THE

AMERICAN

JOURNAL OF SCIENCE AND ARTS.

[THIRD SERIES.]

ART. LVL-On Jupiter and its Satellites; by MARIA MITCHELL, Professor of Astronomy in Vassar College. (With a plate.)

THE observations referred to in the following notes were made at a time when the mechanism of the instrument in use was receiving improvements, and the records are therefore those of eye estimates and not of accurate measurements. They do not, however, rest wholly on my own impressions. Separate drawings and estimates were made in several cases by three of my students, Misses Abbot, Glover, and Mead, and my own were checked by theirs.

The characteristics of the phenomena seen on Jupiter's disc in the Fall of 1870 and the Winter of 1870-71 differed from those of the preceding year. The rosy tinge of the equatorial belt was less marked; the dark spots on the same belt were less decided, and the white spots more numerous. In the notes of Jan. 9, 1870, I use the expression "very rosy" in describing the upper edge of the broad belt; words which I could not use in regard to the color, at any time of observation since that date. The instrument used was the large Equatorial telescope, the object-glass of which is 12 inches in diameter.

The oval marking given in fig. 1 was seen Jan. 19, 1870. It is without doubt the same seen by Prof. Mayer at an earlier date. It is at least a singular coincidence, that a group of small spots on the sun, arranging themselves in the same figure, is recorded on Jan. 22. I record on June 18, 1870, a rosy tinge on the extreme northern edge of Saturn's ring; the night being remarkably good.

AM. JOUR. SCI.-THIRD SERIES, VOL. I, No. 6.—JUNE, 1871.

Nov. 1, 1870.-Two shadows and one satellite were seen at once on Jupiter's disc. The satellite, which was the first, was faint, brown, and ill defined, when first seen; near the center of the planet, its shape was oval. A large spot could be seen on the fourth satellite.

Nov. 10, 1870.-The narrow southern dark belt has a violet tinge.

Nov. 16, 1870.-A short and broad violet marking was seen near the northern border.

it.

Nov. 28, 1870.-The broad equatorial belt could be seen out to the following edge. There were three large white spots upon The third satellite was warmer in tint than the other three. Dec. 3.-The broad belt was seen again, quite out to the following limb. The first satellite was seen on the planet near the preceding limb, of dazzling whiteness, resembling the polar regions of Mars. White spots were noticed on the broad belt. The fourth satellite seemed to have a halo around it.

Dec. 10, 1870.—I began early to watch for the entrance of the first satellite and its shadow. The planet was low and the air full of tremor. I lost the satellite at once, when it entered upon the planet, and did not see it until 8 P. M., when I found it near the center of the disc. It seemed to be round and was dusky in color. Only twelve minutes later, this round figure had enlarged and become an irregular oval figure, stretching into the broad equatorial belt; it was then very distinct. As the shadow and satellite were thrown upon the planet nearly at the same time, it is probable that they were seen together, but there was no distinction of color. It seemed as large as the third satellite plus about its third. The whole shrunk suddenly, and then disappeared a few minutes after 9 P. M., and at 9h 32m P. M. the satellite reappeared, still on the planet, round and very white. At 10h 5m the broad dark equatorial belt was seen out to the following edge. I have given the changes of the first satellite in figs. 3 and 4. No inequality of brightness could be detected in the bright belt along which the course of the satellite lay.

Dec. 16, 1870.-The noticeable peculiarities are, the continuance of the dark equatorial belt quite out to the limb of the planet, both preceding and following, but more strongly in the latter case, and the unequal brightness of the southern bright belt, presenting in a coarse way something of the appearance of the sun's disc. The third satellite was brighter at the center than at the edges.

Jan. 7, 1871.-A very marked phenomenon is represented in figs. 5 and 6. A white oval spot was seen preceded by two brownish spots at 6h 57m. It was afterwards seen only by glimpses,

until 7h 36m when it reappeared, but with a change of relative position; it was then between the brown spots. The shadow of the second satellite, very black at first, dimmed as it came to the center, and became elongated.

Jan. 13, 1871.—Two white spots seen near the northern edge seemed to change in relative position in the course of half an hour.

Jan. 16, 1871.-The passages of the first satellite and of the shadow were carefully watched. The satellite was seen through nearly the whole transit; it was lost for about 20 minutes when near the center-at this time the shadow became elongated. When the shadow entered upon Jupiter, it seemed to have a penumbra around it, and to be preceded by a small white spot. A large white spot was seen above the most northern dark belt.

Jan. 19, 1871.-Three large white spots were seen on the dark equatorial belt. The third satellite was on the planet and was smaller than any one of these spots.

In watching the changes of Jupiter's bright cloudy belts or of its dark bands, one is continually reminded of the changes in the sun's photosphere. Although the variations are less marked on Jupiter than on the sun, yet it is true of both, that drawings made at intervals of half an hour would show different relations of dark and light spots.

The changes in the satellites seem to me peculiar. I have watched the first satellite a good deal, and while I have never seen it enter upon, or leave the disc as other than a white circular object, either I have lost it at the center, or it has changed in shape and color,-becoming elongated toward the polar regions, and assuming a reddish-brown hue. The impression made upon the observer is that of the interposition of some medium through which it is seen dim and distorted.

ART. LVII.-On the Physical Constitution of the Sun; by Professor W. A. NORTON.

AMONG the recent theories of the Physical Constitution of the Sun, based on the later discoveries, astronomical and spectroscopic, that propounded a few years since by M. Faye has been most favorably received. It is an essential feature of this theory that the sun's mass consists wholly, or in a great degree of gases or vapors; and that a process of interchange of solar matter between the interior and the photosphere is in incessant operation, in ascending and descending currents, by which the

* Comptes Rendus, vol. lx, pp. 89 and 138.

solar radiation is maintained. In a paper by J. Homer Lane, on the Theoretical Temperature of the Sun, &c., published in this Journal, July, 1869, it is elaborately argued, and appears to be successfully maintained, that the great rapidity of circulation required by this theory cannot subsist consistently with the received laws of gaseous circulation. Quite recently another theory of the sun's physical constitution has been propounded by Professor F. Zöllner of Leipzig; based mainly on the well established fact that the solar protuberances, conspicuously visible in total eclipses, and observable at all times with the aid of a spectroscope, are most of them due to violent eruptions of masses of incandescent hydrogen.* He remarks that "it is impossible, without passing beyond the well known analogies necessary for the explanation of cosmical phenomena, to assign any other cause to these eruptions than the difference of pressure of the gases emanating from the interior and from the surface of the sun. To make such a difference of pressure possible it is necessary to admit the existence of a separating stratum between the inner and outer strata of hydrogen-the latter of which, as is well known, forms an important portion of the solar atmosphere. In reference to the physical constitution of this stratum, we must furthermore assume that it cannot be gaseous, and must therefore be either solid or liquid." He remarks further, that "with regard to the inner masses of hydrogen bounded by that stratum, two suppositions are possible, viz: 1. The whole interior of the sun is filled with incandescent hydrogen gas, which would make the sun an immense bubble of hydrogen surrounded by a liquid glowing envelope. 2. The masses of hydrogen, bursting out into protuberances, are local collections in bubble-like caverns, which form in the superficial layers of a liquid glowing mass, and burst through when the pressure of the confined gas increases." Prof. Zöllner adopts the latter supposition as the more probable of the two.

This theory may furnish an adequate supposable cause for the observed eruption of incandescent masses of hydrogen, but its fundamental hypotheses have no secure ground to rest upon. The notion, that the sun's photosphere is in the liquid state, is irreconcilable with the astonishing rapidity with which changes often occur on the sun's surface, and also with the fact that the vast elevated masses, seen as the faculæ, occasionally retain the same position for several days; and though suggested long since, has not, to my knowledge, been adopted by any astronomical observer. No hypothesis of the possible origin of the sun's spots, upon this idea, has been framed that affords a satisfactory explanation of even their more conspicuous features and phe

*Journal of the Franklin Institute, Nov., 1870; also Philosophical Magazine, Nov., 1870.

nomena. To this remark the theory advanced by Prof. Zöllner, viz: that "the nucleus of the solar spots is a scoriaceous product of local cooling on a liquid surface, and the penumbræ clouds of condensation, which surround at a certain height the coasts of these islands of slag, offers no exception." It is not new, and has been already overthrown by the investigations of M. Faye.* The other fundamental hypothesis of Professor Zöllner's theory of the physical constitution of the sun, viz: that the masses, which burst out into protuberances, are local collections in bubble-like caverns, which form in the superficial layers of a liquid glowing mass, does not derive any support from analogical facts. It must be regarded as a pure hypothesis, unsustained by any inherent probability, or by any known fact, other than that which it is framed to explain. Besides, an hypothesis which brings the hydrogen in eruptive, or streaming masses, to the sun's surface, does not suffice. Another arbitrary hypothesis is required to dispose of the hydrogen which has thus been accumulating above the sun's photosphere for an indefinite period of time.

Such being the state of the case with regard to the recent attempts to discover the secret of the sun's physical constitution, in the light of the late remarkable discoveries, we seem to be in this dilemma; whatever conception is formed of the condition of the sun's photosphere-whether liquid or gaseous-it appears to be contradicted by received principles, or controverted by established facts. We are thus naturally led to suspect that either some physical cause has been hitherto left out of account, which plays an important part in solar phenomena, or else the conception adopted of the mechanical condition of the solar vapors is radically at fault. It appears to me that good and sufficient reasons may be urged that will justify both these grounds of suspicion; and that a new point of view may be gained from which we may obtain a deeper insight into the physical processes in operation on the sun.

It is a little remarkable that it should have been hitherto assumed that the facts and laws of Terrestrial Physics can alone furnish a true philosophical ground for a theory of Solar Physics; and that no serious attempt should have been made to obtain additional light from known processes in operation on a cosmical scale in the regions of space. There is a class of bodies, some of which in their periodical excursions through the fields of space approach quite near the sun, and which are in general conspicuously subject to influences of a powerful nature exerted by the sun, besides the force of gravitation. It cer

* Comptes Rendus, vol. Ixi, p. 1089.