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About four months before I had to revise this essay on “The Constitution of the Sun,” while staying near Pewsey, in Wiltshire, I was fortunate enough to witness a phenomenon which furnished, by analogy, a verification of the above hypothesis, and served more especially to elucidate one of the traits of solar spots, otherwise difficult to understand. It was at the close of August, when there had been a spell of very hot weather. A slight current of air from the West, moving along the line of the valley, had persisted through the day, which, up to 5 o’clock, had been cloudless, and, with the exception now to be named, remained cloudless. The exception was furnished by a strange-looking cloud almost directly overhead. Its central part was comparatively dense and structureless. Its peripheral part, or to speak strictly, the two-thirds of it which were nearest and most clearly visible, consisted of converging streaks of comparatively thin cloud. Possibly the third part on the remoter side was similarly constituted; but this I could not see. It did not occur to me at the time to think about its cause, though, had the question been raised, I should doubtless have concluded that as the sky still remained cloudless everywhere else, this precipitated mass of vapour must have resulted from a local eddy. In the space of perhaps half-an-hour, the gentle breeze had carried this cloud some miles to the East; and now its nature became obvious. That central part which, seen from underneath, seemed simply a dense, confused part, apparently no nearer than the rest, now, seen sideways, was obviously much lower than the rest and rudely funnel-shaped–nipple-shaped one might say; while the wide thin portion of cloud above it was disk-shaped: the converging streaks of cloud being now, in perspective, merged together. It thus became manifest that the cloud was produced by a feeble whirlwind, perhaps a quarter to half-a-mile in diameter. Further, the appearances made it clear that this feeble whirlwind was limited to the lower stratum of air: the stratum of air above it was not implicated in the cyclonic action. And then, lastly, there was the striking fact that the upper stratum, though not involved in the whirl, was, by its proximity to a region of diminished pressure, slightly rarified; and that its precipitated vapour was, by the draught set up towards the vortex below, drawn into converging streaks. Here, then, was an action analogous to that which, as above suggested, happens around a sun-spot, where the masses of illuminated vapour constituting the photosphere are drawn towards the vortex of the cyclone, and simultaneously elongated into striae: so forming the penumbra. At the same time there was furnished an answer to the chief objection to the cyclonic theory of solar spots. For if, as here seen, a cyclone in a lower stratum may fail to communicate a vortical motion to the stratum above it, we may comprehend how, in a solar cyclone, the photosphere commonly fails to give any indication of the revolving currents below, and is only occasionally so entangled in these currents as itself to display a vortical motion.

Let me add that apart from the elucidations furnished by the phenomenon above described, the probabilities are greatly in favour of the cyclonic origin of the solar spots. That some of them exhibit clear marks of vortical motion is undeniable; and if this is so, the question arises–What is the degree of likelihood that there are two causes for spots? Considering that they have so many characters in common, it is extremely improbable that their common characters are in some cases the concomitants of vortical motion and in other cases the concomitants of a different kind of action. Recognizing this great improbability, even in the absence of a reconciliation between the apparently conflicting traits, it is, I think, clear that when, in the way above shown, we are enabled to understand how it happens that the vortical motion, not ordinarily implicating the photosphere, may consequently be in most cases unapparent, the reasons for accepting the cyclonic theory become almost conclusive.]

FOOTNOTES:

[Footnote 25: If the “rice-grain” appearance is thus produced by the tops of the ascending currents (and M. Faye accepts this interpretation), then I think it excludes M. Faye’s hypothesis that the Sun is gaseous throughout. The comparative smallness of the light-giving spots and their comparative uniformity of size, show us that they have ascended through a stratum of but moderate depth (say 10,000 miles), and that this stratum has a definite lower limit. This favours the hypothesis of a molten shell.]

[Footnote 26: I should add that while M. Faye ascribes solar spots to clouds formed within cyclones, we differ concerning the nature of the cloud. I have argued that it is formed by rarefaction, and consequent refrigeration, of the metallic gases constituting the stratum in which the cyclone exists. He argues that it is formed within the mass of cooled hydrogen drawn from the chromosphere into the vortex of the cyclone. Speaking of the cyclones he says:–“Dans leur embouchure evasee ils entraineront l’hydrogene froid de la chromosphere, produisant partout sur leur trajet vertical un abaissement notable de temperature et une obscurite relative, due a l’opacite de l’hydrogene froid englouti.” (Revue Scientifique, 24 March 1883.) Considering the intense cold required to reduce hydrogen to the “critical point,” it is a strong supposition that the motion given to it by fluid friction on entering the vortex of the cyclone, can produce a rotation, rarefaction, and cooling, great enough to produce precipitation in a region so intensely heated.]