ERUPTIVE ACTION IN THE SUN.

That the forces operating in each of these distant bodies present striking points of analogy is equally clear. The sun is of far greater dimensions than our earth, and is still in great part, if not entirely, in a gaseous condition. The great movements in the outer envelopes of the sun exhibited in the 'sun-spots' and 'solar prominences,' recall to the mind the phenomena of volcanic activity upon our globe. But the vast energy still existing in the intensely heated mass of the sun, and the wonderful mobility of its gaseous materials, give rise to appearances beside which all terrestrial outbursts seem to sink into utter insignificance. Vast cavities of such dimensions that many globes of the size of our earth might be swallowed up in them are formed in the solar envelopes in the course of a few days or hours. Within these cavities or sun-spots incandescent vapours are observed, rushing upwards and downwards with almost inconceivable velocity.

The drawings made by Secchi, and reproduced in figs. 89 and 90, will give some idea of the appearances presented by these great holes in the solar envelopes.

Fig. 89.—A group of Sun-spots. (After Secchi.)

In fig. 89 a group of sun-spots is represented and, in their circular outlines and tendency to a linear arrangement, they can scarcely fail to remind anyone familiar with volcanic phenomena of terrestrial craters, though their dimensions are so much greater.

In fig. 90 the sun-spot represented shows the presence of large floating masses of incandescent materials rushing upwards and downwards within the yawning gulf.

PHENOMENA OF SUN-SPOTS.
Fig. 90.—A Sun-spot, showing the great masses of incandescent vapour rising or falling within it. (After Secchi.)
Fig. 91.—The edge of a Sun-spot, showing a portion of the prominent masses of incandescent gas (A), which detached itself at E and floated into the midst of the cavity.

From fig. 91, taken from a drawing by Mr. Norman Lockyer, we may understand the movements of these great protuberances of incandescent gas which are seen on the sides of the sun-spots.

The so-called solar prominences present even more striking resemblances to the volcanic outbursts of our globe.

Two drawings made by Mr. Norman Lockyer will serve to give some idea of the vast dimensions of these solar prominences, and of the rapid changes which take place in their form.

Fig. 92.—Drawing of a Solar prominence, made by Mr. Norman Lockyer on March 14, 1869, at 11 H. 5 M. A.M.

The masses of incandescent gas were estimated as being no less than 27,000 feet in height, yet in ten minutes they had totally changed their form and appearance, as shown in fig. 93.

Even still more striking are the changes recorded by Professor Young, of New-Haven, in a solar prominence, which he observed on September 7, 1871.

Fig. 93.—The same object, as seen at 11 H. 15 M. on the same day.
SOLAR PROMINENCES.

That astronomer described a mass of incandescent gas rising from the surface of the sun to the height of 54,000 miles. In less than twenty-five minutes he saw the whole mass torn to shreds and blown upwards, some of the fragments being in ten minutes hurled to the height of 200,000 miles above the sun's surface. The masses of incandescent gas thus hurled upwards were of enormous dimensions, the smallest being estimated as having a greater area than the whole of the British Islands, and the force with which they were urged upwards was so great that they acquired a velocity of 166 miles per second. The accompanying woodcut shows the successive appearances presented by this grand eruptive outburst on the surface of the sun.

Fig. 94.—Drawings of a Solar prominence at four different periods on Sept. 7, 1871. (After Young.)

EXTINCT VOLCANOES OF THE MOON.

The moon, which is of far smaller size than our earth, exhibits on its surface sufficiently striking evidences of the action of volcanic forces. Indeed the dimensions of the craters and fissures which cover the whole visible lunar surface are such that we cannot but infer volcanic activity to have been far more violent on the moon than it is at the present day upon the earth. This greater violence of the volcanic forces on the moon is perhaps accounted for by the fact that the force of gravity on the surface of the moon is only one-sixth of that at the surface of the earth; and thus the eruptive energy will have a much less smaller resistance to overcome in bursting asunder the solid crust and accumulated heaps of ejected materials on its surface. But the volcanic action on the moon appears now to have wholly ceased, and the absence of both water and atmosphere in our satellite suggests that this extinction of volcanic energy may have been caused by the complete absorption of its gaseous envelope. The appearance presented by a portion of the moon's surface is shown in fig. 95.

The sun and the moon appear to exhibit two widely separated extremes in the condition assumed during the cooling down from a state of incandescence of great globes of vaporised materials. The several planets, our own among the number, probably exhibit various intermediate stages of consolidation.

Fig. 95.—A group of Lunar craters (Maurolycus, Barocius, etc.), the largest being more than 60 miles in diameter.
ERUPTIVE ACTION IN THE SUN, EARTH AND MOON.

Our earth is, as we have seen, closely allied to the other bodies of the solar system in its movements, its relations, and its composition; and a true theory of terrestrial vulcanicity, when it is discovered, may be expected not only to afford an explanation of the phenomena displayed on our own globe, but to account for those displays of internal energy which have been manifested in other members of the same great family of worlds.