I.

Ir is not necessary for our present purpose either that we should re-open the discussion as to the real discoverer of the solar spots, or that we should attempt to realize the strange and overwhelming mixture of wonder and awe, not to say delight and terror, with which the announcement must have been received. Man with a wondrous "opticktube" had at last dared to peer into the secrets of the sun, and had, all unconsciously, by so doing dealt a deathblow at the fundamental Aristotelian doctrine of the immutability and incorruptibility of the heavens. The secret had been surprised; the sun was no longer the exemplar of spotless purity.

It is not astonishing, therefore, that whether we regard Galileo or Fabricius or Scheiner as the real discoverer, the secret was kept for many months before either of them gave it to the world; or that the latter, a Jesuit, was only permitted by his ecclesiastical superiorwho, so runs the record, remarked to him that he had read Aristotle's writings from end to end many times without finding any mention of solar spots-to publish his discovery at last under a nom de plume. These facts tell as strongly as anything can do of the mixed emotions of those pioneers in the field of solar research. The secret divulged, however, the schoolman. was soon merged in the investigator, and the problem was attacked with a closeness and ardour which are almost models for modern observers. Witness Galileo's first letter to Welser, the chief magistrate of Augsburg, dated May 4, 1612, and Scheiner's last to the same personage, dated July 25, 1612, under the signature of Apelles latens post tabulam.

It would seem, indeed, that everything which could be reaped by the in

struments at their command was immediately garnered. The motion of the spots across the sun's disc from east to west; the period in which they performed a complete circuit; their changes from day to day; the fact that they appeared for the most part in two zones, one north, the other south of the equator,—are samples of the secrets which the sun was at once compelled to yield up. What the spots were not was a question amply and closely discussed both by Galileo and Scheiner; but as to what they were, agreement was more difficult : Galileo at one time declared for clouds in an invisible atmosphere of the sun, Scheiner for a density and opacity equal to that of the moon-in fact for planets separated from the sun's surface, and revolving round him like Mercury and

Venus.

From the time of Galileo to 1769, or during nearly a century and a half, our knowledge was not increased by any new fact of importance, although in 1630 Scheiner managed to write a book of 784 pages1 on the work which had been done in the two decades which had

then elapsed since the discovery. It is

true that Delambre has declared that there is not matter in this ponderous folio for fifty pages, but we hold that Delambre's dictum is harsh to a degree, and that when he made it he had

entirely left out of sight the con

ditions under which the book had been written.

In the year 1769 there was a very large spot visible upon the sun, and Dr. Wilson, of Glasgow, observed it very carefully, and demonstrated subsequently 2 that the spot was a cavity

1 "Rosa Ursina, sive Sol ex admirando facularum et macularum suarum," &c. 2 "Phil. Trans.," 1774.

a conclusion which, although combated by Lalande at once, and by others in quite recent times, maintained and still maintains its ground. He also showed that the surface of the sun was probably of a cloudy nature.

Wilson, the author of the important observation to which we have just referred, was also the first to put forward an elaborate theory of the origin and nature of sun-spots which much influenced the subsequent work till quite recently. These theories, subsequently taken up by Bode and Sir William Herschel, possess, however, but an historical interest, and it is no part of our present purpose to enlarge upon them. It must suffice to say that they were based on the assumption that the sun itself was a habitable, cool, glade-bedeckt globe beneath the luminous atmosphere, and that the appearance of a sun-spot was due to a gaseous eruption breaking through the cloudy envelopes of the solid globe while La Hire held that they were purely surface-phenomena, and Lalande, that they were actual elevations.

:

Before we allude to the more recent discoveries in solar physics, it will be well to describe as briefly as possible the actual general appearance presented by the sun in a powerful telescope; always remembering that our mighty luminary is some 91,000,000 miles removed, that its diameter is 100 times that of our earth, and that the chasms we call sun-spots are sometimes large enough to swallow us up, and half a dozen of our sister planets besides; while if we employ the finest telescope, under the most favourable atmospheric conditions, we are only enabled to observe the various phenomena as we should do with the naked eye at a distance of 180,000 miles.

We may begin by saying that the whole brilliant surface of the sun, called the photosphere, except those portions occupied by the spots, is coarsely mot tled; the surface is in fact principally made up of luminous masses-described by Sir William Herschel as corrugations

-and small points of unequal light, imperfectly separated from each other by rows of minute dark dots, called pores, the intervals between them being extremely small, and occupied by a substance decidedly less luminous than the general surface. Mr. Nasmyth has stated that these luminous objects are of an exceedingly definite shape and general uniformity of size (at least as seen in projection, in the central portions of the disc), something like the oblong leaves of a willow-tree. According to other observers, however, these luminous masses present almost every variety of irregular form: they have been stated to resemble "rice grains," "granules or granulations," "untidy circular masses," things "twice as long as broad," "three times as long as broad," and so on. With regard to the general surface of the sun, therefore, it is not so easy to reconcile the conflicting opinions to which we have alluded.

The word "willow-leaf," however, very well paints the appearance of the minute details sometimes observed in spots details made up of elongated masses of unequal brightness, so arranged that for the most part they point like so many arrows to the centre of the spot. At other times, and even in the same spot, the jagged edge has caused the interior edge of the penumbra to be likened to coarse thatching with straw, the edges of which have been left untrimmed. But other appearances are assumed, depending upon the amount and kind of action going on in the spot at the time.

Further, we know that on the bright surface of the sun rests an absorptive atmosphere, for the luminosity is remarkably less bright near the borders.

The first things which strike us-the most salient phenomena-on the sun's surface are the spots, which always make their appearance on the same side of the sun, travel across it in about fourteen days, and then disappear on the other side. This is not all: if observed in June, they describe a straight line across the sun's face or disc with a dip downwards; if in September, they

cross in a curve; in December they go straight across again, with a dip upwards; and in February their paths are again curved, this time with the curve in the opposite direction. These were the phenomena which proved to Galileo that the apparent motion is due to the sun's actual rotation, and from their motions the position of the sun's axis has been determined with the greatest

accuracy.

Spots generally exhibit three shades of darkness, and float as it were in the general bright surface or photosphere, the darkness increasing from the general surface till the apparent centre of the spot is reached; we have first the penumbra, then the umbra, then the nucleus. But sometimes the darker portions are excentric, and very irregular in outline.

We next come to the brighter portions of the general surface, which are well visible near the edge of the solar disc, and especially about spots approaching the edge. They are bright streaks of diversified form, quite distinct in outline, and either entirely separate or uniting in various ways into ridges and network. These appearances, which have been termed faculæ, are the most brilliant parts of the sun. Where, near the limb, the spots become invisible because they are hollows, these undulated shining ridges still indicate their place-being more remarkable thereabout than elsewhere on the limb, though everywhere traceable in good observing weather. Faculæ appear of all magnitudes, and range from barely discernible, softly-gleaming narrow tracts, 1,000 miles long, to continuous, complicated, and heapy ridges, 40,000 miles and more in length, and 1,000 to 4,000 miles broad.

Let us next inquire into the nature of this brilliantly shining envelope. As first imagined by Wilson and afterwards asserted by Herschel, it is doubtless of a cloudy structure. It is impossible to observe the sun, near a spot, under good atmospheric conditions, without being quite convinced of this:

in fact diligent observation of the umbra and penumbra reveals the fact that change is going on incessantly in the region of the spots. Sometimes, after the lapse of an hour even, changes are noticed here a portion of the penumbra is seen setting sail across the umbra; there a portion of the umbra is melting from sight; here, again, is an evident change of position and direction in masses which retain their form. The two following woodcuts show the changes which took place, in two days, in the great sun-spots of October 1865.

The more minute features-the granules -are most probably the dome-like tops of smaller cloud masses, bright for the same reason that the faculæ are bright, but to a less degree; the fact also that the granules lengthen out as they approach the umbra of a spot is similar to the effect observed in the clouds in our own sky lengthened out when they are drawn into a current.

It is seen, therefore, that the surface of the sun is uneven, and that change of form is perpetually going on: these are conditions impossible in either a liquid or solid surface, such as land or ocean, possible in a surface of cloud or gas.

The cloud-like nature of the sun's surface follows, moreover, from the nature of the sun's light. This increase of our knowledge we owe to those immortal discoverers Kirchhoff and Bunsen, whose wonderful generalization of the results of spectrum analysis has given the present century a new fulcrum wherewith to move the great unknown by the lever of inquiry, and bring it into the light.

Their beautiful discovery has happily been described so often and so clearly that the readers of Macmillan do not require a detailed notice of it here. Suffice it to remark that not only does it enable us to define the sun as the nearest star and to detect some ten terrestrial elements as existing in a state of vapour in its surrounding, absorbing, and therefore cooler, atmosphere; but it enables us to state, as a proved fact, that the light of the sun proceeds from solid or liquid particles in a state of intense incandescence or glowing heat.

We shall shortly have occasion to refer again to this method of research: the more recent work regarding the spots demands attention, however, beforehand in order that we may follow as much as possible the order of time. It has already been stated that the early observers detected that the apparent motion of the spots was due to the real motion of rotation of the sun. But this account of their motion we now know is not all the truth. In addition

to this motion they have a motion of their own of such a nature that the nearer a spot is to the sun's equator the faster it travels; in fact the rate of this proper motion depends upon the latitude of the spot. This was one of the chief results deduced by Mr. Carrington from an elaborate daily investigation of the sun extending over six years, a stupendous work unsurpassed in the acumen and patience brought to the task, and rarely equalled in the results achieved.

This discovery of the proper motion of the spots at once explained the strange discrepancies in the time of the sun's rotation as given by different observers-discrepancies so great that Delambre declared it was useless to continue observations.

Mr. Carrington's work did not stand alone about this time. The great Schwabe had previously determined that if the spotted area were taken at any one time, its amount varied from year to year, that is, that the spots themselves were periodical; having periods of maximum and periods of minimum, the interval between two maximum or minimum periods being about eleven years. The lamented Dawes and Father Secchi largely increased our knowledge of the solar surface, the latter determining specially that there was less heat radiated from a spot than from the general surface.

Some time after Mr. Carrington's book appeared, M. Faye took up the question of solar physics with his usual elaborate treatment, and communicated to the Paris Academy of Sciences two papers of great value, in which, inter alia, he broached a new theory to account for the observed phenomena, and especially to explain the dark appearances presented by the spots.

M. Faye regards the interior of the sun as consisting of the original nebula, from which our whole system has been slowly condensed, in a state of dissociation; that is, at such an intense heat that chemical combinations are impos

1 "Observations on Solar Spots." By R. C. Carrington.

sible; and he looks upon the photosphere as the surface at which this heat is so acted upon by the cold of space as to allow chemical combinations and solid and liquid particles to exist. He goes on to remark that, if the molecular and atomic forces of cohesion and affinity cease to act in the interior of the mass, they come into play on the surface, where, in a gaseous mixture of the most varied elements, the operations of these forces will give rise to precipitations (Herschel), clouds (Wilson), and nongaseous particles capable of incandescence, of which our brilliant terrestrial flames offer so many examples. These particles, obeying the force of gravity, will, in falling, regain the temperature of dissociation, and will be replaced in the superficial layer by ascending gaseous masses, which will act in the same manner. The general equilibrium, therefore, will be disturbed in the vertical direction only by an unceasing exchange going on between the interior and the exterior.

Having in this manner accounted for the photosphere and for the incessant change which is observed, M. Faye goes on as we translate him :

"The formation of the photosphere "will now enable us to account for the (( spots and their movements. We have 66 seen that the successive layers are "constantly traversed by vertical currents, both ascending and descending. "In this perpetual agitation we can "readily imagine that where the ascend

"May not the falling behind of faculæ" (ample evidence of which is given in the paper)" be the physical reaction of "the proper motion of spots observed "by Carrington? so that while the "current passing upwards falls behind, carrying the luminous matter with it, the current coming down moves forward, carrying the spot with it; and may not this current coming from a "colder region account for the deficient "luminosity which characterises a spot?"

We see at once that on these points there is a perfectly clear issue between the two theories. M. Faye holds the spot to radiate feebly because it is hotter-in fact because it unfolds to us the interior of the sun in a state of dissociation. The Kew observers hold that it is less lumi

1 "Researches on Solar Physics." By Warren De la Rue, Balfour Stewart, and B. Loewy (Proc. Royal Society, vol. xv. p. 37).