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 cur"rents, both ascending and descending. "In this perpetual agitation we can readily imagine that where the ascend

ing current becomes more intense the "luminous matter of the photosphere is "momentarily dissipated. Through this "kind of unveiling it is not the solid cold "and black nucleus of the sun that we "shall perceive, but the internal ambient, gaseous mass."

In this quotation we have the two most important points of M. Faye's theory; namely, that the spots are caused by an uprush, and that their dark appearance is due to feeble radiation from a gaseous surface.

M. Faye also considers that the faculæ, like the spots, are due to ascending currents, and he then attempts to account

means of vertical currents, we must "conclude that the ordinary laws of “rotation in a fluid mass in a state of equilibrium are strangely altered, since "this equilibrium is constantly disturbed " in a vertical direction. The ascending masses which spring from a great depth arrive at the top with a linear velocity of rotation less than that of "the surface, because the layers whence they are derived have a smaller radius. "Hence a general lagging in the move"ment of the photosphere."

These remarks of M. Faye will be found in the Comptes Rendus for 16th and 23d Jan. 1865. During the same month, a paper1 was read at the Royal Society, in which certain results derived from the photographs taken at Kew, and certain theories based therefrom, were discussed. We limit ourselves to the two most typical passages in this paper:

"Since the central or bottom part of a spot is much less luminous than the "sun's photosphere, it may perhaps be "concluded that the spot is of a lower 'temperature than the photosphere..."

"May not the falling behind of facula" (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?"

66

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).

nous because it is colder. Again, M. Faye holds that a spot is due to an uprush the Kew observers, that it is due to a downrush.

At the outset there were many arguments against M. Faye's hypothesis. The law of exchanges was utterly against his idea of the darkness of a spot, for if it were the interior of the sun which we saw, and its radiation were feeble, then its absorption would have been equally feeble and the sun would be spotless; for where the photosphere was torn away on the side nearest us, we should be able to see, through the sun, the lower surface of the photosphere on the opposite side.

Again, the arguments in favour of an uprush, in the case both of spots and faculæ, are not very clear, nor have we a satisfactory explanation of the falling behind of the faculæ. But we had not long to wait for facts which, as far as we can see, have entirely settled the question. First, as to the downrush into a spot. In 1865 two observersone in France, the other in Englandcarefully observed the fine spots from time to time visible on the sun's disc in that year; and the observations of both tend to show the absolute certainty that if spots are not caused by downrushes, they are, at all events, fed by them.

Let us hear the French observer first:1 "La rapidité des changements est telle,

The spot had a tongue of facula stretching half-way over it. When the observation commenced at 11:30 on April 2, this tongue of facula was extremely brilliant; by 1 o'clock it had become less brilliant than any portion of the penumbra: at the same time the faculous mass seemed to be giving out its end, veiling the umbra gradually with a kind of stratus cloud evolved out of it, which after a time again condensed into masses resembling the willow-leaves in the penumbra, only less distinct.

The argument for the downrush is to be found in the fact of the diminution of brightness; accepting as proved, first, that the faculæ are higher than the general surface, and, secondly, that a spot is a cavity. But it does not wholly depend upon this, for the masses or granulations on the general surface of the sun appear to lengthen out when they reach the penumbral region, as if they were acted upon by a current, and this may also explain the constantly observed difference in the shape of the

"Willow-leaves" detaching themselves from the penumbra. A very faint one at F.

cloud masses on the general surface and in the penumbra. In this connexion it is worthy of remark, that when a solitary willow-leaf is seen over the centre of a spot, it is often observed to be nearly circular, as if its longer axis were tipped down. It is fair to add, however, that observations of the requisite delicacy can be very rarely made, owing to the many coincident conditions necessary.

The fact that a spot is due to absorption has next to be considered. On M. Faye's theory, as it will doubtless have already suggested itself to the

reader, could a sun-spot be observed by means of a spectroscope; as, by hypothesis, we have radiation from a gas in a state of dissociation, the resulting spectrum would be a gaseous one- -that is, it would consist of bright lines. We, in fact, should get from a spot a spectrum absolutely different from that which belongs to the light emitted from the general surface, the latter being a band of rich colourgoing from red through yellow, green, blue, indigo, to the intensest lavender, crossed by innumerable black lines of different intensity, the former consisting only of three or four thin bands of light, located in the green portion of the spectrum.

On the absorption-hypothesis there would be none of these bright lines; we should get a spectrum in the particular region of the spot similar to the average solar one, but, showing evidence of greater absorption. was put to the test in 1866.1

This

The method adopted was to apply a direct-vision spectroscope to a 61-inch equatorial, so that it was possible to observe at one time the spectra of the umbra of a spot and of the adjoining photosphere or penumbra.

On turning the telescope and spectrum-apparatus, driven by clock-work, on to the sun, the solar spectrum was observed in the field of view of the spectroscope with its central portion (corresponding to the diameter of the umbra falling on the slit) greatly enfeebled in brilliancy.

All the absorption-bands visible in the spectrum of the photosphere, above and below, were visible in the spectrum of the spot; but they appeared thicker where they crossed the spot spectrum. There was not the slightest indication of any bright bands.

The dispersive power of the spectroscope employed was not sufficient to enable it to be determined whether the decreased brilliancy of the spot-spectrum was due in any measure to a greater number of bands of absorption.

1 "Spectroscopic Observations of the Sun," by J. Norman Lockyer (Proceedings of the Royal Society, vol. xv. p. 256).

The Royal Society at once recognised the importance of this discovery, although it was put forward with much hesitation, as the instrument employed was not of sufficient dispersive power, and the spot itself was not a very favourable one for the experiment. larger instrument has now been constructed, and detailed observations are now about to be commenced under the auspices of that body. In the meantime, however, this settlement of the long-debated question has recently been entirely endorsed by Mr. Huggins, whose discovery of the physical constitution of nebulæ, and spectroscopic observations of the fixed stars, make his opinion of the greatest possible weight.

We have thus, as briefly as possible, traced up our knowledge of the sun's surface from the times of Galileo to our own. That surface, we have learnt, is of a cloudy nature, the light and heat being derived from the solid incandescent particles of which the clouds are composed. Further, there are exchanges perpetually going on between the cooler exterior and the interior. The descending current is accompanied by a spot, the ascending one by a facula; and finally, the dark appearance of a spot, like the darkening of the limb, is due to the absorptive properties of the sun's atmosphere.

Let us, for one moment, compare the sun's envelope with our own, and observe the action of the latter when the sun is withdrawn.

The general surface of the ground is a good radiator. On the other hand, the atmosphere is at once a feeble absorbent and a feeble radiator. When the sun's influence is withdrawn from the earth's surface, and the sky is clear, the general surface of the ground and the leaves of plants give off their heat, which is radiated into space unimpeded by the very feeble absorbing power of the air; on the other hand, the air, being a feeble radiator, gives back little or nothing in return.

As far as radiation is concerned, there