it is called, is the largest and most permanent of any yet discovered. It appeared and vanished eight times between the years 1G65 and 1708, then continuing invisible till 1713. The spot has been frequently observed since, some times remaining visible for two or three years in succession, at others being unseen for as long or longer. It would be rash, however, to assume that the same spot has been seen, especially as there is reason to believe that a dark spot on J upiter, instead of being a permanent feature of his surface (and only concealed from time to time by clouds), resembles rather a large spot upon the sun, an opening formed either by cyclonic disturbance in a deep atmosphere, or by some process of disturbance acting from beneath such an atmo sphere. From various sets of observations made upon spots seen for long periods, the following rotation-periods have
been deduced:— | A table should appear at this position in the text. See Help:Table for formatting instructions. |
Jupiter s notation-Period, h. m. 8. Cassini (1665).. Silvabelle , Schrbter (1786) Airy Madler (1855) . 9 56 9 56 9 55 33 9 55 25 9 55 26-6
Miidler s rotation-period is commonly regarded as the most reliable. It was based on observations commenced on November 3, 1834, and continued on every clear night until April 1835, a period including 400 revolutions. Two dark spots were visible when these observations were made. It need hardly be said, however, that very little reliance can be placed on the seconds in Miidler s, Airy s, or Schrotcr s determinations. In fact, there is clear evidence that spots on Jupiter are subject to a proper motion, like that which affects the spots on the sun. Schmidt, in No. 1973 of the Astronomische Nachrichten, gives a number of cases of such proper movements of spots, ranging in velocity from about 7 miles to about 200 miles an hour. It may be noted, also, that from a series of observations of one spot made between March 13 and April 14, 1873, with the great Rosse reflector, a period of 9 h. 55 m. 4 s. was deduced, while observations of another spot in the same interval gave a rotation-period of 9 h. 54m. 55 4 s.
The equator of Jupiter is inclined only 3 5 30" to million the planet s orbit, so that there can be no appreciable P lter - seasonal changes. We should expect, therefore, a great calm to reign in the atmosphere of this planet, the more so that the sun s heat pourei upon each square mile of it is (on the average) less than the 27th part of that received by each square mile of the earth s surface. Moreover, the seasons of Jupiter last nearly twelve times as long as ours, so that we should expect all changes in his atmosphere produced by solar action to take place with exceeding slowness. When, instead, we find signs of verv rapid changes in the aspect o f the belts, implying remark;, ble changes in the condition of the Jovian atmosphere, we seem compelled to recognise the operation of causes much more active than the heat poured by the sun on the distant orb of Jupiter. It seems natural to supposo that Jupiter s mass is itself intensely heated, and that such inherent heat produces these changes in the condition of his atmosphere. We find also in this theory an explanation of several remarkable circumstances, the significance of which has been somewhat strangely overlooked.
So soon as we institute a comparison between Jupiter and the earth, on the supposition that Jupiter is sur rounded by an atmosphere bearing the same relation to his mass that the atmosphere of the earth bears to her mass, we find that a state of things would prevail in no sort resembling what we are acquainted with on earth. For the mass of Jupiter exceeds the earth s more than three hundredfold, while his surface exceeds hers little more than a hundredfold ; sc that the quantify of atmosphere above each square mile of suria-;? would be three times greater on Jupiter than on the earth, and, owing to the greater force of gravity, the atmospheric pressure would still more largely exceed that on the earth. In fact, the density of the Jovian atmosphere at the surface would be more than six times as great as tha density of our air at the sea level Yet the extension of the atmosphere would be very much less in the case of Jupiter ; for in our air the density is halved for each vertical ascent of 3 miles, whereas in the case of a Jovian atmosphere similarly constituted, the density would be halved for each vertical ascent of 1^ miles. At a height, therefore, of 10 (7 times 1^) miles from the surface of Jupiter the pressure would only exceed 7 (= -j-^gth) that at his surface, or say 3 th that at the surface of our earth; whereas, at a height of 101 (3 times 3iJ) miles from our sea level the atmospheric pressure is still equal to j (= ^th) that at the sea level, or 2 J times as dense as the Jovian atmosphere at the same height under the supposed conditions.
We see that, in the case of Jupiter, under any assump tion of resemblance in atmospheric constitution and con dition, we should infer great density at the surface of the planet, but an exceedingly shallow atmosphere, the density diminishing much more rapidly with vertical height than in the case of our atmosphere ; and if aqueous clouds formed in such an atmosphere, as in ours, they would occupy much shallower layers. Yet everything in the telescopic aspect of Jupiter implies that the cloud-layers are of great depth. In fact, their appearance shows that they are far deeper than the terrestrial cloud-layers. The dis appearance of dark spots at a considerable distance from the limb of the planet, as observed by Madler (who found that the two great spots by which he timed the planet s rotation became invisible at from 56 to 57 from the centre of the disk), would indeed imply that the darker inner region lay at an enormous distance below the upper light-reflecting layer. Yet even a depth of thirty or forty miles below the upper cloud-layer would not be consistent with any hypothesis of resemblance between the condition of Jupiter and the earth. Assuming a depth of only thirty miles, the atmospheric pressure would be increased 2 20 times, or more than a millionfold, and the density in the same degree, if the atmosphere could retain the properties of a perfect gas at that enormous pressure. But such a density would exceed some fiftyfold the density of platinum, and of course no gas could exist as such at a thousandth part of this pressure.
Even when we place on one side the difficulty of reconciling observed appearances with the behaviour of gases under conditions with which we are familiar, a difficulty remains which cannot be removed without regarding Jupiter as utterly unlike our earth. If Jupiter had a shallow atmosphere, so that his solid globe were apparently of the same dimensions as the orb we measure, the mean density of such a globe would far surpass the earth s mean density, if the materials of the two globes were similar ; for the pressure within the globe of Jupiter would be enormously greater. Yet we know that, instead of the density of Jupiter being greater than that of the earth, it is not one-fourth of hers. The theory that Jupiter s globe is hollow is inadmissible, because no shell of Jupiter s mass could resist the pressure generated by its own gravity. The alternative theory suggested by Brewster, that the substance of Jupiter and his fellow-giants of the outer planetary family may be porous, like pumice stone, is equally untenable for a like reason.
