Observations on the Solar Spots. By Alexander Wilson, M. D. Professor of Practical Astronomy in the University of Glasgow. Communicated by the Rev. Nevil Maskelyne, Astronomer Royal

I. Observations on the Solar Spots. By Alexander Wilson, M.D. Professor of Practical Astronomy in the University of Glasgow. Communicated by the Rev. Nevil Maskelyne, Astronomer Royal. Redde, Apr. 29, 1773. WHEN the great Copernicus had revived the true system of the world, it was objected against it, that the planet Venus ought to be seen with different phases like the moon; to which he answered, that some time or another that very thing would perhaps come to pass. Who then would have thought, that this prophecy was so near being fulfilled, and that, by means of combining a convex and a concave glass, the common uses of which had been known for near 300 years before, the sight of man was to be extended almost to the remotest parts of the universe? This instrument, which nothing but its being so common hinders us from regarding with the greatest wonder, was invented, about 170 years ago, by one of those happy accidents, to which we owe many of the finest discoveries. At that time flourished the famous Galileo, who was the first that constructed a telescope from some knowledge of the refraction of the rays of light. Having finished one in 1610, he directed it to the heavens, and thus entered upon the most pleasing and most striking field of observation, which any philosopher ever enjoyed. The name of Galileo will always be familiar to us, so long as there remains any remembrance of those celebrated discoveries, which he then made by the help of his glasses. One of the most remarkable of them was that of the solar spots. So strange a discovery, relating to the sun, commanded a great deal of attention. At that time the authority of Aristotle, by controlling the reason of men, governed their opinions in all parts of philosophy; and one of the absurd doctrines, then taught in the schools, was, that the matter of the heavens was ungenerated, incorruptible, and subject to no alteration. But the arguments, which Galileo now produced against that notion, were founded upon the evidence of sense, and it was impos- impossible to oppose them. The spots upon the sun, and the many strange variations, to which they were subject, convinced all, who were able to lay aside the prejudices of a vain philosophy, that there was not so great a difference, betwixt celestial and sublunary bodies, as had been imagined. To such as were so reconciled to it, the discovery appeared grand and striking; and great hopes were entertained, that, by duly attending to the phænomena of the spots, something curious and important might be determined concerning the nature and constitution of the sun itself. We accordingly find, that many astronomers, of the first note, were very early engaged in this inquiry. Of all those who applied themselves this way, Scheiner and Hevelius deservedly hold the first place, and nothing but the charms of so noble an investigation could have induced them to prosecute their observations with so much assiduity. Scheiner began his in the year 1624, 14 years after Galileo had first made the discovery. In the year 1630, he at last published his Rosa Ursina, in which we have a detail of his labours during that long interval of time. Hevelius came after Scheiner, and diligently watched the appearances of the spots for two years, the result of which application he has given us in his Selenographia and Cometographia. But notwithstanding these attempts, so worthy of men actuated by a true desire of knowledge, it must be confessed, that nothing of moment hath been derived from them. If we except a few conclusions concerning the rotation of the sun round its axis, and the inclination of its axis to the plane of the ecliptic, ecliptic, every thing else, which hath been inferred from the phænomena of the spots, seems altogether to be matter of conjecture. Hevelius, from his great fondness of the subject, and from a desire to avail himself of that long course of observation, to which he had so patiently submitted, has been led into many speculations concerning the spots and the nature of the sun's body. The following quotations furnish us with a remarkable instance of this, and will serve to give us a view of the ideas, which he came to entertain upon these subjects. In his Cometographia, p 360, speaking of the solar spots he expresses himself thus: "Hæc materia nunc ea ipsa est evaporatio et exhalatio (quia aliunde minime oriri potest) quæ ex ipso corpore solis, ut supra ostensum est, expiratur et exhalatur: quæ penitus ex diversis partibus heterogeneis, certos gradus opacitatis et densitatis habentibus, constat, modo extenuioribus, modo opacioribus, modo siccioribus, modo magis viscosis, glutinosis (ut nostro loquar more usitato) sive bituminosis. Unde etiam diversa procreantur phænomena; extenuioribus, purioribus, et siccioribus, umbræ videlicet faculæ et luculæ; ex impurioribus vero crassioribusque illis viscosis, maculæ eorundemque nuclei progignuntur. Cum ergo solis exhalationes ejusmodi partes sint heterogeneæ, quidni etiam illud ipsum corpus, quod eas ejaculatur, prout in praecedentibus fusa assertum est, ex diversis partibus heterogeneis constabit? Ad hæc, quia tales admirabiles et manifestissimæ generationes, mutationes, alterationes, condensationes, rarefactiones, coitiones, diductiones, imo motus "motus locales corporum, ac rerum materiarumque "ex ipso sole ejectarum evaporatarumque, pene "continenter, ad instar nubium ac meteororum sub- "lunarium exterrâ promanantium, peragi videmus; "nonne clare ex eo evincitur, solem habere suam "atmosphæram, in eâque dari, pro virtutis suæ "indole, generationes et corruptiones rebus sublu- "naribus haud dissimiles rarioresque? imo, non- "nunquam longe frequentiores, et insigniores ac- "cidere (si præsertim observationes macularum a "Scheinero nobisque habitas exacte examinaveris) "quam prope hanc terram unquam possint?" And again,. when speaking of the sun's body: "Atque "ita hic liquor igneus (ut sic loqui liceat) est quasi "vastissimum luminum pelagus et mare igneum, "quod suos habet abyssus, occultos meatus, voras- "gines atque vortices; quod, ad instar maris nostri, "perpetuis fluctuum quibuidam voluminibus agi- "tatur, et suo modo evaporationes et exhalationes "jugiter evomit atque exhalat: eoque magis aut "minus, quo magis vel minus materia ista in vis- "ceribus ejus, atque intimis recessibus, igneum "llud pelagus, illiusque fluctus commoventur ac "concitantur." But all that we here find, however plausible and ingenious, can be regarded only as conjecture. It does not appear, that any who have followed HEVELIUS have met with more success. Their observa- tions seem not to differ from his in any remarkable circumstance; nor do we find, that their inferences from them, although sometimes different, have any better pretensions to the truth. The many strange and variable circumstances of the spots, which were disco discoverable from a minute observation, still remained unaccountable; and we often find them at a loss, in framing any hypothesis, which could fully satisfy the mind concerning them. In process of time, astronomers began to withdraw their attention from a subject, which remained so dark and perplexing, and, for many years, all researches of this sort have been, in a great measure, laid aside. Chance, or a happy concurrence of circumstances, hath sometimes effected more, than could have been expected from the most promising measures: a remark which, it is hoped, will, in some degree, be found justified in the sequel of this paper. The observations upon the solar spots, which I now proceed to relate, appear to be totally different from any hitherto to be found, and such as seem to open a new and curious field of speculation into the whole of this subject. Astronomers will remember, that a spot of an extraordinary size appeared upon the sun, in the month of November 1769. The first notice, I had of it, was by a letter from a friend at London, but the weather continued dark and cloudy for some days after; at length, on the 22d day, I had a view of the sun through an excellent Gregorian telescope, of 26 inch focus, which magnified 112 times. I then beheld the spot, which at that time was not far from the sun's western limb, and below his equatorial diameter. The atmosphere being now very clear, and free from all tremor and undulation, it was pleasant to see the nucleus of the spot, and the shady zone or umbra which surrounded it, so very distinct. It being afternoon when when I first got sight of it, I had not an opportunity of observing it long that day. Next day being the 23d, I had a curiosity to see it again, and so repaired to my telescope, in order to examine, if any alterations, in the size and figure, had taken place since last observation. The air was still favourable, and I again saw the spot, it having its nucleus and umbra very sharply defined. I now found however a remarkable change; for the umbra, which before was equally broad all round the nucleus, appeared much contracted on that part which lay towards the center of the disc, whilst the other parts of it remained nearly of their former dimensions. This change of the umbra seemed somewhat extraordinary, as it was the very reverse of what I expected from the motion of the spot towards the limb. But next day, at 10 o'clock, I had another observation, and discovered changes, which were still more unexpected. The distance of the spot from the limb was now about $24''$. By this time, the contracted side of the umbra above mentioned had entirely vanished; and the figure of the nucleus was now remarkably changed, from what it had been the preceding day. This alteration of the figure appeared evidently to have taken place upon that side which had now lost the umbra, the breadth of the nucleus being thereby more suddenly impaired than it ought to have been, by the motion of the spot across the disc. Fig. 1, 2, [Tab. I.] represent the appearance which the spot had on the 23d and 24th days. Regarding Regarding these circumstances as new, I began to consider, what might be the cause of them. One of two things seemed necessarily to be the case; either, that they were owing to some physical alteration or wasting of the spot, and of that part of it where the deficiency of the umbra was observed; or else, that they were owing to the nearer approach of the spot to the limb, by the sun's rotation on his axis. The last of these two ideas had no sooner struck me, than I began to suspect, that the central part, or nucleus of this spot, was beneath the level of the sun's spherical surface; and that the shady zone or umbra, which surrounded it, might be nothing else but the shelving sides of the luminous matter of the sun, reaching from his surface, in every direction, down to the nucleus: for, upon this supposition, I perceived, that a just account could be given of the changes, of the umbra and of the figure of the nucleus, above described. The opinion therefore, which I ventured to form from what I had seen this day, was, that this spot might, probably, be a vast excavation in the luminous matter of the sun; the nucleus, commonly so called, being the bottom, and the umbra the shelving sides of the excavation: and, moreover, that the umbra, next the center of the disc, although out of my view, did still however exist, and was rendered invisible by its present position only; and further, that the sudden alterations, now discernible in the figure of the nucleus, were occasioned by some part of it also being hid, by the interposition position of the edge of the excavation, between the nucleus and the eye. These views, which now presented themselves, I remember to have communicated, that afternoon, to my son; when I then told him, that, if they were well founded, there would be room to verify them, if the spot should again return upon the opposite side of the disc. I was however uncertain, if it would last so long upon the sun's body, as to be again visible after the time of half a revolution; a circumstance which I wished to take place, as I was aware, that my present observations might justly be deemed insufficient, for establishing so singular an opinion concerning the nature of this spot; and that, notwithstanding all which I had seen, we might still imagine, that these changes were produced by certain physical alterations of the spot itself. These considerations made me attentively wait its return. At last, on December 11th, I again discovered it, on the opposite side of the disc, it having by that time advanced a little way from the eastern limb, being distant from it $1'30''$. And now I could only perceive three sides of the umbra, namely, the upper and under sides, and that towards the limb, which was the side that formerly had vanished. The side towards the center of the disc was not as yet visible; but I concluded, upon the same grounds as formerly, that it was hid from my sight, by its averted position only, and that, after the spot had advanced a little further, it would make its appearance. Accordingly, the next day, being December 12th, at ten o'clock, it came into view. view, and I saw it distinctly, though narrower than the other sides. After this, my observations were interrupted, by unfavourable weather, till the 17th, when the spot had passed the center of the disc, the umbra now appearing to surround the nucleus equally. Fig. 3, 4, and 5, represent the spot, as it appeared on December 11th, 12th, and 17th, when it came upon the disc for the second time. All the foregoing appearances, when taken together, and when duly considered, seem to prove in the most convincing manner, that the nucleus of this spot was considerably beneath the level of the sun's spherical surface. The next thing, which I took into consideration, was to think of some means, whereby I could form an estimate of its depth. At the time of the observation I had on December 12th, I had remarked, that the breadth of the side of the umbra, next the limb, was about $14''$; but, for determining the point in question, it was also requisite, to know the inclination of the shelving side of the umbra to the sun's spherical surface. And here it occurred, that, in the case of a large spot, this would in some measure, be deduced from observation. For, at the time when the side of the umbra is just hid, or begins first to come in view; it is evident, that a line joining the eye and its observed edge, or uppermost limit, coincides with the plane of its declivity. By measuring therefore the distance of the edge from the limb, when this change takes place, and by representing it by a projection, the inclination or declivity in some measure may be ascertained. For in fig. 5. [Tab. II.] let ILDK be a portion of the sun's limb, and A B C D a section of the spot, S L the sun's semidiameter, L G the observed distance from the limb, when the side of the umbra changes, then will the plane of the umbra, C D, coincide with the line, E D G, drawn perpendicular to S L, at the point G. Let F H be a tangent to the limb, at the point D, and join S D. Since G L, the versed sine of the angle L S D, is given by observation, that angle is given; which, by the figure, is equal to F D E, or G D H; which angle is therefore given, and is the angle of inclination of the plane of the umbra to the sun's spherical surface. In the small triangle therefore C M D, which may be considered as rectangular, the angle M D C is given, and the side D C, equal to A B, is given nearly, by observation; therefore the side M C is given, which may be regarded as the depth of the nucleus, without any material error. I had not an opportunity, in the course of the foregoing observations, to measure the distance G L, not having seen the spot, at the time when either of the sides of the umbra changed. It is however certain, that, when the spot came upon the disc for the second time, this change happened sometime in the night between the 11th and 12th of December; and I judge that the distance of the plane of the umbra, when in a line with the eye, must have been about $1'35'$ from the sun's eastern limb; from which we may safely conclude, that the nucleus of the spot was, at that time, not less than a semidiameter of the earth, below the level of the sun's spherical surface, and made the bottom of an ama- zing cavity, from the surface downwards, whose other dimensions were of much greater extent. Being thus persuaded of the depression of this great spot below the surface, I immediately set about examining smaller ones, in order to discover if they were of the same kind. With this view, I began a course of observations, that from them I might either make the inference universal, or limit it, as the phenomena should point out. I was not long engaged in this pursuit, before I perceived in them the same changes of their umbræ, which have been described above at so much length. This was manifest in spots of any considerable size, when the air was favourable, and the telescope well adjusted for distinct vision. The first, which I saw undergo this change upon its near approach to the limb; was on January 17th, 1770, as represented in Fig. 1. [TAB. II.] The figures 2, 3, 4, in this plate, and 1, 2, 3, 4, [TAB. III.] are so many other instances taken from the register of the observations at that time. But, as this appearance, of the umbra changing, is evident from a bare inspection of the figures, we have omitted any unnecessary description of them. These eight cases are taken out from above 40 others of the same kind; and in them all, the nuclei were in the middle of the umbræ, before their approach to the limb. It may be remarked, that in fig. 2. [TAB. II.] the same spot traversed the disc and had its umbra changed, both in the coming on and going off. In general, we have found that the umbra thus changes, when a spot is about a minute distant from the limb, at a medium. From all these observations, may we not safely conclude, that every spot consisting of a nucleus and surrounding umbra, as defined by Scheiner and Hevelius, is of the same kind with those above described? But as, in researches of this sort, we can never be too cautious in making a general conclusion, so I would humbly recommend these observations to every lover of the subject, to those especially who are provided with large and good telescopes; for, without this advantage, I have found by repeated trials, that I could not discern the minute changes of the small spots. In the course of the foregoing observations, I had occasion to remark, five different times, another extraordinary circumstance of the spots, which I have not seen mentioned, by any one who has written upon the subject. It consists of changes, which seem to arise from a disturbing force, when one spot breaks out in the neighbourhood of another. The first case of this sort which I met with, was on Nov. 9th, 1770, when the umbra of the spot $m$, fig. 1. [Tab. IV.] although a great way from the limb, was deficient towards the right hand, at which side, and very near it, there lay another spot much smaller. In like manner, the two spots $n$ and $o$, fig. 2. [Tab. IV.] which lay very near one another, had each of them that side of its umbra, which faced the other, taken away. But it was remarkable, that, three days after, the spot $o$ had nearly vanished, when the side of the umbra of the spot $n$, which faced it, began now to dilate. In fig. 3, the spot $p$, had its umbra flattened upon opposite sides, by three small spots on one hand, and one upon the other. other. Again, in fig. 4, the two spots \( q, r \), had their umbrae deficient, by the intervention of some small spots, that lay between them. Now it must here be particularly remarked, that though a spot, when undisturbed, will, when near the sun's limb, exhibit the change of the umbra formerly mentioned, yet it is plain, that a case may now and then occur, when this change will be counteracted, by means of the phenomenon which we have just now described. For if we should suppose, for example, the spot \( m \), fig. 1. Tab. IV. to have been on November 9th, near the western limb, it is evident, that we should have formed a different judgment concerning the change of the umbra. And accordingly, in the course of the observations formerly mentioned, I in reality met with three cases, when this change did not take place. I am sensible, that it may be thought strange, that none of the observers, who had looked at the solar spots with so much attention, should ever have taken notice of the gradual changes above described. This partly may be accounted for from the following considerations. We have already seen, that conjectures, concerning the nature of the sun, were early indulged in the course of this inquiry. His body was thought to be an immense globe of fire, which was for ever raging with the most fervent heat. Hence the first observers, reflecting upon the perpetual generation, changes, and decay of the spots, and that through so wide an extent of his surface, very naturally imagined, that they could consist of nothing but smoke and groser grosser exhalations, or such transient and perishable materials. This hypothesis had at least the air of being supported by a very plausible analogy. The minds of men being carried away by such prepossessions, it would less readily occur, that successful observations were only to be made, by an accurate and critical attention to those minute changes, which the spots sometimes undergo. But what would still more conduce to this oversight, was the method, which most of them followed, in making their observations. This was by the camera obscura, which both Scheiner and Hevelius often used, and which we find greatly extolled by them, and described at great length in their writings. But spots, when seen in this way, have nothing of that distinctness, which is so remarkable, and so pleasing, when they are viewed directly through a good telescope armed with an helioscope, or glats properly smoaked. PART II. It appears then that the solar spots are immense excavations in the body of the sun; and that what hitherto hath been called the nucleus is the bottom, and what hath been called the umbra the sloping sides of the excavation. It also appears, that the solar matter, at the depth of the nucleus, does not emit light, or emits so little, as to appear dark compared to that resplendent substance at the surface; that this beauteous substance is at the surface, most fulgid; and when any of it is, seen below the general level, forming the sides of an excavation, that then its lustre is somehow impaired, so as to give the appearance of a surrounding umbra. Here our induction ends. To proceed further would be to carry it beyond its true limits, and to intermix with conclusions, which are certain and manifest, the suggestion of hypotheses, which at best are precarious and liable to error. But from what we have now seen, many curious speculations do naturally present themselves. By what mysterious process is it, that those astonishing excavations are at first produced? What is the nature of that shining substance, which is thereby perpetually disturbed? To what are we to ascribe the darkness of the nucleus, and the diminished lustre of the umbra? And what conceptions are we to form of the many strange changes, and at length of the final decay of all these appearances, whereby those regions of the sun, that were so hurt and disfigured, again undergo a renovation? We We often find Scheiner and Hevelius mentioning many things concerning the spots, which appeared to them very inexplicable. Hevelius, when speaking of the vast number of spots which break out upon the sun, and of the prodigious size of some of them, admires how from his single body so much matter, exhalations, &c. could be generated, as in any degree to be adequate to so many and so vast phenomena. "Nuclei autem," (says he, Cometograph. p. 401) macularum "sicilicet partes densiores, saepius unam partem cen- "tesimam, imo quinquagesimam, de solari dia- "metro occupant; sic, ut paucis dicam, immania "et admiranda sunt corpora. Adhaec, intellectum "humanum fere superat, quomodo unquam, ex "unico isto corpore solis, tantum materiae, totque "exhalationes vaporesque erumpere ac produci "queant, quo talia vastissima phænomena procreari "possint." Again, Scheiner, when speaking of that property of the spots, where a large nucleus is often divided into two or more nuclei, seems greatly puzzled to account for it. Sch. Rosa Ur- fina, p. 498, says: "Ex uno saepe magno nucleo "fiunt duo, tres, pluresve, non locali partium ip- "sius distractione, sed intervallorum necio qua "exinanitione. Manent enim illorum centra in- "ter se æqualiter dissita." Many instances of this sort could be brought; and indeed, considering the contrariety betwixt any hypothesis they had formed, and what would now seem to be the real condition of the phænomena, it is no wonder that such difficulties should occur. Every theory, how ingenious however, which is founded upon a misapprehension of things, is apt to be pressed with many difficulties; and whenever palpable contradictions appear, they may be regarded certainly as proofs of our having fallen into error. Upon this principle, I think, we might freely have rejected any theories, hitherto entertained, concerning the solar spots, tho' their falsity had not become manifest from more enlarged experience and observation. It must indeed be acknowledged, that it is very disadvantageous to science, to indulge much in hypotheses, the truth being rarely hit upon in this way, and very often misled. Sometimes, however, it may not be improper, to throw out hints and conjectures, when we can attain to nothing better, provided we are at due pains to distinguish betwixt such and that real knowledge, which we derive, by strict induction, from incontestable principles. The best way therefore, of preserving so proper and necessary a distinction, will be to propose what further remains to be said, upon this subject, in the form of queries; because, however plausible they may appear, they are at best but matter of conjecture. Hints, when propounded in this way, are freed from the danger of making us rest in any error, whilst, sooner or later, they may become helps in leading us to a right understanding of the subject. The queries which we shall proceed to make, are chiefly founded upon the following phenomena of the spots, as described by Scheiner and Hevelius. 1. Every spot which hath a nucleus, hath also an umbra surrounding it. Vid. Scheiner, p. 496. Hev. p. 409. 349. 2. The boundary betwixt the nucleus and umbra is always distinct and well defined. Vid. Scheiner, p. 497. 3. The encrease of a spot is gradual, the breadth of the nucleus and umbra dilating at the same time. Vid. Scheiner, p. 491, &c. 4. In like manner the decrease of a spot is gradual, the breadth of the nucleus and umbra contracting at the same time. Sch. p. 491. 498. 5. The exterior boundary of the umbra never consists of sharp angles, but is always curvilinear, how irregular soever the out-line of the nucleus may be. Sch. 511. 6. The nucleus of a spot, whilst on the decrease, in many cases changes its figure, by the umbra encroaching irregularly upon it; insomuch that, in a small space of time, new encroachments are discernible, whereby the boundary, betwixt the nucleus and umbra, is perpetually varying. Sch. 514. Hev. 412. 7. It often happens, by these encroachments, that the nucleus of a spot is divided into two or more nuclei. Sch. 498. 8. The nuclei of spots vanish sooner than the umbræ. Many instances of this sort are to be seen in Hevelius' plates, and the same is affirmed by Mr. Derham in The Philosophical Transactions. 9. Small umbræ are frequently seen without nuclei. Sch. p. 497. 10. An umbra of any considerable size is seldom seen without a nucleus in the middle of it. Ibid. 11. When a spot, which consisted of a nucleus and umbra, is about to disappear, if it is not succeeded ceeded by a facula, or more fulgid appearance; the place, which it occupied, is soon after not distinguishable from any other part of the sun's surface. This is certain from the accounts of all observers. **Queries and Conjectures, tending to explain the above properties of the spots.** When we consider, that the solar spots, some of whose properties have just now been enumerated, are so many vast excavations in the luminous substance of the sun, and that, wherever such excavations are found, we always discern dark and obscure parts situated below; is it not reasonable to think, that the great and stupendous body of the sun is made up of two kinds of matter, very different in their qualities; that by far the greater part is solid and dark; and that this immense and dark globe is encompassed with a thin covering of that resplendent substance, from which the sun would seem to derive the whole of his vivifying heat and energy? And will not this hypothesis help to account for many phenomena of the spots in a satisfactory manner? For if a portion of this luminous covering were by any means displaced, so as to expose to our view a part of the internal dark globe, would not this give the appearance of a spot? In this case, would not that part of the dark globe, which is now laid bare, correspond to the nucleus, and the sloping sides of the luminous matter to the umbra? And is not this consonant to that property of a spot mentioned in the first article; article; for would it not hence follow, that every spot, having a nucleus, should also have an umbra surrounding that nucleus, a natural account being at the same time suggested, for the boundary betwixt the nucleus and umbra being always distinctly defined, as mentioned in the second article. Although we may never have a competent notion of the nature and qualities of this shining and resplendent substance, or of the means by which the excavations in it are formed; we however discover, in their production, the agency of some mighty, though unknown, cause, which is there often exerting itself. Although we manifestly behold its effects, yet the mode of its operations may perhaps remain unsearchable. But if we were here to venture a conjecture, might we not suppose, that the luminous matter is so disturbed, and the excavations in it occasioned, by the working of some sort of elastic vapour, which is generated within the dark globe? And might not this elastic principle, by its expansion, swell into such a volume, as to reach up to the surface of the luminous matter, which would thereby be separated and laid aside in all directions? And for as much as there is no regularity in the time of a spot's enlarging, compared to the time of its decreasing, some enlarging quickly, and decreasing slowly, and vice versa, may we not imagine, that this is owing to the duration and quantity of the elastic principle now mentioned? and in general, may we not from hence form some idea of the production and subsequent enlargement of a spot, as mentioned in the 3d article? But But to proceed. As we know, from experience, that the spots are of a transient nature, not lasting upon the sun for a long space of time, does it not seem reasonable to think, that their gradual decrease, as mentioned in article 4th, is occasioned by the luminous matter encroaching again upon that part of the dark globe, which had been uncovered? And from this may we not infer, that the luminous matter gravitates, and is in some degree fluid; for thereby would it not have a tendency to flow down, in all directions, and encroach, so as at last to cover the nucleus? And do not these things appear further probable, when we reflect upon that uniform inclination, which the sides of the umbra, or excavation, have to the external surface of the sun's body? For does not this indicate a fluid sort of matter gradually yielding to the force of gravity? And again, is not this notion further supported, when we consider the property mentioned in the 5th article, namely, that the exterior boundary of the umbra never consists of sharp angles or turnings, but is always curvilinear, and, most frequently, of a round form: for we know, that this boundary is nothing else but the lip of the excavation, which, on supposition that the luminous matter possesses some degree of fluidity, will not be disposed, either in enlarging or contracting, to become irregular by sudden or sharp turnings? Upon supposition that the surface of the dark globe of the sun is smooth and level, it may be urged, that the nucleus of a spot, whilst upon the decrease, should, according to the present view of things, always acquire a figure, at least nearly, circular, cular, and that the luminous matter, continuing to flow down on all sides by an equal gravity, should so encroach upon the nucleus, as to make it retain that figure, till at last it be entirely overflowed. But this not being the case, and because it most frequently happens, that the encroachments of the umbra upon the nucleus are extremely variable, as mentioned in the 6th article, may we not from this infer, that the surface of the internal dark globe of the sun, is by no means smooth and level, but on the contrary very irregular, for, upon this supposition, if for example the area of the nucleus of a great spot were to diversified by mountains and vallies, would not the encroachments of the luminous matter be consequently irregular; and, according as it was more or less retarded or accelerated, at different places, by being contiguous to prominencies or hollows, would not all the alterations in the figure of the decreasing nucleus, how variable soever, be thus plainly accounted for? and because it often happens, that the nucleus of a spot, whilst on the decrease, is gradually cut in pieces by a luminous zone or zones, which wander across it, as mentioned in the 7th article, does not this look like the gradual flowing in of the luminous matter, as it were, into deep channels, which would thus appear to abound in the surface of the sun’s dark body? If we reflect upon the irregularities, which are upon the surface of this earth, and upon the enormous mountains and cavities, which are in the moon, may we not, from such analogy, imagine, that there may be the like, or much greater, irregularities in the surface of the sun? sun? I may here take notice of a curious instance of a nucleus being divided into two parts, which fell under my own observation. On October 8th, 1770, the spot s, Tab. IV. had a small jutting in of the luminous matter, upon the opposite sides of the nucleus, which, by the 10th day, had advanced contrary ways, so as to meet, by which means the nucleus was now divided into two parts, as represented by fig. t. It was here very remarkable, that the parts of the umbra opposite to this cut in the nucleus, were dilated as in fig. t. May not this dilatation have been occasioned by the rapid flowing in of the luminous matter into the deep channel below? Is not the property mentioned in the 8th article, namely, that the nucleus of a spot vanishes sooner than the umbra, also agreeable to the present views? from this state of the phenomenon, we suppose that that part of the sun's dark body, which had been uncovered and exposed to our view, when the spot first broke out, is now again just overflowed by the gradual inundation of the luminous matter. But, after the nucleus thus disappears, may there not however, in many cases, be still left a cavity in the luminous matter, large enough to be perceived? and will not this cavity, so long as it continues, give the appearance of a small undivided umbra? and will not this umbra still be perceivable, till the luminous matter, by continuing to flow in, has filled up the cavity? after which, will not the place of the umbra acquire the same lustre with the rest of the sun's surface, and thus will not all traces of the spot vanish vanish from his body? And do not the particulars mentioned in the 9th, 10th and 11th articles seem agreeable to what is now said? Both Scheiner and Hevelius seem to think, that spots sometimes alter their place upon the disc, not only by the sun's rotation round his axis, but also by a motion, which they impute to the spots themselves. This I could never observe. It is very true, that when a number of small spots lie near one another, there may be from time to time a change of their relative situation, but it is plain, that this may proceed entirely from some of them increasing and others diminishing irregularly. But what would further contribute towards forming a judgment of this kind is, the apparent alteration of the relative place, which must arise from the motion across the disc on a spherical surface; a circumstance which I am uncertain if it has been sufficiently attended to. What has been advanced, in the course of the foregoing queries, may perhaps be rendered still more probable, by considering the observations related in the first part of this paper, concerning the changes which are made upon the figure of a spot, when another breaks out in its neighbourhood; and which seem to arise from a disturbing force. For, from the cases there laid down, would it not appear, that when a spot is breaking out, the luminous matter is then forced, in all directions, from the nucleus, and is affected much in the same manner, as it would be, were it a fluid matter encompassing the sun's dark body? As to the particular nature and qualities of this luminous matter, we have been sometimes apt to imagine, that it cannot well be any very ponderous fluid, but that it rather must resemble as to its consistence a very dense and thick fog, which broods on the surface of the sun's dark body. How far will this idea tend to facilitate our conceptions of the various phenomena of the spots above described? It has been gathered from many observations, that the time which the spots take to traverse the whole disc, is nearly equal to the time that they are hid by being on the opposite surface. It is plain, that the time of their appearing upon the disc must be some small matter shorter than that of their being hid behind it, on account of our not seeing a complete hemisphere of the sun. But further, it must now be considered, that when a spot just enters the disc, the part, which is first visible, is the farthest umbra, by which time the spot has really advanced a whole diameter of itself upon the disc. And again, when the same spot goes off the disc, it is evident, that the part, which is last visible, is then the farthest umbra, on which account the continuance of the spot upon the disc will be shortened by an interval of time, which corresponds nearly to the whole breadth of it. This, as well as the other appearances, described in the first part of this paper, concerning the change of the umbra and figure of the nucleus, when spots approach the limb, are all well illustrated, by making, in a sphere, an excavation similar to what we have described, the bottom of which may be painted black. black to represent the nucleus, and the floping sides shaded, if the sphere be of a light colour. According to the view of things given in the foregoing queries, there would seem to be something very extraordinary in the dark and unignited state of the great internal globe of the sun. Does not this seem to indicate, that the luminous matter, which encompasses it, derives not its splendor from any intensity of heat? For, if this were the case, would not the parts underneath, which would be perpetually in contact with that glowing matter, be heated to such a degree, as to become luminous and bright? At the same time it must be confessed, that although the internal globe was in reality much ignited, yet when any part of it, forming the nucleus of a spot, is exposed to our view, and is seen in competition with a substance of such amazing splendor, it is no wonder, that an inferior degree of light should, in these circumstances, be unperceivable. In order to obtain some knowledge of this point, I think an experiment might be tried, if we had an opportunity of a very large spot, by making a contrivance in the eye-piece of a telescope, whereby an observer could look at the nucleus alone with the naked eye, without being in danger of light coming from any other part of the sun. In this case, if the observer found no greater splendor, than what might be expected from a planet very near the sun, and illumined by as much of his surface as corresponds to the spots umbra, we might reasonably conclude, that the solar matter, at the depth of the nucleus, is in reality reality not ignited. But, from the nature of the thing, doth there seem any necessity for thinking, that there prevails there any such raging and fervent heat, as many have imagined? It is proper here, to attend to the distinction betwixt this shining matter of the sun, and the rays of light which proceed from it. It may perhaps be thought, that the re-action of the rays upon the matter, at their emission, may be productive of a violent degree of heat. But whoever would urge this argument, in favour of the sun being intensely heated, as arising from the nature of the thing, ought to consider, that all polished bodies are less and less disposed to be heated, by the action of the rays of light, in proportion as their surfaces are more polished, and as their powers of reflection are brought to a greater degree of perfection. And is there not a strong analogy betwixt the re-action of light upon matter, in cases where it is reflected, and in cases where it is emitted? It may perhaps be expected, that, in this paper, mention should be made of the other appearances, that are discernible upon the surface of the sun, besides the spots properly so called; I mean the faculae, luculi, &c. as described by Scheiner and Hevelius. But all these phenomena seem to be so different from any thing we have considered, and so unconnected with the present discovery, that little assistance can be brought from that quarter towards a right conception of them. As to the faculae, or brighter parts of the sun, we are at a loss for their origin. It may in general be remarked, that although we have obtained an experimental rimental proof, that the luminous matter acquires some degree of shade, when forming the sides of an excavation, yet it is uncertain, if this be merely the effect of position, and much more so, if any different modification of position could ever dispose it to put on a brighter or more fulgid appearance. Yet, after all, may not these faculae, &c. depend upon some irregularities in the bright surface of the sun? For may not the luminous matter, by being agitated by the same cause to which the spots owe their origin, though in a less degree, have its surface perpetually disturbed, and made irregular, and thus give occasion to a variety of light and shade, sufficient perhaps to produce the phenomena under consideration? And does not this conjecture receive further confirmation, when we consider, that these faculae, &c. are found only in that zodiac, within which the spots appear, and that they always abound most in the neighbourhood of the spots themselves, or where spots recently have been? For in those undisturbed regions of the sun which lie towards his poles, and where no spots ever appear, and which Scheiner calls the plagæ aquabiles, we never discover any diversity of appearance. Thus we have endeavoured to give a general idea of the production, changes, and decay of the solar spots, considered as excavations in the body of the sun; a thing which seems to be established from the observations described in the first part of this paper. But concerning the nature of that mighty agency, which occasions those amazing commotions in the luminous matter, or concerning ing the density, viscosity, and other qualities of this matter, or the manner in which it is disturbed in the middle zone only, and not at the polar regions, and many such other questions, I freely confess, that they far surpass my knowledge. To conclude; as what hath now been said may open a new field of inquiry into this subject, so a discussion of these curious points may, some time or other, fall to the share of abler men, whose love of philosophy may induce them to pursue so noble an investigation.