ocean, both Sun and Earth being half immersed in it. We shall see further on that this level-this plane-called the

FIG. 9.-Curve showing the period of rotation of the photosphere in different latitudes north and south from Carrington's observations; 851' of solar longitude per diem = rate of rotation in lat. 15° N. The vertical lines represent differences of 10' of longitude + to the right, to the left, of the line cutting the curve in lat. 15° N.

plane of the Ecliptic-is used by Astronomers in precisely

the same way as we commonly use the sea level. We say, for instance, that such a mountain is so high above the level of the sea. Astronomers say that such a star is so high above the plane of the ecliptic.

106. Well then, we have imagined the Earth and Sun to be floating in an ocean up to the middle-which is the

meaning of half immersed. Now, if the Sun were quite upright, the spots would always seem at the same distance above the level of our ocean. But this we have not found to be the case. From two opposite points of the Earth's path (the points it occupies in June and December) the spots are seen to describe straight lines across the disc, while midway between these points (September and March) their paths are observed to be sharply curved, in one case

with the convex side downwards, in the other with the convex side upwards. A moment's thought will show that these appearances can only arise from a dipping down of the Sun's axis of rotation. Now this we find to be the case. The Sun's axis inclines towards the point occupied by the Earth in September. When we come to deal with the Earth and the other planets, we shall find that their axes also incline in different directions.

107. It has been found that the spots, besides having an apparent motion, caused by their being carried round by the Sun in its rotation, have a motion of their own. This proper motion, as distinguished from their apparent motion, has recently been investigated in the most complete manner by Mr. Carrington. What he has discovered shows that there need be no wonder that different observers have varied so greatly in the time they have assigned to the Sun's rotation. As we have already shown (Art. 104), this rotation has been deduced from the time taken by the spots to cross the disc; but it now seems that all sun-spots have a movement of their own, and that the rapidity of this movement varies regularly with their distance from the solar equator,- that is, the region half-way between the two poles of rotation. In fact, the spots near the equator travel faster than those away from it (Fig. 9), so that if we take an equatorial spot we shall say that the Sun rotates in about twenty-five days; and if we take one situated half-way between the equator and the poles, in either hemisphere, we shall say that it rotates in twenty-seven and a half days.

108. We have now considered the distance and size of the Sun; we have found that it, like our Earth, rotates on its axis, and we have determined the direction in which the axis points. We must next try to learn something of its appearance and of its nature, or, as it is called, its physical constitution. Here we confess at once that our knowledge on this subject is not yet complete. This, however, is little to be wondered at. We have done so

much, and gleaned so many facts, at distances the very statement of which is almost meaningless to us, so stupendous are they, that we forget that our mighty Sun, in spite of its brilliant shining and fostering heat, is still some 93,000,000 miles removed ;-that its diameter is 100 times that of our Earth; and that the chasm we call a sun-spot is yet large enough to swallow us up, and half a dozen of our sister planets besides; while, if we employ the finest telescope, we can only observe the various phenomena as we should do with the naked eye at a distance of 180,000 miles.

To look at the Sun through a telescope, without proper appliances, is a very dangerous affair. Several astronomers have lost their eyesight by so doing, and our readers should not use even the smallest telescope without proper guidance.

Lesson VIII.-Telescopic Appearance of the Sun-spots. Penumbra, Umbra, Nucleus, Faculæ, Granules. Red Flames.

109. We have already said that the first things which strike us on the Sun's surface, when we look at it with a powerful telescope, are the spots. In Plate IV. we give drawings of a very fine one, visible on the Sun in 1865. We shall often refer to them in the following description. The spots are not scattered all over the Sun's disc, but are generally limited to those parts of it a little above and below the Sun's equator, which is represented by the middle lines in Fig. 8. The arrows show the direction in which the spots, carried round by the Sun's rotation, appear to travel across the disc.

110. The spots float, as it were, in what, as we have already seen in the case of the stars, is called the photosphere; the half-shade shown in the spot is called the penumbra (that is, half shade); inside the penumbra is a still darker shade, called the umbra, and inside this

E

again is the nucleus. Diagrams 3 and 4 of Plate IV. will render this perfectly clear. The white surface represents the photosphere; the half tones the penumbra; the dark, irregular central portions the umbra; and the blackest parts in the centre of these dark portions, the nucleus.

111. Sun-spots are cavities, or hollows, eaten into the photosphere, and these different shades represent different depths. Their occasional appearance as notches on the Sun's limb, or edge (as shown in Fig. 4), renders this absolutely certain.

112. Diligent observation of the umbra and penumbra, with powerful instruments, reveals to us the fact that change is going on incessantly in the region of the spots. Sometimes changes are noticed, after the lapse of an hour even: here a portion of the penumbra is seen setting sail across the umbra; here a portion of the umbra is melting from sight; here, again, an evident change of position and direction in masses which retain their form. The enormous changes, extending ever tens of thousands of square miles of the Sun's surface, which took place in the great sun-spot of 1865, are shown in Plate IV.

113. Near the edge of the solar disc, and especially about spots approaching the edge, it is quite easy, even with a small telescope, to discern certain very bright streaks of diversified form, quite distinct in outline, and either entirely separate or uniting in various ways into ridges and network. These appearances, which have been termed faculæ, are the most brilliant parts of the Sun. Where, near the edge, the spots become invisible, undulated shining ridges still indicate their place-being more remarkable thereabout than elsewhere, though everywhere traceable in good observing weather. Faculæ may be of all magnitudes, from hardly visible, softly-gleaming, narrow tracts 1,000 miles long, to continuous complicated and heapy ridges 40,000 miles and more in length, and 1,000 to 4,000 miles broad. Ridges of this kind