and their surroundings, a perplexing series of problems is suggested. These problems are indeed so perplexing as abundantly to justify the disagreement hitherto found among theorisers. Admitting the spots to be depressions, what is the real disposition of the matter which produces the appearances we call facula-penumbra-umbra-nucleus ? The penumbra may belong to a lower layer; but the general arrangement according to which the willow-leaves on the penumbra point inwards towards the umbra seems to indicate a real connection between the penumbra and the faculous bordering. This arrangement is indeed sometimes so marked that one is led to imagine that the so-called willow-leaves are filamentous bodies which usually hang in a nearly vertical position and so appear nearly round, but when thrust aside during the formation of a spot hang nearly horizontally, the ends which had been lowest floating like streamers towards the region whence they had been removed. If we could but conceal from ourselves a large portion of the evidence we have (or else explain it away) this view might be insisted upon with pleasing confidence; but as a matter of fact it merely serves to indicate the impression produced by certain phenomena, and has at present no value whatever.*

A great difficulty lies in the fact that we have no

* Lest I should here be supposed to be too curtly criticising the views of others, let me hasten to say that the fancy thus summarily rejected is my own, and, so far as I know, as original as it is probably valueless. Yet I have not introduced it without a purpose. There is at least as much evidence in its favour as in favour of many theories which have been very confidently put forward.

clear evidence to show whether the Sun-spots are formed by forces acting from without or from within. Here I set on one side the theory that in a spot we see a region where a great heat has dissolved solid or liquid or cloudlike matter forming the photosphere, and that thus the intensely hot, but feebly radiating gaseous nucleus of the Sun (according to this theory) is disclosed. Kirchhoff has fairly disposed of this theory by showing that this intensely hot nucleus would be transparent to the light from the farther side of the Sun, and that therefore no spot could appear unless two openings on opposite sides of the Sun happened to be in the same visual direction.* refer now, not to this or similar theories, but to the definite problem, whether the seat of that action which leads to the formation of a spot lies below or above the level of the photosphere. The spectroscope shows that a spot is a region where certain gases exist at a lower temperature than in other parts of the Sun. But whether this low temperature results from the expansion of compressed gas erupted from the Sun, or from the fact that matter has reached the Sun from outer space, remains as yet altogether unknown.

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* Fr. Secchi was the original propounder of this theory (not M. Faye, to whom it is usually ascribed). The theory really does account for many observed features of the solar spots, but it is none the less untenable. Fr. Secchi's answer to Kirchhoff's objection would seem to indicate that he has not recognised the exact force of that objection. He says it is not true that a gaseous nucleus would be perfectly transparent to rays from the further side of the Sun, for we see that our own atmosphere absorbs light as well as heat. Kirchhoff's argument is that the solar nucleus would be transparent on account of its existing at a higher temperature than the photosphere-according to the theory at ast which he deals with.

Again, as to the prominences, it seems to be demonstrated that they are due to some form of eruption and only assume the cloud form after the eruption which gave them birth has ceased. But what are the circumstances which give birth to these eruptions, what the nature of the layer (Zöllner's Trennungschicht') beneath which the eruptive action is prepared, and what the actual depth whence the erupted matter springs, we have very little to show.

And lastly, as to the corona and the general relations involved in the access of external matter from the interplanetary and intersidereal spaces to the neighbourhood of the Sun's globe, we have, I apprehend, small means of forming an opinion. The condition, indeed, of the space which lies immediately around the Sun is very little understood by us. It may be that in the study of the corona during total eclipses we may find a means of answering the many perplexing questions associated with this matter. It may even be that new appliances may enable us to study the corona when the Sun is not eclipsed, and so to learn whether systematic processes affecting the Sun's economy are at work in the region immediately surrounding him. At present our information on this subject is meagre in the extreme; and our means for acquiring information are far from promising. Here, as in so many matters related to the physical constitution of the Sun, we must perforce wait until our experimental knowledge and our instrumental means have been very largely increased.

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CHAPTER VIII.

THE SUN OUR FIRE, LIGHT, AND LIFE.

FEW of the results of modern scientific research are more remarkable than the recognition of the real extent of the influence which the Sun exerts upon the Earth. Of old the Sun's power as ruler over the seasons, his action upon vegetation, and other like influences, were recognised in a vague and general way. But men were far from regarding the Sun as the true source of many forms of force which seem almost equally important. Still less were they prepared to trace his influence in nearly every kind of action or mode of motion taking place upon our globe. It is the most striking feature of recent scientific research that it has taught us to see in nearly all terrestrial phenomena the action of a certain proportion of Sunforce.

We owe to the greatest astronomer of our timeSir John Herschel-the first definite enunciation of this great principle. The Sun's rays,' he wrote in 1833, are the ultimate source of almost every motion which takes place on the surface of the Earth. By its heat are produced all winds, and those disturbances in

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the electric equilibrium of the atmosphere which give rise to the phenomena of lightning, and probably also to terrestrial action and the aurora. By their vivifying action vegetables are enabled to draw support from inorganic matter, and become in their turn the support of animals and man, and the source of those great deposits of dynamical efficiency which are laid up for human use in our coal strata. By them the waters of the sea are made to circulate in vapour through the air, and irrigate the land, producing springs and rivers. By them are produced all disturbances of the chemical equilibrium of the elements of nature, which by a series of compositions and decompositions give rise to new products, and originate a transfer of materials. Even the slow degradation of the solid constituents of the surface, in which its chief geological change consists, is almost entirely due-on the one hand to the abrasion of wind or rain and the alternation of heat and frost, on the other to the continual beating of sea-waves agitated by winds, the results of solar radiation. Tidal action (itself partly due to the Sun's agency) exercises here a comparatively slight influence. The effect of oceanic currents (mainly originating in that influence), though slight in abrasion, is powerful in diffusing and transporting the matter abraded; and when we consider the immense transfer of matter so produced, the increase of pressure over large spaces in the bed of the ocean, and diminution over corresponding portions of the land, we are not at a loss to perceive how the elastic force of subterranean fires, thus