Sun, the central ruling body of the planetary system, and the great source of light and heat. The visible orb of the sun, as distinguished from the complex structure of which that orb is but a part, is a globe about 853,000 m. in diameter. So far as observation extends, this globe is spherical in shape, no difference having been detected in the polar and equatorial diameters. In fact, no single set of measurements, however carefully made, could lead to the conclusion that there is any compression in the solar orb, since the equality of the diameters results not from a single set of measures, but from comparisons between many thousands of observations made at Greenwich, Paris, Washington, and other leading observatories. The volume of the sun exceeds the earth's nearly 1,253,000 times. His mean density is almost exactly equal to one fourth of the earth's, so that his mass exceeds hers about 316,000 times. Gravity at the visible boundary of the solar globe exceeds gravity at the earth's surface about 27.1 times; and a body dropped from rest near the sun's surface would fall through 436 ft. in the first second, and have acquired a velocity of 872 ft. a second, or about 10 m. a minute. The sun's mass exceeds the combined mass of all the planets about 750 times.

His mean distance from the earth has been estimated at about 91,430,000 m.; though we may expect that the results obtained during the late transit of Venus (December, 1874) and to be obtained during the coming transit (December, 1882) will lead to some correction of this estimate. It already appears probable that the sun's estimated mean distance must be increased to about 92,000,000 m. The greatest and least distances of the sun from the earth (assuming his mean distance to be 91,430,000 m.) are respectively 92,963,000 and 89,897,000 m.; and his apparent diameter varies from 31' 31'8" to 32' 36.4" as he passes from his greatest to his least distance. - The sun has an apparent motion among the stars from west to east along the great circle called the ecliptic (see Ecliptic), making a complete circuit of the heavens in 365 days, 6 hours, 9 minutes, and 9.6 seconds, though the passage from vernal equinox to vernal equinox (first point of Aries) occupies only 365d. 5h. 48m. 48.6s., because of the precession of the equinoxes. (See Precession.) These two periods are called respectively the sidereal year and the tropical year.

There is one other astronomical year (besides the civil, Julian, and lunar years) known as the anomalistic year, being the interval separating successive passages of the perigee of the solar path, viewed geometrically; its length amounts to 365d. 6h. 13m. 49.3s. The apparent motion of the sun is not uniform in the ecliptic, owing to the eccentricity of the earth's orbit; it is greatest about Dec. 31 to Jan. 1, when he moves through 1° 1' 9.9" in 24h., and least about June 30-July 1, when he only moves through 0° 57' 11.5" in 24h. The sun has also three real motions : 1, an axial rotation, the nature of which will presently be described; 2, a motion about the centre of gravity of the whole solar system, but in consequence of the great superiority of his mass over that of all the other bodies this centre of inertia is always within the sun's volume; 3, a progressive motion in space toward the direction of the constellation Hercules, the rate of which has been estimated at 150,000,-000 m. per annum, but on evidence exceedingly questionable. The fact of solar motion toward Hercules is as nearly certain as possible, but the rate of this motion is not known.

Recent researches suggest that it is far greater than the rate just mentioned, great though that rate may appear. - Examined with a telescope, the sun's surface, which appears very nearly uniform to the naked eye, is seen to be brightest near the centre, and to grow progressively darker toward the circumference. It is also marked by various irregularities, spots, faculae, mottling, besides other appearances requiring greater telescopic power for their detection. The spots on the sun were independently discovered by Galileo, Fabri-cius, Scheiner, and Harriot. It was soon perceived that they move in such a way as to indicate that they are real surface markings, not bodies passing between the earth and the sun, and that therefore the sun's rotation can be measured by observing them. It was found that the sun rotates in a period of about 25½ days; and as the spots do not at all times pass on straight lines across the sun's face, but sometimes on a course slightly bowed upward and at others on a course slightly bowed downward, it was seen that the sun's axis of rotation is not quite upright as referred to the plane of the ecliptic, but slightly inclined.

The following elements of the sun's rotation belong to the astronomy of recent times, having been deduced from results obtained by Carrington and Sporer, reduced to the year 1869 :

It will be perceived that a mean rotation is indicated. Carrington's observations have shown that spots in different solar latitudes travel at different rates, varying in fact from a daily rotation through about 122/3° in lat. 50° (nearly the highest in which spots have been observed) to a daily rotation through nearly 14½° at the solar equator (where, however, spots are very rarely seen). Carrington gives the following formula for the rotation in different solar latitudes : daily rotation = 14° 25'-(2° 45') sin.7/4 lat.; but this formula is purely empirical. The curious point about this variation in the rate of turning is that, taking two parts of the visible solar surface in the same longitude, but one in lat 45° (say), the other on the equator, the latter will advance further and further in longitude from the former, gaining daily about two degrees, so that in the course of about 180 days it will have gained a complete revolution. That is to say, the sun's equator makes about two revolutions more per annum than the regions in 45° north and south solar latitude.

The spots on the sun have usually a dark central region called the umbra, within which is a still darker part called the nucleus, while around this there is a fringe of fainter shade than the umbra, called the penumbra. Although the umbra and nucleus appear dark, however, it is not to be supposed that they are really dark; in fact, Prof. Langley of Pittsburgh has succeeded in examining the light from the nucleus alone, and he finds that though the nucleus looks perfectly black by contrast with the general surface, it shines in reality with a light unbearably brilliant when viewed alone, while his thermal measurements show that the heat from the nucleus is even greater proportionately than the light, and not very greatly below the heat of the surrounding surface. The boundary between the umbra and the penumbra is in general well defined; and commonly the inner part of the penumbra nearest to the umbra is brighter than the exterior portion. Many spots are of enormous size, so as to be visible with the naked eye. Sir W. Herschel saw one in 1779 which had a diameter exceeding 50,-000 m., and many far larger than this have since been seen.

The spots are not scattered over the whole surface of the sun, but are for the most part confined to two belts between lat. 5° and 30° on either side of the solar equator. An equatorial zone 6° wide is almost entirely free from spots. Owing to this peculiarity of arrangement, Sir J. Herschel suggested the existence of motions in the solar atmosphere corresponding to our trade winds; but the circumstances of the solar orb and atmosphere differ so entirely from those of our earth and air, that such comparisons are unsafe. Dr. Wilson of Glasgow was the first to show that the umbra of a spot is below the level of the penumbra. He observed that a spot, visible in 1769, changed in shape as it traversed the solar disk, precisely as it would . if the spot had been a depression below the general surface of the sun. The penumbra was markedly wider on the side nearest the edge of the solar disk than on the other side, whereas the reverse should have been the case if the spot had been a surface marking. Sir W. Herschel in 1777 began a series of solar observations which before long confirmed Wilson's views. He was led to explain the spots by the theory that the sun's globe is surrounded by two layers of clouds, suspended in an atmosphere at different elevations.

Ho supposed the upper cloud stratum to be self-luminous, and to be the source of the solar light, or the true photosphere (to use a convenient term invented by Schroter). The lower layer ho regarded as opaque, and as owing whatever light it appears to possess to the reflection of light received from the upper layer. He supposed that when an opening is formed in the outer layer we see merely a penumbral spot; but that when the inner layer also is displaced we see the true surface of the sun, which he supposed to be solid, and not necessarily so heated as to be unfit for habitation. Modern researches show this part at least of Herschel's theory to be wholly untenable, everything tending to prove that the whole mass of the sun to its innermost core is intensely heated. The recognition of a nucleus within the umbra would seem to indicate that a third cloud layer exists within the second or internal layer of Herschel's theory. But the observations of Prof. Langley show that most probably all the features of the solar photosphere yet observed are phenomena of cloud envelopes, since he has been able to recognize cloud forms at one level floating over cloud forms at a lower level, while even in the (relatively) darkest depths of the nucleus clouds are still to be perceived, though so deep down that their outlines can be barely discerned.

The study of the solar spectrum (see Spectrum Analysis), while revealing much respecting the constitution and physical condition of the solar orb, has thrown some light also on the nature of sun spots. Mr. Huggins, for instance, has found that several of the absorption bands belonging to the solar spectrum are wider in the spectrum of a spot, a circumstance indicative of increased absorption so far as the vapors corresponding to such lines are concerned. Spots are more numerous in some years than in others, and occasionally no spots are visible for many successive days. Schwabe of Dessau began to study this peculiarity in 1S26, and after many years recognized a remarkable periodicity in the frequency of sun spots. They are found gradually to increase in number during a certain period, and then to decrease until at length there are no spots; then they increase again, and so on. According to Schwabe's earlier investigations, the cycle lasts 10½- years; but Wolf of Zurich has found by examining earlier observations that the true average period is about 11 .11 years. (See Magnetism, Terrestrial.) Various, minor cycles have been suspected, besides a long cycle of about 56 years.

Wolf in 1859 presented a formula by which the frequency of spots is connected with the motions of the four bodies, Venus, the earth, Jupiter, and Saturn. Prof. Loomis of Yale college has since advocated a theory (suggested by the present writer in 1865, in " Saturn and its System," p. 168, note) that the long cycle of 50 years is related to the successive conjunctions of Saturn and Jupiter. But the association is as yet very far from being demonstrated, to say the least. - Besides the spots, the telescope reveals minute dark dots or pores mottling the surface, which have been lately found to be the intervals separating numberless cloud-like forms, apparently minute, but in reality from 200 to 1,000 m. in diameter, the brilliancy of which so greatly exceeds that of the intervening spaces that they must be recognized as the principal radiators of the solar light and heat. These are. found to be in constant fluctuation, and Sir J. Herschel compares their appearance to the slow subsidence of some flocculent chemical precipitates in a transparent fluid when viewed perpendicularly from above. Near the great spots or groups of spots there are often seen streaks more luminous than the neighboring surface, called faculoe. They are oftenest seen toward the borders of the disk.

Mr. Dawes saw, on Oct. 22, 1859, -in a large mass of faculae, one bright streak forming the very edge of the sun, and projecting irregularly beyond the circular contour, reminding him of a ridge of low hills often seen at the enlightened limb of the moon. M. Chacornac, a most diligent French investigator, observed on one occasion a sudden transformation of the luminous part of the photosphere into dark parts; luminous bridges were seen crossing the spots, and then gradually becoming dark. As these luminous bridges darkened, they at the same time plunged into the deeper parts, and became covered with other luminous bridges which formed above them. - The phenomena witnessed during total solar eclipses are next to be considered. The red prominences were first seen during the solar eclipse of July 8, 1842. In the eclipse of July 28, 1851, it was shown that they belong to the sun, since the advancing moon visibly concealed those on one side and disclosed those on the other side. During the eclipse of June 18, 18G0, Secchi and De la Rue photographed the prominences at two stations in Spain, and thenceforth the solar nature of these appendages was admitted by all.

As some of them were seen to extend fully 3' from the edge of the sun on that occasion, it became manifest that they are objects of enormous dimensions, since 3' at the sun's distance corresponds to an extension of about 80,000 m. In the Indian eclipse of August, 1868, the prominences were examined with the spectroscope by Col. Tennant, Capt. Herschel, and Messrs. Janssen and Rayet. The spectrum was found to consist of bright lines, showing that the colored prominences are masses of glowing gas, the bright lines of hydrogen were recognized, and an orange-yellow line was ascribed (mistakenly, however) to sodium. But on the following day Janssen applied a new method of research, the principle of which had been indicated earlier by Huggins (" Report of Council of Astronomical Society," "Monthly Notices," February, 1868). Since prismatic dispersion reduces the brightness of the solar spectrum, but only throws the lines of a gaseous spectrum further apart, it follows that by directing a tele-spectroscope toward the place of a prominence, the light from the air which usually obliterates the prominence light can be so reduced by sufficient dispersion that the prominence lines may be rendered visible.

Janssen found this to be the case, and by noting the indications thus afforded he was able to determine the presence and even the shape of prominences at various parts of the sun's edge. Two months later, but before the news of Janssen's success had reached England, Mr. Lockyer obtained a similar result. Before long Huggins, who had been the first to enunciate the principle of the method, showed how by opening the slit of the spectroscope the whole of a prominence could be seen at once. Since then the prominences have been successfully studied by Zollncr, Respighi, Secchi, and others. Prof. Young of Dartmouth college has been particularly successful in applying this method of research. - Even before the prominences were discovered, it was known that a border of red light surrounds the solar disk; it had been seen on the eastern side at the beginning of total eclipse, and on the western side at the end. In 1860 this envelope was very clearly seen, and even photographed. It has been designated as the sierra, because of its serrated appearance; but recently the name chromosphere (for chromatosphere) has been given to it.

The observations of prominences and sierra as summarized by Secchi indicate the following results: " The sierra presents four aspects: 1, smooth, with defined outline; 2, smooth, but no defined outline; 3, fringed with filaments; and 4, irregularly fringed with small flames. The prominences may be divided into three orders, heaps, jets, and plumes. The heaped prominences need no special description. The jets are those to which alone the following description by Respighi (erroneously given as generally applicable to all prominences) can be applied: 'They originate generally in rectilinear jets either vertical or oblique, very bright and very well defined. They rise to a great height, often to a height of at least 80,000 m., and occasionally to more than twice that; then bending back, fall again upon the sun like the jets of our fountains. Then they spread into figures resembling gigantic trees more or less rich in branches.' Their luminosity," proceeds Secchi, "is intense, insomuch that they can be seen through the light clouds into which the sierra breaks up. Their spectrum indicates the presence of many elements besides hydrogen. When they have reached a certain height they cease to grow, and become transformed into exceedingly bright masses, which eventually separate into fleecy clouds.

The jet prominences last but a short time, rarely an hour, frequently but a few minutes, and they are only to be seen in the neighborhood of the spots. Wherever there are jet prominences there also are faculaa. The plume prominences are distinguished from the jets in not being characterized by any signs of an eruptive origin. They often extend to an enormous height; they last longer than the jets, though subject to rapid changes of figure; and lastly they are distributed indifferently over the sun's surface. It would seem that in jets a part of the photosphere is lifted up, whereas in the case of plumes only the sierra is disturbed." (It is here of importance to remark that these eruptive prominences, particularly associated with spots, are of late becoming recognized as chiefly due to metallic vapors, in distinction from the "plume" forms, which are largely composed of hydrogen.) This account would be incomplete without a description of the remarkable solar explosion actually witnessed by Prof. Young on Sept. 7, 1871. Fig. 1 represents a cloud prominence he had been observing on the eastern limb of the sun. It was about 100,-000 m. long by 54,000 m. high.

He was called away at 12h. 30m., and on returning at 12h. 55m. "found that the whole thing had been literally blown to shreds by some inconceivable up-rush from beneath." Fig. 2 represents the appearance when the up - rushing hydrogen had attained its greatest height, exceeding 200,000 m. "The whole phenomenon," he says, "suggested most forcibly the idea of an explosion under the great prominence, acting mainly upward, but also in all directions outward, and then after an interval followed by a corresponding inrush." A strange circumstance remains to bo mentioned : "The same afternoon a portion of the sierra on the opposite limb of the sun was for several hours in a state of unusual brilliance and excitement, and showed in the spectroscope more than 120 bright lines whose position was determined and catalogued - all that I had ever seen before and some 15 or 20 besides." Before passing from the prominences it may be well to indicate the laws of their numerical distribution, as determined by Secchi and others.

This is shown in fig. 3. On the left side the results of Carrington's observation of 1,414 spots between 1853 and 1861 are indicated, and on the right the result of Secchi's observations of 2,767 protuberances in 1871, the number of spots or prominences being of course shown by the length of the radial lines. The dotted line on the right-hand side represents in the same manner the distribution of the larger prominences, viz., those exceeding 1' or 27,000 m. in height. - During a total eclipse there appears around the black body of the moon a halo or glory of light, bright, close to the place of the concealed sun, but gradually fading away outward, until its light is lost in the general tint of the sky. In this glory of light, which is called the solar corona, radiations are also sometimes seen, and under favorable atmospheric conditions complicated series of streaks can be seen extending to a considerable distance outward from the prominence region. Various theories were advanced in former times to explain the corona. According to one theory, it is a phenomenon caused by the solar light falling on our own atmosphere; another theory ascribed it to a lunar atmosphere.

In the opinion of Lever-rier and Foucault (among others), the corona is an example of the interference of light (see Light), the phenomenon being analogous to the colored fringes seen on a screen in a darkened room when a solar beam is admitted through a chink. To this theory Airy raised the objection that if, in order to make the analogy perfect, the eye is placed in the position of the screen, no colored fringes are seen. It is shown that the corona is partly polarized, and hence partly consists of reflected light. It has been further proved that the plane of polarization passes through the sun and the observer. This was regarded by Airy as pointing to the existence of an atmospheric medium capable of reflecting light, and extending from the earth to the moon. But in more recent times astronomers began to perceive that no other theory can be admitted than that which regards the corona as a true solar appendage. (Of course, it must be admitted that a portion of the light around the eclipsed sun comes from our own atmosphere, which must necessarily be illuminated by the true corona during eclipse, precisely as it is illuminated by the sun when there is no eclipse; but it will readily be understood that this portion of reflected light is very small in amount.) During the solar eclipse of August, 1869, Profs. Young and Harkness discovered that certainly one bright line exists in the spectrum of the corona, and two other lines were suspected.

European astronomers expressed doubt as to the accuracy of this observation; but it was confirmed during the Mediterranean eclipse of December, 1870, when Young thus summed up his own and other observations: " There is surrounding the sun, beyond any further reasonable doubt, a mass of self-luminous gaseous matter, whose spectrum is characterized by the green line 1,474 Kirchhorf. The precise extent of this it is hardly possible to consider as determined, but it must be many times the thickness of the red hydrogen portion of the sierra, perhaps on an average 8' or 10', with occasional horns of twice that height. It is not at all unlikely that it may even turn out to have no upper limit, but to extend from the sun indefinitely into space." During the same eclipse, Brothers of Manchester and Willard of Philadelphia (the latter acting under the directions of Prof. Win-lock of the Harvard observatory) photographed the corona successfully from two distant stations, Willard being near Jerez in Spain, Brothers near Syracuse in Sicily. The views thus obtained agreed so closely (save in circumstances depending on photographic conditions) as to leave no doubt that the corona is a solar phenomenon.

Doubts were still expressed, and it was not until the solar eclipse of December, 1871, that these were finally removed. On that occasion the spectroscopic and photographic results were alike decisive. Jans-sen with the spectroscope not only recognized the bright lines before seen and others less bright, but also a faint solar spectrum, which, since our atmosphere during total eclipse is certainly not illuminated by sunlight, must have been reflected by matter in the solar corona, such as vaporous clouds, meteor flights, or the like. Mr. Davis, a photographer sent out at Lord Lindsay's expense, obtained five excellent photographs of the corona, all agreeing perfectly inter se, excepting in extent. This proved certainly that the features of the corona do not change as they would if the phenomenon depended on the passage of light rays athwart lunar inequalities, to fall upon scattered matter at a less distance than the moon. Again Col. Tennant obtained six photographs, similarly accordant inter se, and also agreeing perfectly with Mr. Davis's at Doda-betta, a station far removed from Davis's, Bai-cull. Since, also, Dodabetta is near the highest peak of the Neilgherries, about 9,000 ft. above the sea level, while Baicull is close to the seashore, it will be manifest that if the features of the corona depended on the illumination of our own atmosphere, the pictures of Tennant's series would have differed altogether from those of Davis's series.

Thus, independently of the spectroscopic evidence, the photographs proved that the corona is a solar appendage, at least as far as those features shown in the two series extend. But they extend from the sun in places to a distance exceeding his own diameter, and amounting in fact to more than a million miles. There is reason to believe that the true solar corona extends much further, and that in reality the zodiacal light (see Zodiacal Light) forms the outer part of the solar corona; so that if the light of the sun could be for a time obliterated without rendering his appendages invisible, we should see the corona merging gradually into the faint glow of the zodiacal light. Mr. Arthur W. Wright of Yale college has succeeded in showing that this light is not emitted from incandescent gas, but reflected from particles or small bodies, and hence derived from the sun. - Another important discovery made during total solar eclipses relates to a solar atmosphere underlying even the sierra. Secchi had observed in 1869 that close to the sun's limb the solar spectrum becomes continuous; this he considered to be due to the existence of a relatively very shallow atmosphere, consisting of the vapors which cause the dark lines of the solar spectrum.

For if the brightness of the lines of these vapors corresponds very closely to the brightness of the ordinary solar spectrum for the parts near to the sun's edge, the dark lines of the latter spectrum would be cancelled, and so a continuous spectrum would be produced. For another reason, the present writer had adopted the theory that the atmosphere producing the absorption lines of the solar spectrum must be shallow, compared at least with the dimensions of the sun's globe; for he showed that a shallow and not a deep atmosphere is to be inferred from the darkening of the solar disk near its edge. The opinion thus advanced on theoretical grounds was shown to be correct by the observations of Prof. Young during the total eclipse of December, 1870; for, "directing his analyzing spectroscope to the part of the sun's limb which was to disappear last, he found that at the instant when totality commenced the solar spectrum was suddenly replaced by a spectrum consisting of a thousand soft bright lines." In other words, the vapors which by their absorptive action produce the dark lines of the ordinary solar spectrum were for the moment shining with their own light, and thus produced a spectrum of bright lines.

This spectrum continued visible for a few seconds only, showing that the complex atmosphere producing it cannot be more than two or three hundred miles in depth. The observation was successfully renewed during the eclipse of December, 1871, and again during the annular eclipse of June, 1872. - How to account for the supply of the prodigious amount of heat constantly radiated from the solar surface has offered a boundless field of hypothesis. One conjecture has been that the sun is now giving off the heat imparted to it at its creation, and that it is gradually cooling down; another ascribed it to combustion, and a third to currents of electricity. Newton and Buffon conjectured that comets might be the aliment of the sun, and of late years a somewhat similar theory (first broached by Mr. Waterston in 1853) has been in vogue, viz., that a stream of meteoric matter constantly pouring into the sun from the regions of space supplies its heat, by the conversion into it of the arrested motion. As the sun may indeed derive a small amount of heat from this cause, it deserves more attention than previous conjectures.

But conjecture and hypothesis may be said to have given place to views which claim a higher title, as it is now becoming generally recognized, in accordance with modern physical theories of heat, that in the gravitation of the sun's mass toward its centre, and in its consequent condensation, sufficient heat must be evolved to supply the present radiation, enormous as this undoubtedly is. It appears to be susceptible of full demonstration that a contraction of the sun's volume of a given definite amount, which is yet so slight as to be invisible to the most powerful telescope, is competent to furnish a heat supply equal to all that can have been emitted during historical periods. According to this theory then (which is due largely to the development by llelmholtz of Mayer's great generalization), the sun's mass remains unaltered, and its temperature nearly constant, while its size is slowly diminishing as it contracts; so slowly, however, that the supply may bo reckoned on through periods almost infinite as measured by the known past of our race, and which are in any case to be counted by millions of years.

It would appear from early measurements of Secchi that the different portions of the solar disk do not radiate heat in uniform degrees, and his tables show that the equatorial regions are slightly hotter than the polar. It has been explained that the rapid decrease of brightness toward the edge of the sun obliges us to admit the existence of a shallow atmosphere around it. Prof. Langley has recently published tables from more extended measurements, showing the rate of absorption both of heat and light, the latter being greater than the former. As ho does not now find the difference between the equatorial and polar heat observed by Sec-chi in 1852, the latter concludes from a comparison of his own observations with Lang-ley's, that great changes occur in the distribution of the heat on the sun's surface. Prof. Langley has further shown that this atmosphere absorbs one half of the sun's total radiation, and he considers that its function in the solar emission is of great importance to us. A slight alteration in the thickness of this obscuring envelope would induce changes on the earth greater than those known to have occurred in its climate in past geologic epochs, which may themselves not impossibly have been due to this hitherto unrecognized cause.

M. Fizeau has found that the chemical rays are similarly reduced in amount toward the edge of the solar disk, a fact which is also abundantly shown by the darkening near the edge of photographic sun pictures, like those by Rutherfurd and De la Rue. - To sum up briefly the received hypotheses of the physical constitution of the sun: Of its internal structure we know nothing, but we can infer from the low density of the solar globe as a whole that no considerable portion is solid or liquid. The regions we examine appear to consist of cloud layers at several levels floating in a complex atmosphere, in which probably most of the elements are known to us, and certainly many of them exist in the form of vapor. Outside this complex atmosphere extend envelopes of simpler constitution, though into them occasionally arise the vapors which ordinarily lie lower down. The sierra, for instance, consists in the main of glowing hydrogen gas, and that gas, whatever it may be, which produces the line near the orange-yellow sodium lines. The prominence region may be regarded as simply the extension of the sierra.

The inner corona is still simpler than the sierra so far as its gaseous constitution is concerned; but here meteoric and cometic matter appears, extending to the outer corona and to great distances beyond even the visible limits of the zodiacal. Returning to the photosphere, we find it subject to continual fluctuations, both from local causes of agitation and from the subjacent vapor acting by its elasticity to burst through it; the faculae, which are found to be above the general level of the photosphere, are taken to he heapings up of the luminous matter like the crested surges of the sea. All the strata are suhject to great movements, which sometimes have the character of uniform progression analogous to our trade winds, and sometimes are violent and resemble in their effects our tornadoes and whirlwinds. Eruptive action appears to operate from time to time with exceeding violence, but whether the enormous velocities of outrush are due to true explosive action (which would compel us to believe that the sun is enclosed by a liquid shell, so as to resemble a gigantic bubble), or to the uprising of lighter vapors from enormous depths, as heated currents rise in our own atmosphere, is not as yet certainly known.

Fig. 1. - Prominence as it appeared at half-past 12 o'clock, Sept. 7, 1871.

Fig. 2. - As the above appeared half an hour later.

Fig. 3 - Relative Frequency of Protuberances and Sun Spots.