247. During the total eclipse observed in America on July 29, 1878, equatorial streamers were seen proceeding from the Sun to a distance of fully 10,000,000 miles, and

something of the same extraordinary appearance had also been noticed in 1867. Now, it is remarkable that both these years were epochs of sun-spot minimum, so that the shape of the corona has been assumed to be in some way

connected with the phases of the Sun's activity. The view has, however, lately been put forward that these apparent coronal extensions may really indicate a permanent solar appendage in a ring of cooled material revolving partly within the limits of the solar atmosphere. The fall of matter from this ring upon the Sun's surface, resulting from inevitable disturbances, would produce spots and prominences, and account for many of the best-marked features of the sun-spot period. It would also explain the acceleration of spots near the equator.

247a. The spectroscope tells us that the corona is in part made up of two kinds of gas-hydrogen, and an unknown gas emitting green light exclusively; in part of solid or liquid particles reflecting sunlight.

248. When totality has commenced, apparently close to the edge of the Moon, and therefore within the corona, are observed fantastically-shaped masses, full lake-red, fading into rose-pink, variously called red flames and red prominences. Two of the most remarkable of these hitherto noticed were observed in the eclipse of 1851. They have thus been described by the Rev. W. R. Dawes :—

"A bluntly triangular pink body [was seen] suspended, as it were, in the corona. This was separated from the Moon's edge when first seen, and the separation increased as the Moon advanced. It had the appearance of a large conical protuberance, whose base was hidden by some intervening soft and ill-defined substance. . . To the north of this appeared the most wonderful phenomenon of the whole a red protuberance, of vivid brightness and very deep tint, arose to a height of perhaps 1' when first seen, and increased in length to 2′ or more, as the Moon's progress revealed it more completely. In shape it somewhat resembled a Turkish cimeter, the northern edge being convex, and the southern concave. Towards the apex it bent suddenly to the south, or upwards, as seen in the telescope. To my great astonishment, this marvellous object continued visible for about five seconds,

as nearly as I could judge, after the Sun began to reappear."

249. It has been definitely established that these prominences belong to the Sun, as those at first visible on the eastern side are gradually obscured by the Moon, while those on the western are becoming more visible, owing to the Moon's motion from west to east over the Sun. The height of some of them above the Sun's surface is upwards of 90,000 miles, and occasional uprushes have been watched, to elevations of more than 200,000 miles.

250. These red prominences are composed of incandescent hydrogen gas and metallic vapours lying outside the photosphere; and their spectroscopic examination indicates that the pressure in the high prominences is small. It indicates also that the matter composing them is in exceedingly rapid motion, upward movements up to 250 miles a second, having been detected by this means. (See Art. 504b.)

LESSON XIX.-The other Planets compared with the Earth. Physical Description of Mars.

251. We are now in a position to examine the other planets of our system, and to bring the facts taught us by our own to bear upon them. In the case of all the planets we have been able to ascertain the facts necessary to determine the elements of their revolution round the Sun; that is to say, the time in which the complete circuit round the common luminary is accomplished, and the shape and position of their orbits with regard to our own. Now, the shape of the orbit depends upon the degree of its ellipticity —for all are elliptical—and its position upon the distance of the planet from the Sun, and the degree in which the plane of each orbit departs from that of our own. When we have, in addition to these particulars, the position of the

nodes the points in which the orbit intersects the plane of our orbit-and the position of the perihelion points, we have all the materials necessary for calculation or for making a diagram of the planet's path.

252. Still, however satisfactory our examination of the planets has been with regard to their revolution round the Sun, we are compelled to state that when we wish to inquire into their rotations on their axes, the length of their days, their seasons, and their physical constitutions, the knowledge as yet acquired by means of the telescope is far from complete. Thus, of the planets Mercury and Venus we have nothing quite certain to tell on these matters; they are both so lost in the Sun's rays, and so refulgent in consequence of their nearness to that body, that our observation of them, of Mercury especially, has been to a great extent baffled. Nevertheless, the observations made in the beginning of this century by a German astronomer named Schröter have been so far confirmed in recent years, that we may pretty safely admit a period of about 25 hours for Mercury's rotation on his axis. That is, his day is an hour or so longer than ours. And the period assigned to Venus of 23h. 21m. is probably still more nearly exact. This has been confirmed by the measurement of photographs taken during the transit of Venus in 1882, which show that the figure of the planet is nearly identical with that of the Earth. It is bulged just to the same extent (see Art. 196); consequently it rotates at just about the same rate.

252a. The same class of facts in the case of Uranus and Neptune are equally hard to get at, but for a different reason. At our nearest approach to Uranus we are nearly 1,700,000,000 miles away from that planet; at our nearest approach to Neptune we are about 2,700,000,000 miles away, and we cannot be surprised that our telescopes almost fail us in delicate observations at such distances. Still, on the small disk of Uranus some delicate cloudmarkings have lately been detected, evidently connected

with his rotation; and the reappearances of a bright spot seem to give for that rotation a period of about 10 hours.

253. With regard however to Mars, Jupiter, and Saturn, the planets whose orbits are nearest to our owr, our information is comparatively full and complete. For instance, we can for these planets give the following information in addition to that stated in Arts. 139 and 140, and Table II. of the Appendix :

The first column requires no explanation. We see, however, at once that the day in Mars is nearly equal to our own, while in the large planets, Jupiter and Saturn, it is not half so long. Now the revolutions of these planets round the Sun are accomplished as follows:-

We can therefore easily find the number of days according to the period of rotation of each planet, which go to make each planetary year: thus in Saturn's year there are 24,584 Saturnian days, or 67 times more days than in our own.

254. In the second column the inclination of the planets' axes of rotation is given. It will be recollected that the inclination of our own is 2310, and that it is on this inclination that our seasons depend. It will be seen at once therefore that Mars and Saturn are much like the Earth in this respect, and that Jupiter is a planet almost without seasons, for the inclination of its axis is only 3o, while that of Venus is 53°. The axis of rotation