Faye and some other astronomers are disposed to support a theory according to which the spots are nothing else than aërial cyclones, but this does not seem admissible. If the fundamental principle of a spot is that it arises from a whirling movement, the rays (so to speak) which compose the penumbræ must always be crooked, or the theory falls to the ground. It is quite true that indications of cyclonic action do sometimes appear, but they are at any rate very rare, for only a small percentage exhibit in a distinct manner a spiral structure. Moreover, when such a structure is seen it does not endure for the whole lifetime of the spot but only for a day or two: the spot may last a long time after it has lost its spiral features, if it ever had any. Sometimes even the whirling movement, after having slackened, begins again, but in the contrary direction. Under these circumstances, though this occasional spiral structure is very curious and interesting, we are not justified in taking it as the basis of a theory which has any pretensions to explain the general nature of sun-spots. [Illustration: Fig. 7.—Sun-spot seen as a Notch.] When we examine the Sun with instruments of large aperture and high magnifying power, we notice that its surface is far from being as smooth and uniform as it appears in a small telescope. On the contrary, it presents an irregular undulating appearance like a pond or other sheet of water agitated by the wind. Careful scrutiny with a powerful eye-piece reveals the fact that the Sun’s surface is marked by a multitude of wrinkles and irregularities which it is well-nigh impossible to describe in words. More or less everywhere there is a general mottling visible; it is more distinct in some places than others, and especially so towards the centre of the disc. This peculiar appearance varies very much from time to time, and its distinctness seems to depend a great deal on the state of the Earth’s atmosphere, for it becomes invisible when the air is disturbed; but these variations depend also on real variations of the photosphere—a fact which observations made in very calm weather are thought clearly to indicate.