Jupiter has the same general appearance with Mars, only that the belts on his surface are much larger and more permanent. They are not to be seen but by a good telescope, and their appearances differ very much at different times, and even at the same time in telescopes of different powers. Their number is very variable, as sometimes only one, and at others there are as many as eight. They are generally parallel to one another, but not always so; and their breadth is likewise variable, one belt having been observed to grow narrow, while another in its neighbourhood has increased in breadth. They seem almost always to be of an uniform tint; but in very favourable weather, they sometimes seem to consist of a number of curved lines, something like strokes made with the graver. The time of their continuance is very uncertain, sometimes remaining unchanged for three months; at others, new belts have been found, in an hour or two. In some of these belts, large black spots have occasionally appeared, which were observed to move swiftly over the disk from east to west, and to return in a short time to the same place. The belts of Jupiter were first observed at Naples by Zuppi and Bartoli, two Jesuits; and about the year 1660, they were observed by Campani, with refracting telescopes of his own construction, and not much inferior in distinctness to those of the present day; the great modern improvement in refracting telescopes consisting rather in the reduction of their size than in the increase of their magnifying power. The figure of Jupiter is that of an oblate spheroid, the longest diameter being to the shortest in the proportion as 13 to 12; his rotation being from west to east, like that of the Sun, and the plane of his equator being very nearly coincident with that of his orbit, so that there can be scarcely any difference of seasons in that planet. His rotation has been observed to be somewhat quicker in his aphelion than his perihelion. The axis of rotation is nearly perpen dicular to the plane of the ecliptic, and the planet makes one diurnal revolution in 9 h. 55 m. 37 s. The changes in the appearance of his spots, and the difference in the time of their rotation, make it probable that they do not adhere to Jupiter, but are clouds transported by the winds with different velocities in an atmosphere subject to violent agitations. The apparent diameter of this planet differs from 46" to 31"; but his mean apparent diameter is equal to 36". Jupiter is accompanied by four satellites, which were first discovered by Galileo on the 8th of January 1610. The relative situation of these small bodies changes at every instant: they oscillate, as it were, on each side the planet, and it is by the extent of these oscillations that the rank of the satellites is determined, that being called the first satellite whose oscillation is the least. They are sometimes seen to pass over the disk of the planet, and project a shadow in the form of a well defined black spot, when they describe a chord over his disk. Hence it is inferred that Jupiter and his satellites are opaque bodies enlightened by the Sun; and when the latter interpose between the Sun and Jupiter, they produce real solar eclipses precisely similar to those which the Moon occasions on the earth. This phenomenon leads to the explanation of another which the satellites present. They are often observed to disappear, though, at some distance from the disk of the planet, the third and fourth reappear sometimes on the same side of the disk. The shadow which Jupiter projects behind it is the only cause that can explain these disappearances, which are perfectly similar to eclipses of the Moon. The circumstances which accompany them leave no doubt of the reality of the cause. The satellites are always observed to disappear on the side of the disk opposite to the Sun, and consequently on the same side to which the conical shadow is projected. They are eclipsed nearest the disk when the planet is nearest to its opposition. Lastly, the duration of these eclipses answers to the time which should elapse while they traverse the shadow of Jupiter. Thus, it is evident that these satellites move from west to east. Their mean sidereal and synoidical revolutions, as seen from the centre of Jupiter, are very accurately determined by comparing eclipses at long intervals from each other, and observed near the opposition of the planet. It is thus discovered that the motion of Jupiter's satellites is almost circular and uniform, because this hypothesis corresponds very nearly with those eclipses which happen when we see the planet in the same position relatively to the Sun. Therefore, the positions of the satellites at every instant, as seen from the centre of Jupiter, may be determined. From hence results a simple and exact method of comparing with each other the distances of Jupiter and of the Sun from the earth, a method which antient astronomers could not possess. They only judged of its distance by the time of its revolution; as they estimated those planets to be farthest from us whose period of revolution was the longest. It may further be observed, that, when Jupiter is in quadrature with the Sun, the earth is farthest out of the line that passes through the centres of the Sun and Jupiter, and therefore the shadow of the planet is then most exposed to our view: but, even then, the body of the planet will hide from us one side of that part of the shadow which is nearest to it, through· which the first satellite passes; which is the reason that, though we see the entrance of that satellite into the shadow, or its coming out from that thence, as the earth is situated on the east or west side thereof, we cannot see them both; whereas the other satellites going through the shadow at a greater distance from Jupiter, their ingress and egress are both visible. The Naturalist's Diary. Arise, ye winds, 'tis now your time to blow, NATURE having perfected her seeds, her next care is to disperse them: the seed cannot answer its purpose while it remains confined in the capsule. After the seeds, therefore, are ripened, the pericarpium opens to let them out; and the opening is not like an accidental bursting, but, for the most part, is according to a certain rule in each plant. What I have always thought very extraordinary nuts and shells, which we can hardly crack with our teeth, divide and make way for the little tender sprout which proceeds from the kernel. Handling the nut, I could hardly conceive how the plantule was ever to get out of it. There are cases, it is said, in which the seed-vessel, by an elastic jerk, at the moment of its explosion, casts the seeds to a distance'. We all, however, know that many seeds (those of the most composite flowers, as of the thistle, dandelion, &c.) are endowed with, what are not improperly called, wings; that is, downy appendages, by which they are enabled to float in the air, and are carried oftentimes, by the wind, to great distances from, the plant which produced them.. It is the swelling, also, of this downy tuft within the seed-vessel, that seems, to overcome the resistance of its coats, and to open a passage for the seed to escape. Some seeds which are furnished with hooks or spines, attach themselves to the rough coats of ani 'This is particularly instanced in the tonch-me-not (impatiens), and the cuckoo flower (cardamine). mals, and thus promote their dispersion'. Others are contained in berries, and, being swallowed by birds, are again committed, without injury, to the earth, in various places. It is a curious circumstance, and not generally known, that most of those oaks, which are called spontaneous, are planted by the squirrel. This little animal has performed the most essential service to the British navy. A gentleman, walking one day in the woods belonging to the Duke of Beaufort, near Troy House, in the county of Monmouth, his attention was diverted by a squirrel, which sat very composedly upon the ground. He stopped to observe his motions. In a few minutes, the squirrel darted like lightning to the top of a tree, beneath which he had been sitting. In an instant, he was down with an acorn in his mouth, and began to burrow in the earth with his hands. After digging a small hole, he stooped down, and deposited the acorn; then covering it, he darted up the tree again. In a moment he was down with another, which he buried in the same manner. This he continued to do as long as the observer thought proper to watch him. The industry of this little animal is directed to the purpose of securing him against want in the winter; and, as it is probable that his memory is not sufficiently retentive to enable him to remember the spots in which he deposits every acorn, the industrious little fellow, no doubt, loses a few every year. These few spring up, and are destined to supply the place of the parent tree. Thus is Britain, in some measure, indebted to the industry and bad memory of a squirrel for her pride, her glory, and very existence !2 her Horse-beans are set by jays and pies, who hide them among the grass and moss, and afterwards, 'The common bur belongs to this class. 2 Philosophy of Nature, vol. i, pp. 30, 31. |