depths of the umbra, resembling gigantic cataracts. I have seen narrow branches of faculæ, which, after having fallen to great depths in the umbra, floated across it and disappeared under the photosphere on the opposite side. I have also seen luminous bridges, resembling cables, tightly stretched across the spots, slackening slowly, as if loosened at one end, and gently curving into the umbra, where they formed immense loops, large enough to receive our globe.

It is to be remarked that, in descending under the photospheric shell, the bright faculæ and the luminous bridges gradually lose their brilliancy. At first they appear grayish, but in descending farther they assume more and more the pink color peculiar to the rosy veils. The pinkish color acquired by the faculæ when they reach a certain depth under the photosphere, is precisely the color of the chromosphere and of the solar protuberances, as seen during total eclipses of the Sun—a fact which furnishes another proof that the faculæ are of the same nature as the protuberances.

I record here an observation which, at first sight, may appear paradoxical; but which seems, however, to be of considerable importance, as it shows unmistakably that the solar light is mainly, if not entirely, generated on its surface, or at least very near to it. On May 26, 1878, I observed a large group of Sun-spots at a little distance from the east limb of the Sun. The spot nearest to the limb was partly covered over on its eastern and western sides by bright and massive faculæ which concealed about two-thirds of the whole spot, only a narrow opening, running from north to south, being left across the middle of the spot. Owing to the rotundity of the Sun, the penumbra of this spot, although partly covered by the faculæ, could, however, be seen on its eastern side, since the sight of the observer could there penetrate sidewise under the faculæ. Upon that part of the penumbra appeared a strong shadow, representing perfectly the outline of the facular mass situated above it. The phenomenon was so apparent that no error of observation was possible, and a good drawing of it was secured. If this faculæ had been as bright beneath as it was above, it is evident that no shadow could have been produced; hence the light of these faculæ must have been mainly generated on or very near their exterior surfaces. This, with the well-proved fact that the bright faculæ lose their light in falling into the interior of the Sun, seems to suggest the idea that the bright light emitted by the faculæ, and very probably all the solar light, can be generated only on its surface; the presence of the coronal atmosphere being perhaps necessary to produce it. Several times before this observation, I had suspected that some faculæ were casting a shadow, but as this seemed so improbable, my attention was not awakened until the phenomenon became so prominent that it could not escape notice.

With due attention, some glimpses of the phenomenon can frequently be observed through the openings of some of the faculæ projecting over the penumbra of Sun-spots. It is very seldom that the structure of the penumbra is seen through such openings, which usually appear as dark as the umbra of the large spots, although they do not penetrate through the photosphere like the latter. It is only when the rents in the faculæ are numerous and quite large, that the penumbral structure is recognized through them. Since these superficial rents in the faculæ do not extend through the photosphere, and appear black, it seems evident that the penumbra seen through them cannot be as bright as it is when no faculæ are projected upon it, and therefore that the faculæ intercept light from the exterior surface, which would otherwise reach the penumbra.

While the matter forming the faculæ sometimes falls into the interior of the Sun, the same kind of matter is frequently ejected in enormous quantities, and with great force, from the interior, through the visible and invisible openings of the photosphere, and form the protuberances described in the following section of this manual (Solar protuberances) It is not only the incandescent hydrogen gas or the metallic vapors which are thus ejected, but also cooler hydrogen gas, which sometimes appears as dark clouds on the solar surface. On December 12, 1875, I observed such a cloud of hydrogen issuing from the corner of a small Sun-spot. It traveled several thousand miles on the solar surface, in a north-easterly direction, before it became invisible.

Solar spots are formed in various ways; but, for the most part, the apparition of a spot is announced beforehand, by a great commotion of the solar surface at the place of its appearance, and by the formation of large and bright masses of faculæ, which are usually swollen into enormous bubbles by the pressure of the internal gases. These bubbles become visible in the spectroscope while they are traversing the solar limb, as they are then presented to us sidewise. Under the action of the increasing pressure, the base of the faculæ is considerably stretched, and, its weakest side finally giving way, the facular mass is torn in many places from the solar surface, and is perforated by holes of different sizes and forms. The holes thus made along the border of the faculæ appear as small black spots, separated more or less by the remaining portion of the lacerated faculæ, and they enlarge more and more at the expense of the intervening portions, which thus become very narrow. This perforated side of the faculæ, offering less resistance, is gradually lifted up, as would be the cover of a box, for example, while its opposite side remains attached to the surface. The facular matter separating the small black holes is greatly stretched during this action, and forms long columns and filaments. These appear as luminous bridges upon the large and perfectly-formed spot, which is then seen under the lifted facular masses. The spots thus made visible are soon freed from the facular masses, which are gradually shifted towards the opposite side.

In such cases the spots are undoubtedly formed under the faculæ before they can be seen. This becomes evident when such spots, not yet cleared from the faculæ covering them, are observed near the east limb; since in this position the observer can see through the side-openings of the faculæ, and sometimes recognize the spots under their cover.

It frequently occurs that the spots thus formed under the faculæ continue to be partly covered by the facular clouds, the forces at work in them being apparently too feeble to shift them aside. In such cases these spots are visible when they are in the vicinity of the limb, where they are seen sidewise; but when observed in the east, in being carried forward by the solar rotation towards the centre of the disk, they gradually diminish in size, and finally become invisible. The disappearance of these spots, however, is only apparent, being due to the fact that, as they advance towards the centre of the disk, our lateral view of them is gradually lost, and they are finally hidden from sight by the overhanging faculæ which then serve as a screen between the observer and the spot. This class of spots may be called Lateral Spots, from the fact that they can only be seen laterally, and near the Sun's limb.

Solar spots are also formed in various other ways. Some, like those represented in Plate I., appearing without being announced by any apparent disturbance of the surface, or by the formation of any faculæ, form and develop in a very short time. Others, appearing at first as very small spots having an umbra and a penumbra, slowly and gradually develop into very large spots. This mode of formation, which would seem to be the most natural, is, however, quite rare. Spots of this class have a duration and permanence not observed in those of any other type. These spots of slow and regular development are never accompanied by faculæ or luminous bridges, nor have they any gray or rosy veils in their interior; a fact which may, perhaps, account for their permanent character.

Another class of spots, which is also rare, appear as long and narrow crevasses showing the penumbral structure of the ordinary spots; but these rarely have any umbra. These long, and sometimes exceedingly narrow fissures of the solar envelopes, with their radiated penumbral structure, strongly suggest the idea that the photosphere is composed of a multitude of filamentary elements having the granulations for summits. Such a crevasse is represented on Plate I., and unites the two spots which form the group.

The duration of Sun-spots varies greatly. Some last only for a few hours; while others continue for weeks and even months at a time, but not without undergoing changes.

The modes of disappearance of Sun-spots are as various as those of their apparition. The spots rarely close up by a gradual diminution or contraction of their umbra and penumbra. This mode of disappearance belongs exclusively to the spots deprived of faculæ and veils. One of the most common modes of the disappearance of a spot is its invasion by large facular masses, which slowly advance upon its penumbra and umbra and finally cover it entirely. It is a process precisely the reverse of that in which spots are formed by the shifting aside of the faculæ, as above described. In other types, the spots close up by the gradual enlargement of the luminous bridges traversing them, which are slowly transformed into branches of the photosphere, all of the characteristics of which they have acquired. In many cases, the spots covered over by the faculæ continue to exist for some time, hidden under these masses, as is often proved, either by the appearance of small spots on the facular mass left at the place they occupied, or even by the reappearance of the same spot.

Apart from the general movement of rotation of the solar surface, some of the spots seem to be endowed with a proper motion of their own, which becomes greater the nearer the spots are to the solar equator. According to the observations of Mr. Carrington, the period of rotation of the Sun, as deduced from the observations of the solar spots during a period of seven years, is 25 days at the equator; while at 50° of heliocentric latitude it is 27 days. But the period of rotation, as derived from the observations of spots occupying the same latitude, is far from being constant, as it varies at different times, with the frequency of the spots and with the solar activity, so that at present the law of these variations is not well known. From the character of the solar envelope, it seems very natural that the rotation should differ in the different zones and at different times, since this envelope is not rigid, but very movable, and governed by forces which are themselves very variable.

Although it is a general law that the spots near the equator have a more rapid motion than those situated in higher latitudes, yet, in many cases, the proper motion of the spots is more apparent than real. For the most part, the changes of form and the rapid displacements observed in some spots are only apparent, and due to the fact that the large masses of faculæ which are kept in suspense above them are very unstable, and change position with the slightest change in the forces holding them in suspension. Since in these cases we view the spots through the openings of the faculæ situated above them, the slightest motion of these objects produces an apparent motion in the spots, although they have remained motionless. Accordingly, it has been remarked that of all the spots, those which have the greater proper motion are precisely those which have the most faculæ and luminous bridges; while the other spots in the same regions, but not attended by similar phenomena, are comparatively steady in their movement. These last spots are undoubtedly better adapted than any others to exhibit the rotation of the Sun; but it is probable that this period of rotation will never be known with accuracy, simply because the solar surface is unstable, and does not rotate uniformly.

The Sun-spots have a remarkable tendency to form into groups of various sizes, but whatever may be the number of spots thus assembled, the group is nearly always composed of two principal spots, to which the others are only accessories. The tendency of the Sun-spots to assemble in pairs is general, and is observed in all latitudes, even among the minute temporary groups formed in the polar regions. Whenever several are situated quite close together, those belonging to the same group can be easily recognized by this character. Whatever may be the position of the axis of the two principal spots of a group when it is first formed, this axis has a decided tendency to place itself parallel to the solar equator, no matter to what latitude the group belongs; and if it is disturbed from this position, it soon returns to it when the disturbance has ceased.

It is also remarkable that the spots observed at the same time remain in nearly the same parallel of latitude for a greater or less period of time; but they keep changing their position from year to year, their latitude decreasing with the activity of the solar forces.

Among the Sun-spots, those associated with faculæ form the groups which attain the largest proportions. When such groups acquire an apparent diameter of 1' or more, they are plainly visible to the naked eye, since for a spot to be visible to the naked eye on the Sun, it need only subtend an angle of 50". I have sometimes seen such groups through a smoky atmosphere, when the solar light was so much reduced that the disk could be observed directly and without injury to the sight.

The largest spot which ever came under my observation was seen during the period from the 13th to the 19th of November, 1870. This spot, which was on the northern hemisphere of the Sun, was conspicuous among the smaller spots constituting the group to which it belonged, and followed them on the east. On November 16th, when it attained its largest size, the diameter of its penumbra occupied fully one-fifth of the diameter of the Sun; its real diameter being, therefore, not less than 172,000 miles, or nearly 22 times the diameter of the Earth. As the umbra of this spot occupied a little more than one-third of its whole diameter, seven globes like our own, placed side by side on a straight line, could easily have passed through this immense gap. To fill the area of this opening, about 45 such globes would have been needed. This spot was, of course, very easily seen with the naked eye, its diameter being almost eight times that required for a spot to be visible without a telescope.

Ancient historians often speak of obscurations of the Sun, and it has been supposed by some astronomers that this phenomenon might have been due in some cases to the apparition of large spots. A few spots on the surface of the Sun, like that just described, would sensibly reduce its light.

Besides the ordinary Sun-spots already described, others are at times observed on the surface of the Sun, which show some of the same characteristics, but never attain so large proportions. They always appear as if seen through a fog, or veil, between the granulations of the solar surface. On account of their vagueness and ill-defined contours, I have proposed for these objects the term, "Veiled Spots." Veiled spots have a shorter duration than the ordinary spots, the smaller types sometimes forming and vanishing in a few minutes. Some of the larger veiled spots, however, remain visible for several days in succession, and show the characteristics of other spots in regard to the arrangement of their parts.

The veiled spots have no umbra or penumbra, although they are usually accompanied by faculæ resembling those seen near the ordinary spots. They are frequently seen in the polar regions, but are there always of small size and of short duration. The veiled spots are larger, and more apt to arrange themselves into groups, in the regions occupied by the ordinary spots, and it is not rare to observe such spots transform themselves into ordinary spots, and vice versa. The veiled spots, therefore, seem to be ordinary spots filled up, or covered over by the granulations and semi-transparent gases composing the chromospheric layer. That it is so, becomes more evident, from the fact that large Sun-spots in process of diminution are sometimes gradually covered with faint and scattered granules which descend in long, narrow filaments, and become less and less distinguishable as they attain greater depth. This phenomenon, associated with the fact that the luminous bridges seen over the Sun-spots which are closing up are sometimes transformed into branches which show the characteristic structure of the photosphere, goes far to prove that the solar envelopes are mainly composed of an innumerable quantity of radial filaments of varying height.