the sun, and Dr. Wilson, of Glasgow, observed it very CHAP. VI. carefully, and demonstrated subsequently that the spot was a cavity-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.

From the description of the telescopic appearance of the sun given in Chapter II., it is obvious 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, but 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 on which to move the great unknown by the lever of inquiry, and bring it into the light.

1 Phil. Trans.," 1774. See ante, page 10.

CHAP. VI.

Their beautiful discovery not only enables 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 liquid1 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 Motion of spots demands attention, however, beforehand in order spots. 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 we now know is not all the truth. In addition to this apparent motion they have a real 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 Sun-spot 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 mini

beriod.

1 Or densely gaseous, according to Frankland's researches on hydrogen, and later work. (Note added 1873.)

mum periods being about eleven years. The lamented CHAP. VI. 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.

can exist in the interior of the sun.

M. Faye regards the interior of the sun as consisting of the original nebula, from which our whole system has been binations slowly condensed, in a state of dissociation; that is, at such an intense heat that chemical combinations are impossible; 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,1 where, in a gaseous mixture of the most varied elements, the operations of these forces will give rise to precipitations (Herschel), clouds (Wilson), and non-gaseous 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

I shall show in the sequel, from my own researches, that this is most probably not the case. (1873)

CHAP. VI. and for the incessant change which is observed, M. Faye goes on as we translate him:

Faye states that the

spots are uprushes;

The facula

also.

"The formation of the photosphere will now enable us to account for the spots and their movements. We have 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 ascending 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 for the proper motion of the spots by the ascending currents:-"From the continual changes going on between the lower beds of the surface by 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 movement of the photosphere."

These remarks of M. Faye will be found in the Comptes Rendus for 16th and 23d Jan. 1865.1 During the same month, a paper 2 was read at the Royal Society, in which certain results derived from the photographs taken at

1 See also Chapters IV. and V.

24.

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

Kew, and certain theories based therefrom, were discussed. CHAP. VI. 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 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 characterizes a spot?" We see at once that on these points there is a perfectly Difference 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 luminous because it is colder. Again, M. Faye holds that a spot is due to an uprush: the Kew Observers, that it is cue 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 * See note on p. 57, and Comptes Rendus, vol. Ixiii. p. 234, 1866.

between

M. Faye's theory, and that of the

Kew Observers.