If the reader has grasped the broad general outlines now given respecting the Sun and its spots he will perhaps be interested to learn a few further details, but these must be presented in a somewhat disjointed fashion, because the multitude of facts on record concerning sun-spots are so great as to render a methodical treatment of them extremely difficult within the limits here imposed on me. These matters have been gone into in a very exhaustive way by Secchi in his great treatise on the Sun, and in what follows I have made much use of his observations. Let us look a little further into the laws regulating the movement of the spots. If it is not a question of seeing a spot spring into view, but of watching one already in existence, we shall, in general, see such a spot appear on the Eastern limb of the Sun just after having turned the corner, so to speak. The spots traverse the Sun’s disc in lines which are apparently oblique with reference to the diurnal movement and the plane of the ecliptic, and after about 13 days they will disappear at the Western limb if they have not done so before by reason of physical changes in their condition. It is not uncommon for a spot after remaining invisible for 13 days on the other side of the Sun, so to speak, to reappear on the Eastern limb and make a second passage across the Sun; sometimes a third, and indeed sometimes even a fourth, passage may be observed, but more generally they change their form and vanish altogether either before passing off the visible disc, or whilst they are on the opposite side as viewed from the Earth. [Illustration: Fig. 6.—Change of Form in Sun-spots owing to the Sun’s rotation.]