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. [Illustration: 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. [Sidenote: PHENOMENA OF SUN-SPOTS.] [Illustration: Fig. 90.--A Sun-spot, showing the great masses of incandescent vapour rising or falling within it. (After Secchi.)] [Illustration: 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. [Illustration: 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. [Illustration: Fig. 93.--The same object, as seen at 11 H. 15 M. on the same day.]